SWITCHED MODE POWER SYSTEM
Descrição do Produto
ACKNOWLEDGEMENT
I would like to express my profound thanks to Mr. Md Arifuddin, ECIL
Hyderabad who permitted me to do the project under his guidance. I would
like to express to him for his excellent guidance through out our project
work.
I would also like to express my gratitude to Mr. G. Kishan Rao, Head
of Dept. EEE, Syed Hashim College of Science & Technology, Pregnapur in
allowing me to do the project work at ECIL, Hyderabad.
I am very grateful to Mr. Shashi Kumar who, as my internal guide gave
valuable suggestions in making this project work possible. I am thankful to
our faculty members of the college for their kind cooperation.
CONTENTS
1) ABSTRACT.
2) INTRODUCTION TO POWER SUPPLIES.
3) POWER SUPPLY TERMINOLOGY.
4) GENERAL DESCRIPTION OF SMPS.
5) SPECIFICATIONS OF SMPS.
6) TOPOLOGIES IN SMPS.
7) THEORY OF OPERATION OF SMPS.
8) TEST PROCEDURES OF SMPS.
9) STANDARDS.
10) FUNCTIONAL TEST REPORTS.
11) TROUBLE SHOOTING OF SMPS.
12) DATA SHEETS.
ABSTRACT
The SMPS series switched mode power supply is a compact light weight
power supply designed to meet stringent quality requirements and to
withstand rugged environmental conditions.
Each power supply has been designed to deliver 100W or 60W DC O/P
with additional features such as threshold current limit, short circuit
protection, Over voltage protection and under voltage indication.
This project is an introduction and study of switched mode power
supply. The various topologies and general description of switched mode
power supplies are explained and demonstrated. A superficial view of test
procedures and trouble shooting have been explained.
INTRODUCTION TO SMPS
Necessity :
Most of the electronic equipment operate on DC
Voltage where as the electrical energy available from main supply is
alternating in nature. This necessitates a DC power supply.
Classification of Power Supplies :
Power supplies can be categorized as follows :
1. Linear Power Supply.
2. Switched Mode Power Supply.
A brief Discussion of the two configurations is as below :
Linear Power Supplies :
Fig 1(a) shows a series pass regulated power supply. In this
type of power supply, a low frequency(50Hz or 60Hz) transformer is used
to step down the Ac mains to a lower voltage of same frequency. This
secondary voltage is in turn rectified and filtered and the resulting Dc is
fed into a series pass active element.
By sampling a portion of output voltage and comparing it to a
fixed reference voltage, and error signal is generated. This signal
varies the base current of the pass transistor, thereby varying
conductivity of the transistor, i.e, the series element is used as
a form of "variable resistor" to control and regulate the output voltage.
In this method of power supplies, the pass transistor works in the
active region of its characteristics.
Switched Mode Power Supply :
Fig 1(b) shows a high frequency off- the-line
switching power supply.
In this scheme the Ac line is directly rectified and
filtered to produce a raw high Dc voltage, which in turn fed into a
switching element. The switch operating at high frequencies of 20KHz to
1MHz, chopping the Dc voltage into high frequency square wave. This
square wave is fed into the power isolation transformer, stepped
down to a predetermined value, they are rectified and filtered to
produce required Dc output.
A portion of this output is monitored and compared against a
fixed frequency reference voltage and the error signal is used to
control the On/Off times of the switching element, thus regulating the
voltage.
Figure-1 (a) BLOCK DIAGRAM OF LINEAR POWER SUPPLY
Figure-1 (b) Simple Block Diagram of SMPS
COMPARISION BETWEEN LINEAR AND SWITCHED MODE
POWER SUPPLY
"S no "Parameters "Linear PS "SMPS "
"1. "Load regulation "0.01 to 0.05 % "0.05 to 0.1% "
"2. "Line regulation "Better "Good "
"3. "Output Ripple "Minimum(1mv-5mv P.P) "More (25mv- 100mv P.P)"
"4. "Noise "Negligible "More "
"5. "Efficiency "Poor (30 to 50%) "Good (70 to 80%) "
"6. "Power Density "0.5W/cm3 ">2.5W/cm3 "
"7. "Weight "Bulky "Less "
"8. "Hold up time "2ms "30ms "
"9. "Transient "20 us "200 us "
" "Response " " "
"10. "Serviceability "Easy "Complex "
POWER SUPPLY TERMINOLOGY
ELECTRO MAGNETIC INTERFERENCE (EMI) :-
Also called radio-frequency interference(RFI), EMI is unwanted high
frequency energy caused by the switching transistors, output rectifiers,
and zener diodes in switching power supplies. EMI can be conducted through
the input or output lines or radiated through space.
HOLD UP TIME:-
The total time any output will remain with in the regulated band
after the input line voltage has been turned OFF. Typically measured at
full load and nominal line conditions.
INRUSH CURRENT :-
A high surge of input current that occurs in switchers and
occasionally in linear upon initial turn on.
OFF-LINE SWITCHER :-
A circuit combination commonly used in PWM switchers in which the
input rectifiers and filter section sit directly across the A.C input line.
SNUBBER :-
A network consisting of a resistor, capacitor and diode used in
switching power supplies to trap high energy transients and to protect
sensitive components.
SOFT START :-
Input surge current limiting in a switching power supply where the
switching drive is slowly ramped on.
STABILITY :-
The change in output voltage that occurs at constant load, AC input,
the temperature after a given period of time following warm-up. This effect
is related, in part to internal temperature and aging effects.
TRANSIENT RESPONSE TIME :-
The amount of time taken for an output to settle within some
tolerance band, normally following a stated change in load.
BLEEDER RESISTOR :-
A resistor usually connected across a filter circuit to discharge
capacitors when the unit is turned off.
BURN OUT :-
Condition during peak usage periods when electric utilities reduce
their nominal line voltage 10% to 15%.
BURN OUT PROTECTION :-
The ability of the power supply to continue operating within
specification through the duration of burnout.
CONSTANT CURRENT :-
A power supply that regulates current level regardless of changes in
load resistance.
CROSS REGULATION :-
In a multiple- output power supply, the load variation of one output
can cause a voltage change in other outputs. This voltage change divided by
its nominal value is the cross regulation.
CURRENT LIMITING CIRCUIT :-
A bounding circuit designed to prevent overload of constant-voltage
power supply. It can take the form of constant, fold back or cycle-by-cycle
current limiting.
EFFICIENCY :-
The ratio of output power to input power. It is generally measured at
full load and normal line conditions. In multiple-output switching power
supplies, efficiency can be a function of total output power and its
division among the outputs.
EQUIVALENT SERIES CAPACITOR (ESR):-
The amount of resistance in series with an ideal capacitor,
duplicates the performance of a real capacitor. In general, the low ESR,
the higher the quality of the capacitor and the more the effective, it is
as filtering device. ESR is a prime determinant of ripple in switching
power supplies.
FILTER :-
A frequency-sensitive network that attenuates unwanted noise and
ripple components of a rectified output.
FLY BACK CONVERTER :-
Switching power supply configuration using a single transistor and a
fly back diode.
FORWARD CONVERTER:-
Switching power supply configuration using single transistor.
HYBRID SUPPLIES :-
A power supply that combines two or more different regulation
techniques, such as ferro resonant and linear or switching and linear.
ISOLATION :-
The degree of electrical separation between two points. It can be
expressed in terms of voltage (break down), current(galvanic), or
resistance and/or capacitance (impedance), in power supplies. It is
important to maximize the input to output isolation.
LEAKAGE CURRENT :-
Current flowing between the output buses and chassis ground due to
imperfections in electronic components and designs.
LINEAR REGULATOR :-
A common voltage stabilization technique in which the control
device(usually a transistor) is placed in series or parallel with the power
source to regulate the voltage across the load. The term "linear" is used
because the voltage drop across the control device is varied continuously
to dissipate unused power.
NOISE :-
Noise is the aperiodic, random component of undesired deviations in
the output voltage. Usually specified in combination with ripple.
OPTO-ISOLATOR:-
Device that provides electrical isolation and signal path by making
an electrical to optical to electrical signal transformation from its input
to output terminals. This is accomplished with a light-emitting-diode in
close proximity to a photo-transistor. Opto-isolators are used in the
feedback loop to maintain electrical isolation between input to output of
the power supply. Aging may provoke the demanded feed-back response.
OVER VOLTAGE PROTECTION :-
A protection mechanism for the load circuitry that does not allow the
output voltage to exceed a preset level. In most cases, the output voltage
is reduced to a low value, and the input power must be recycled to restore
the power supply output. Often protection is provided by a suppressor diode
across the output, engaging over current limiting.
PULSE WIDTH MODULATION:-
A circuit used in switching regulated power
supplies were the switching frequency is held constant and width of
the pulse is varied, controlling both the line and load changes
with minimum dissipation.
RECOVERY TIME:-
The time required by a transient overshoot or
undershoot in a stabilized output quantity to decay to with in the
specified limits.
REDUNDANCE:-
The ability to connect power supplies in
parallel so that if one fails the other will provide continuous power to
the load this mode is used in systems when power supply failure cannot
be tolerated.
RIPPLE:-
The periodic A.C noise component of the power supply output
voltage.
SCHOTTKY DIODE:-
A device that exhibits a low forward voltage drop(0.6V) and a
fast recovery time. This type of diode is especially useful at high
current, low voltage(typically 5V D.C to D.C), were low losses and high
switching speed are important.
SWITCHING FREQUENCY:-
The rate at which the source voltage in a switching regulator or
chopped in a D.C to D.C converter.
SWITCHING REGULATOR:-
A high-efficiency D.C to D.C converter consisting of inductors and
capacitors to store energy and switching elements(typically transistors
of SCR's), which open and close as necessary to regulate voltage across
a load. The switching duty cycle is generally controlled by a feedback
loop to stabilize the output voltage.
THERMISTOR:-
A device which relatively high electrical resistance when cold and
almost no resistance when at operating temperature. Thermistors are
sometimes used to limit inrush current in off-line switchers.
TYPES OF TOPOLOGIES IN SMPS
There are three types of power converters in SMPS, there are
1) Fly back or Buck boost converter
2) Forward or Buck converter
3) Push Pull or Derived converter
OPERATION OF FLYBACK CONVERTER:
The fly back circuit is shown in fig2. When the switch
is closed, the current flows through inductor L storing energy, because
of the voltage polarity, diode D is reverse biased ,thus no voltage
is present across the load. When the switch is open inductor L
reverses polarity because of collapsing magnetic field, forward biasing
diode D, inducing a current flow Ic. Thus an output voltage of opposing
polarity to the input voltage appears across R. Since the switch
commutates continuous inductor current alternatively between input and
output, both the currents are pulsating in form. Therefore in the buck
boost converter, energy is stored in the inductor L during the switch on
period. Then this energy is transferred to the load during the flyback or
switch off period.
OPERATION OF FORWARD CONVERTER:
Figure3 illustrates the operation of forward converter.
When the switch is
Closed, current I flows in a forward converter through inductor L
producing an output voltage across load. Diode d is also reverse biased due
to the direction of the input voltage.
When the switch opens the magnetic field in the inductor L
changes the polarity forward-biasing diode D and producing a current
through capacitor C. Therefore the output voltage polarity across
resistance R remains the same. Diode D is often called a "freewheeling"
or "fly wheel diode". Because of this switching action the output current
is continuous and non pulsating.
OPERATION OF PUSH PULL CONVERTER:
The circuit of push-pull converter is shown in figure4. It is the
combination of two forward converters operating in push-pull or (push-
push) action, with alternate closing of either switch S1 or S2.
In SMPS we use two transistor forward converter.
FORWARD CONVERTER
In forward converter reduced voltage stress is offered by two
transistor forward converter .Q1 and Q2 are in series with the top
and bottom of the transformer primary. Both of these transistor are
turned ON and OFF simultaneously. When they are on primary and secondary
dot ends of the transformer are positive and power is delivered to the
loads when they are turned OFF, current stored in the transformer
magnetizing inductance reverses polarity of all the windings.
The dot end of the primary of the transformer tries to go far
negative but is caught at ground by diode D1. The no dot end of transformer
primary tries to go far positive but is caught at Vdc by diode d2.
Thus the emitter of Q1 can never be more than Vdc below its
collector, and the collector of Q2 can never be more than Vdc above its
emitter. Leakage inductance spikes are clamped so that the maximum voltage
stress on either transistor can never be more than the maximum DC input
voltage.
Energy stored in the leakage inductance during the ON time is fed
back in to Vdc via D1 and D2. When the transistors turn OFF ,the leakage
inductance current flow out of the no-dot end of primary of transformer
through D2,into the positive end of Vdc, out of the no-dot end ,and up
through D1 back in to the dot end of primary of transformer.
For the reverse polarity voltage across primary of transformer
when the transistor are off is equal to the forward polarity voltage
across it when in on condition. Thus the core will always succeed in
being fully reset with 20% safety margin before the start of next half
cycle and the maximum on time is never required to be greater than 80% of a
half period. Now when Q1, Q2 has turned off, the dot end of secondary go
negative with respect to its no-dot end. Current in output inductor L tries
to decrease .Since current in inductor can not change instantaneously, the
polarity across inductor reverses in an attempt to maintain constant
current. The inductor discharges and the current now continues to flow in
the same direction through the free-wheeling diode back in to the inductor.
ADVANTAGES OF CURRENT MODE
1. Avoidance of flux imbalance in push-pull converters.
2. Instantaneous correction against line voltage changes with out the
delay in a error amplifier voltage feed back forward
characteristics.
3. Ease an simplicity of feed back loop stabilization.
4. A no. of current mode power supplies may be operated in parallel
with each sharing the total load current equally.
5. Improved load current regulation.
CURRENT MODE DEFICIENCIES AND PROBLEMS :
1. Constant peak Vs constant average output inductor problems
2. Response to an output inductor current disturbance.
3. Slope compensation to correct problems in current mode.
4. Slope compensation with a positive going ramp voltage.
5. Implementing the slope compensation
GENERAL DESCRIPTION OF SMPS
SMPS 100/60 Series Switched Mode Power Supply is a compact,
efficient light weight power supply designed and developed at ECIL,
HYD, to meet stringent quality requirement and to withstand rugged
environmental conditions.
Each power supply has been designed to deliver 100w/60w Dc
output with additional features such as threshold current limit, short
circuit protection, over voltage protection, and under voltage indication.
This is an off-the-line power supply. (i.e., input is connected
directly to mains without isolation transformer.)
Power Supply unit consists of two sections:-
1. Power Board Section.
2. Control Section.
Power Board Section
Power Board Section consists of input line filter, rectifier, filter,
switching element , snubber circuit, drive circuit, output power
transformer, high frequency rectifier and filter section.
Control Section consists of three cards
1. Control Card.
2. Error Amplifier Card.
3. Alarm Card.
Control Card consists of UC1846 PWM control IC, it has oscillator circuit,
over voltage protection circuit and short circuit protection.
Error amplifier consists of a comparator for error voltage generation.
Alarm Card consists of under voltage indication circuit , a
rectifier and a filter section.
All the three control section cards are mounted on the power board with the
help of right angled pin connectors.
Input and Output connections are available from power board edge connector
which is a 48 pin heavy duty EURO connector.
FIGURE(1) BLOCK DIAGRAM OF SWITCHED MODE POWER SUPPLY
EXPLANATION OF BLOCK DIAGRAM OF SWITCHED MODE
POWER SUPPLY
The block diagram of switched mode power supply is shown in fig1.
RFI FILTER:
The EMI/RFI filter ensures that the electromagnetic
interference generated in the equipment is not coupled back to supply mains
there by contaminating near by equipment.
INPUT RECTIFIER AND FILTER:
After RFI filter the input is directly fed into a bridge
rectifier where it is rectified and raw D.c output voltage is obtained.
SWITCHING ELEMENT:
This high voltage raw dc is chopped to high frequencies by the
switching element(MOSFET).These transistors are in turn driven by pulses
obtained from a control circuit. Thus the output voltage can be controlled
by varying the on times of switching transistor.
This high frequency square wave is coupled to the output
section through a power transformer.
Either the switch is ON or OFF, energy is dissipated for
only half cycle, there by resulting in a very high overall power supply
efficiency of about 70% to 80%. Load current is sensed and on time
controlled accordingly to limit the output current, there by protecting
the circuits which are powered by this supply.
ISOLATION POWER TRANSFORMER:
This transformer is used to step down the high frequency pulses to
required voltage and to provide isolation. The size of this transformer is
quite small due to high operating frequency. This results in compactness of
the supply.
OUTPUT SECTION:
These high frequency isolated stepped down pulses are in turn require
special components such as schottky or fast recovery rectifiers. Schottky
diodes are used because of their low forward voltage drop, therefore they
provide higher efficiencies. The Ac ripple frequency fed in to the
filters is very high, hence a bank of low ESR capacitors are employed
to reduce the ripple to a minimum voltage.
Output supervisory circuits form part of the output section to
protect the load from power supply failures, which normally consists of
over voltage protection and under voltage protection. It is also possible
to have two or more secondaries so that a range
Of Dc output voltages are possible.
INPUT/OUTPUT ISOLATION:
Input to output isolation is provided by the separating the grounds
of the control section and main circuit. Isolation can be achieved by means
of opto coupler or by pulse transformer.
FEEDBACK AND CONTROL:
A portion of the output is sensed by the control circuit which
consists of the following:
1)Error amplifier
2)Pulse width modulator.
ERROR AMPLIFIER:
It has two inputs, one of its connected to a voltage reference .The
other input is connected at the output of the power supply. The variation
in the output voltage is always sensed and compared with the reference
voltage. A signal proportional to the deviations of output is generated
at the output of error amplifier. Error amplifier is driven to the input
of the PWM.
PULSE WIDTH MODULATOR:
PWM generates a pulse whose on or off period is function of error
amplifier output i.e the pulse width varies in accordance with the changes
in the
1)output load current, 2)output voltage.
The functional block for supervisory circuits such as current limiting ,
sensing, soft start, shutdown of the output under emergencies are
integrated along with the PWM function. A precise voltage reference source
is also provided.
Specifications of SMPS
1. Input 240v AC +/- 10%, 50hz, 1ph
(Long Term Variation)
240v AC –30% , for a Duration
of 20msec.
2. DC Output
Model no Output
ECPS- SM100 (05100) 5V, 20A
3. Output Voltage Tolerance +/-1%
4. Load Regulation +/- 0.1% to 0.6%(Typ). For different voltage
ratings, 1% maximum.
5. Line Regulation 0.1% (Typ) 0.5% Maximum.
6. Ripple + Noise 2% Typ. ,1% +2% max. of nominal output.
7. Temp. Co-efficient 0.2% /deg.c Typ, 0.5% max.
8. Threshold Current 120% approx.
9. Short Circuit Over voltage Protection Fixed Factory set. The
Over Voltage CKT.(approx. at 115%)
is reset by interrupting the input
for a short duration.
10. Drift +/- 0.2% Typ. For 8Hrs, 0.5%max.
11. Transient Response < 5msec for a step change of 50% to 100% of
rated value.
12. Efficiency Better than 65%.
13. Hold Up Time 20m sec at 216v AC (10m sec Typ).
14. Insulation Resistance 100 M OHM minimum @ 100v DC.
15. Power Indication "Ac ON " & "Dc ON" indications thru LED's
on front Panel & "Power ON"
Switch provided on Rear Panel.
16. Fuse Protection Provided on Rear Panel.
17. Input / Output Thru 48 Pin STD. EURO Connector
(SOLDER TYPE) ON LINE HOT PLUG-IN TYPE. Test Jacks provided on
rear Panel for measuring DC output.
18. Paralleling/Load Sharing Multiple units can be paralleled for a
load share.
19. Operating Temp. 0 to 55oC , RH- 55% to 85%.
20. Storage Temp. -25 to +85oC
21. Shock & Vibration Confirms to IEEE 344
(Frequency 1 to 33Hz )
(Acceleration 3.5 in x ,y, z axis)
22. Climatic Conforms to IS 9000 Part II & IV
(Dry Heat: 55oC RH 50% for 4 Hrs.
Damp Heat : 40 +/- 2o C at 90% RH
to 25 +/- 5oC at 98% RH).
23. Dimensions 3U(H) X 20T(W) X 220mm(D).
24. Mounting Bin Mounted, Accommodates into 3UX84TX 320mm(19" STD.
EURO BIN).
THEORY OF OPERATION OF SMPS
Input Ac line is supplied to line filter(LFI) through
ON/OFF switch and a fuse of proper current rating.
To suppress RFI coupling back to A.c mains ,a couple
inductor LF1 is inserted in series with each A.c line. While capacitors
C1,C8,C9 &C10 are placed across lines and across line and ground
conductor.
The filtered 230V ac is now applied to a bridge rectifier
BR1,the output of the bridge rectifier is 325V dc. According to the
below formula.
Vdc=1.414 Vrms.
Capacitors C11 & C12 are smoothing capacitor, R2 is a
bleeder resistor connected across the capacitor bank(C11 & C12) this
arrangement provides approximately 325V dc at R2.
Resistor R1 is thermistor which limits the inrush
current , causes great stress on the input components, switches,
rectifiers and capacitors.R1 offers high resistance during turn-on, there
by limiting in rush currents but after warming up its resistance is
decreased.
Q1, Q2, D5, D6, T2, D2, L2, C5, C6, C7 form the
forward converter configuration.
MOSFET's Q1 & Q2 are connected across the filtered
D.c, Q1 & Q2 are the switching elements driven by the pulses from PWM IC.
Both the transistors Q1 & Q2 are turned ON and OFF simultaneously. When
they are turned on all the primary and secondary dot ends of the
transformer T2 are positive and power is delivered to the loads. When they
turn off currents stored in T2 magnetizing inductance reverses polarity of
all windings. The dot end of primary tries to go negative but is caught at
ground by diode D5, the non dot end of primary tries to go positive but is
caught at Vdc from R2 diode D6.
Thus the emitter of Q1 can be never be more than Vdc below
its collector, collector of Q2 can never be more than the Vdc above its
emitter .Leakage inductance spikes are clamped so that the maximum voltage
stress on either transistor can never be more than the maximum D.c input
voltage.
During On time the power is delivered to the load through
flywheel diode D2, diode D1, L1, L2, C5, C6, C7 and also energy
stored in the inductors. During OFF time the stored energy in the
inductors is delivered to the load through D2, thus constant output is
maintained.
C13,D3 & R88 form snubber circuit for Q1. When Q1 is ON the capacitor C13
is charged through diode D3. When Q1 turns OFF C13 discharges through
resistor R88. Thus the snubber transistor which have dissipate the power in
absence of snubber.
C15,D7,R89 and C14,D4,R90 forms the snubber circuits for Q2 and T2
respectively, does the same function as earlier.
After time duration determined by the control circuit both Q1 & Q2 go to
Off condition. As a result of primary winding, Zener diodes Z1 & Z2,
resistors R6 and R13 clamp the pulses to prevent negative excursions.
A current transformer T3 is connected in series with Q1 & Q2 so that the
current flowing through them can be monitored. The output is fed to the PWM
IC through D15.
T2 is a power transformer which steps down the high voltage pulses to the
required voltage.D2 is a high frequency rectifier which rectifies the high
frequency pulses. C5 ,C6,C7 are bank of capacitors which remove A.c
ripples.
Pulses from PWM IC are applied at q5 , which is an amplifier stage. D12 &
Z3 clamps the pulses to the required level. T4 is a pulse transformer which
produces two secondary output pulses, which are applied to Q3 & Q4.
Function of Q3 & Q4 is to switch, the MOSFET's and reduce the reverse
recovery losses.
An auxiliary transformer T1 with secondary of 17V a.c and 5V a.c. is
connected to mains to provide a separate bias supply for control section.
The two secondaries are connected to alarm card through header connectors
J13 & J14 respectively.
FEEDBACK AND CONTROL CIRCUITS
In pulse with modulation method of feedback control the conduction time of
the switching transistors during on period is varied to regulate the output
voltage to a predetermined value to achieve this, we designed three
individual sections.
1. Control Section
2. Alarm Section
3. Error Amplifier Section
UC 1846 :-
The block diagram of UC 1846 is shown in the figure. There are two feedback
loops.
One is the output voltage sensor (PWM). The current sensor converts the
ramp on a step current to ramp on a step voltage.
Regulation against line and load current changes by variation of the power
transistor on time, determined by both voltage sensing error amplifier
output and the PWM comparator.
The power transistor on time is determined as follows. An internal
oscillator whose time period is determined by external discrete components
Rt, Ct generates a narrow clock pulse Cp. Frequency of the oscillator is
given by
Foc=2.2/(Rt * Ct)
At every clock pulse feed forward to S-R flip-flop Q is reset causing the
output to be low. Now, when the PWM voltage comparator output goes high,
the output at Q is set causing the output to go high. Hence the high time
at A Or B and turns off the power transistor which had been powered on.
Thus an instant at which the PWM comparator output goes high determines the
end of the on time.
Now the PWM comparator compares the ramp on a step current sensing voltage
to the output of the error amplifier, hence when the peak of the current
sensing voltage is equal to the output of E/A then the comparator output
goes high.
A low output from R-S flip-flop (Q) occurs per one clock pulse and goes
back high when the non inverting input of the PWM is equal to E/A output.
Chip output stages A and B are totem poles the output 1800 out of phase
i.e. when top transistor is on the bottom transistor is off and vice-versa.
The steering is done by binary counter (T flip-flop) which is triggered
once per clock pulse by leading edge of the pulse. The positive going
pulses steers the logic gates alternatively. The chip output points A & B
are 1800 out of phase. Positive pulses whose duration is same as that of
negative pulses.
The shutdown circuit consists of a comparator with a 350 mV temperature
compensated offset. .The shut down is accomplished by applying a signal
greater than 350mV to pin-16, causing the output latch to fire and setting
the PWM latch to provide an immediate signal to the outputs.
CONTROL CARD PCB :-
The control card consists of UC 1846 PWM IC whose internal circuit and
explanation is given earlier.
For this IC, regulated 12V Vcc is applied to the pin-13 & pin-15
through J5-6. A regulated precession 5.1V is generated at pin-2 is
connected to J5-4. Pin-12 is ground.
Pin-8 & Pin-9 forms the oscillator, an external potentiometer Rv2 is
connected to generate a constant current into a capacitor C22 which s
connected to pin-8, to produce a
linear saw tooth waveform. The internal oscillator frequency may be
approximated by selecting Rv2 & C22 such that
Foc=2.2/(Rv2*C22)
Pins 4 & 3 are used for current sensing to sense peak switch currents
on a pulse by pulse basis for comparision with an error voltage. Here we
use isolated current sensing. Transformer (T3) coupling can provide
isolation and increase efficiency. The maximum swing on the inverting input
of the PWM comparator is limited to approximately 3.5V by internal
regulated supply. According for a fixed gain of 3, Max differential must be
kept below 1.2V at current sensing inputs. To achieve this, we use a
potentiometer Rv1.
A small RC filter C24 & R20 in series with the input is generally used
to reduce spike to an acceptable level.
The reference voltage 5.1V through J5-4 is reduced to 2.55V with the
help of divider network R52 & R53 is fed to a short circuit potection
comparator IC2/2 which compares this reference voltage with output
current. Normally the output of this comparator remains low.
When the output terminals are short circuited the comparator output goes
high. Therefore Q9 turns on pulling pin-7 of UC 1846 to low. This limits
the output current to safe value and reduces the output voltage.
Another comparator IC2/1 is used for over voltage protection which compares
reduced reference voltage(2.55V). This comparator compares output voltage
with reference voltage. When output voltage goes beyond specified over
limit, it generates a logic high signal at pin-1 of the comparator IC2/1,
which is fed to a pin-16 of UC 1846 and shuts down the output. The output
is appeared at the pin-14 which is a square wave of 10V, this is given to
Q8 base. The amplified output of 12.5V with a phase reversal is fed to
transistor Q5 on main board.
The error amplifier output is fed to the pin-5 of UC 1846 which generates
pulses according to the error voltage.
The reference voltage of 5.1V is reduced to 3.4V by a divider network R22
and R23 is given to pin-1.
ALARM PCB :-
Alarm card produces DC bias needed for control section and consists of
under voltage indication circuit and line failure alarm circuit. The
secondaries of the transformer T1 are connected to circuits through J13 and
J14, from J13 17V Ac is connected to the bridge rectifier. The diodes D29,
D30, D31, D32 form bridge and the capacitors C49, C50 used for filtering.
The IC6 is used for regulation and then the output is filtered by capacitor
C51. This voltage is given to the needed circuits.
Under Voltage Indication:
For under voltage detection a comparator IC5/2 is used, this
comparator has two inputs. The reference voltage is given through a divider
network to the inverting input (pin-6 of IC5/2) and the output voltage is
given to the non-inverting input(pin-5 of IC5/2) through divider network.
The comparator compares both the inputs and the output is given at pin-7 of
IC5. This output is given to relay RL1 through Z7 and Q10, when there is
low output signal the relay operates, indicating the under voltage with an
alarm sound and a LED indication at the EURO connector.
Line Failure Alarm Circuit:
From J14 the step downed voltage of 6V is rectified by a bridge
rectifier formed by D24, D25, D26, D27. This rectified voltage is fed to th
pin-3 of comparator IC5/1 which compares it with reference voltage (from j4-
3)applied at pin-2 of IC5/1, according to the comparator output the
capacitor C43 charges and discharges as following:
1. At normal conditions, the line voltage always exceeds the reference
voltage, that is why the output of the comparator at this condition is in
high state such that the capacitor charges to the peak value. Therefore,
the TTL output is low state.
2. If any variation occurs in line voltage, immediately the reference
voltage exceeds the rectified output. Therefore the output of the
comparator falls
to low state. At this condition the capacitor is discharged through
resistor R78 and the TTL output goes to high state.
To identify this change in state, a special provision of alarm
indication is made in the case of line failure. This alarm is
indicated before 15msec to observe the indication easily.
ERROR AMPLIFIER PCB :-
Error amplifier compares the output voltage and output current with
reference voltage and generates error voltage. This error voltage is fed to
the PWM comparator.
Error amplifier has two inputs, one of its input (pin-3 of IC3/1) is
connected to a reference voltage through a divider network. The other input
(pin-2 of IC3/1) is connected to output voltage. Output voltage is always
sensed and compared with the reference voltage, a signal proportional to
the deviation of output is generated at the output of error amplifier pin-1
of IC3/1. Error amplifier output is driven to the PWM comparator.
For slow, small variations of the output voltage V0 due to either line
input or load changes will be sensed by the inverting input of error
amplifier. It compares to a reference voltage at the non-inverting error
amplifier input. This will change the DC voltage level at the error
amplifier output. It drives the PWM.
TEST PROCEDURES OF SMPS
NOTE:- Voltage adjustment potentiometer and current limit adjustment
potentiometer are located on the top end side of the power supply board.
INITIAL SETUP :-
The power supply unit consists of a 48 pin heavy duty EURO connector,
through which AC mains is applied. For load regulation testing purpose a
rheostat is connected to the same connector, by varying it, the unit tested
at different load currents.
Through this connector AC mains is switched on, and the AC/ON LED is
checked.
VOLTAGE ADJUSTMENTS :-
The output voltage can be adjusted with the help of a potentiometer
(volt adj pot). By rotating the pot either clockwise or anti clockwise, the
output voltage is increased or decreased. With the help of this pot, the
output voltage is adjusted to the required value.
OVER VOLTAGE PROTECTION TRIP :-
First the load current is set around 50%. Then the voltage adjustable
pot is varied to increase the output voltage. If the trim pot is varied
beyond the set value of over voltage, the over voltage protection circuit
is activated and the output voltage goes to zero. This is a latched
condition. To reset, first the output voltage adjustment trim pot should be
reduced to a lower setting. Next, power to the unit is to be switched off.
Subsequently, if power on the unit supplies power and the voltage
adjustment trim pot should be readjusted to give the original set value.
CURRENT LIMIT :-
With the unit powered on, the load current is gradually increased
beyond the rated load by using a suitable rheostat with an ammeter
connected in series. At the point of current limit, a decrease in ammeter
reading is observed. This phenomenon can be further verified by monitoring
the output voltage, which will be low.
LOAD REGULATION :-
DEFINITION:
It is change Eout in steady state value of DC output voltage due to
change in load resistance from open circuit to a value that yields maximum
rated output current (or vice versa).
PROCEDURE:-
a) Adjust AC input voltage to a specified nominal voltage and load
current of full load value.
b) Note DC output voltage VFL.
c) Put off load current and note the DC output voltage VNL.
d) Calculate load regulation as follows :
%Load Regulation =(VNL-VFL)/VFL
e) Ensure % Load Regulation is less than or equal to specified value.
f) Repeat step(a) through (e) at specified AC high line and load line
voltage.
PRECAUTIONS :-
At full load and no load current A.C line voltage must be equal.
LINE REGULATION :-
It is change Eout in steady state value of DC output voltage due to
change in A.C input voltage over the specified range from low line to high
line or vice versa.
PROCEDURE :-
a) Adjust D.C load current to specified full load value.
b) Reduce A.C line voltage to specified high line voltage by variac.
c) Note D.C output voltage (VL).
d) Increase A.C line voltage to specified high line voltage by variac.
Note D.C output (VH).
Calculate line regulation as:
%Line Regulation = (VH-VL)/VN.
Where VN is the nominal voltage.
e) Ensure line regulation is less than or equal to specified value.
SHORT CIRCUIT TEST :-
Keep A.C input voltage at its nominal value. Short the output
terminals with a suitable wire. Remove the short circuit after 30 minutes.
Ensure D.C output returns to its nominal value after removal of short.
EFFICIENCY TEST :-
It is the ratio of out put power ( rated voltage * full load current
) to A.C input power measured when the power supply is delivering full load
current at rated output voltage.
PROCEDURE :-
Connect a watt meter at A.C input side, put on A.C power and adjust
output voltage to rated and current to full load value. Note A.C input
power.
Efficiency can be calculated as :
%Efficiency = (Output voltage * Output Current)/A.C Power.
MODULE STATUS :-
By rotating the output adjust trim pot either side, output D.C
voltage can be increased or decreased. When ever output voltage goes beyond
the limits, the potential free contacts of the relay get opened, which are
usually closed ones. This can be tested with the help of a continuity
tester by sensing the continuity across terminals meant for module status
provided in the EURO connector. As far as the output is out of the limits
the potential free contacts remain opened.
RIPPLE AND NIOSE TEST :-
The power supply unit is switched on, and loaded to rated currents
(i.e. full load current) using a properly compensated probe with a disc
capacitor of at least 0.1uf and an electrolytic capacitor of at least
4.7uf/63V connected in parallel, the ripple is monitored.
LOAD TEST :- (CURRENT SHARING CHECK)
Load test done at nominal A.C input voltage and outputs loaded to
their respective full loads. By loading both the outputs to their
respective loads on power supply can be switched off to check redudency.
Though A.C input fails for one section, other section supports the full
load with out disturbing the power to the system. When both power supplies
are on, current sharing can be checked.
This general test procedure explains how a regulated DC power supply
is to be tested. The specification for each type of power supply shall
be referred to check the power supply performance along with thus
procedure.
2) OBJECT :
s procedure is written to follow a similar method of testing power
supplies of different
specifications.
3) PREPARATION:
Make the connections as shown in figure of relevant test procedure. Put
ON all the test instruments and allow them to warm up. Adjust mechanical
zero of panel meters of unit under test.
4) PRECAUTIONS:
1) Ensure connections are made as per fig.
2) Set test instruments range as shown along with the fig.
3) Adjust load resistance in the setup to maximum value.
4) Ensure input voltage to be applied tallies with marking on the unit.
5) Read the entire test procedure once before starting the testing.
6) Connect load resistor leads to only to load terminals of power supply
in series with on/off switch and ammeter as shown in fig.
7) Connect measuring instrument leads directly to sensing-terminals of
power supply if provided externally, if not to load terminals.
5) ADJUSTMENT OF INPUT:
1) Put ON input power to the unit.
2) Adjust input voltage to specified value.
3) Ensure power ON lamp glows (if provided).
4) Adjust output voltage to rated value by adjusting potentiometer
provided (Voltage Adj.)
5) Adjust load current to rated full load value.
6) Allow the unit to stabilize for about 30 minutes.
6. ADJUSTMENT OF DC OUTPUT VOLTAGE:
Adjust AC input voltage to its normal value. By varying the
potentiometer provided to vary the output voltage ensure it is
possible to get specified minimum and maximum output voltage when the
power supply is delivering full load current.
7. PANEL METERS CALIBRATION :
VOLTMETER :
Note the panel meter reading at rated output voltage. Compute the
accuracy and ensure it is within the specified limits.
NOTE : If the power supply is variable voltage type, accuracy
should be ensured at 5 points as stated in ammeter
calibration.
AMMETER:
Note the panel meter reading at 20%, 40%, 60%, 80% & 100% full load
current. Compute the accuracy and ensure it is within specified
limits.
8. LOAD EFFECT (LOAD REGULATION) :
DEFINITION:
Formerly known as load regulation, is the change in Eout in steady
state value of DC output voltage due to a change in load resistance
from open circuit to a vlaue that yields maximum rated output current
(or vice versa).
PROCEDURE:
a) Adjust AC input voltage to specified nominal voltage and load current
to full load value.
b) Note the DC output voltage VFN.
c) Put OFF load current and note the output voltage VNN.
d) Calculate the load regulation as follows :
%Load Regulation = ((VNN-VFN)/VFN) X 100
Note : Use the similar value of VFN got in step c or d to
calculate worst case load regulation.
e) Ensure %Load Regulation is less than or equal to the specified value.
f) Repeat step (a) thru (f) at specified AC high line and low line
voltage.
PRECAUTIONS :
At full load and no load current maintain AC line voltage to be equal.
9. PARD (RIPPLE AND NOISE)
DEFINITION :
The term PARD replaces former term ripple and noise. PARD is the
periodic and random deviation of DC output voltage from its average
value over a specified band width with all other parameters maintained
constant.
PROCEDURE:
Measure ripple plus noise with DMM (Digital multi meter) in AC mode if
it is specified in RMS value and with oscilloscope if it is specified
in P-P.
Measure PARD at full load and no load current at low and high line
voltages.
Ensure measured PARD is not more than specified.
PRECAUTIONS:
1. Use method B or C depending on the oscilloscope used to measure PARD.
Refer Fig.1.
2. An oscilloscope display showing a 2fL( where fL is line frequency)
fundamental component is indication of clean measurement setup (due to
full wave rectification) while the measure of a fundamental frequency
fL usually means that an improved setup will result in a more accurate
(and lower) value of measured ripple.
3. To ensure no difference of potential exists between supply and the
scope it is recommended they both be plugged to same AC power bus
whenever possible. If the same bus cannot be used both AC groundsmust
be at earth potential.
4. Either a twisted pair or preferably a shielded two wire cable should
be used to connect output terminals of P/S to the vertical input
terminals of the scope. When using a shielded two wire it is
essential for the shield to be connected to ground at one end only so
that no ground current will flow through this shielded preventing
induced noise signals in the shielded leads.
5. In measurements where power supply and oscilloscope cases are
connected to ground it may be necessary to use differential scope with
floating inputs as shown in Fig-C.
6. To be absolutely certain that the measurement setup is free from
extremeous signals turn off power supply and with the scope connected
across +S and –S terminals ascertain that no signals are present on
the oscilloscope.
10. SOURCE EFFECT (LINE REGULATION):
DEFINITION:
Formerly known as line regulation is the change in Eout in the steady
state value of DC output voltage due to a change in AC input voltage
over the specified range from low line to high line or from high line
to low line.
PROCEDURE:
a) Adjust DC load current to the specified full load value.
b) Reduce AC line voltage to the specified low line voltage by variac.
c) Note the DC output voltage (VL).
d) Increase AC line voltage to specified high line voltage by variac.
e) Note the DC output voltage (VH).
f) Calculate the line regulation as follows:
%Line Regulation = (VH - VL) X 100/VN.
Where VN = Nominal voltage.
g) Ensure % line regulation is less than or equal to specified value.
11. THRESHOLD CURRENT:
DEFINITION:
Value of output load current for which limiting can be detected by the
departure of the stabilized current from its specified load effect
band.
PROCEDURE:
Keep the AC input voltage at its low line value. Decrease load
resistance slowly and record the value of current at which limiting
action is first detected as a reduction in the stabilized output
voltage below the load effect band. Reduce current to its full load
value.
Repeat above procedure with high line AC voltage.
SHORT CIRCUIT PROTECTION:
Keep the AC input voltage at its nominal value. Short the output
terminals with a suitable wire. Remove the short after 30 minutes.
Ensure DC output voltage returns to its nominal value after removal of
short.
EFFICIENCY TEST:
DEFINITION:
It is the ratio of output power (rated voltage X full load current) to
the AC input power measured when the power supply is delivering full
load current at rated output voltage.
PROCEDURE:
Connect a wattmeter at the AC input side. Put ON AC power supply and
adjust input voltage to rated value.
Adjust the output voltage to rated value and current to full load
value.
Note the AC input power.
% Efficiency = Output voltage X Output current / AC input power.
Ensure % efficiency calculated is not less than specified value.
12. EFFECT TRNSIENT RECOVERY TIME :
Refer FIG-2
DEFINITION :
The time 'X' for the output to recover and stay within 'Y' milli volts
of nominal output voltage following a 'Z' amp. Set change in load
current.
Where 'Y' is generally of the same order as the load regulation
specified.
Nominal output voltage is defined as the DC level half way between the
steady state output voltage before and after the imposed load change.
'Z' is the specified load current change typically equal to the full
load current rating of the supply.
PROCEDURE:
Adjust the AC input voltage to the power supply to rated value and
output to its specified nominal value. Adjust load current to full
load value. Put OFF the load current and measure the output voltage
transient on the storage oscilloscope, put on the load current and
measure the out put voltage transient. Ensure that the output voltage
spike and recovery time are within specified limits.
PRECAUTIONS:
Use an oscilloscope of higher band width (10MHz or more) to measure
output voltage spike, more accrurately.
BURN-IN-TEST:
1) This test is carried out to weed out infantile mortality of
components used.
2) Keep the unit on for 100 Hrs with nominal AC input voltage and at full
load current.
3) At the end of 100 Hrs put OFF power to the unit, open the cover and
check the components visually and note the observations if any.
4) On one unit, measure the temperature on heat dissipating components
like power transformers, rectifier diodes, power transistors etc. ,
immediately after putting OFF power and opening cover. Ensure
temperature rise is within permissible limits.
5) Put ON power to the unit and check the performance of the unit as
detailed in relevant inspection procedure.
13. DRIFT (STABILITY) :
DEFINITION:
Change in output voltage over a specified period following a specified
warm-up period. During warm-up and measurement interval all parameters
such as load resistance, ambient temperature and input line voltage
are held constant.
PROCEDURE:
a) Keep the setup in a controlled temperature area.
b) Adjust AC input voltage to nominal value and output load current to
its specified full load value.
c) Note the AC input voltage, load current, ambient temperature and DC
output voltage at an interval of 1Hr over a specified time span after
the 1 Hr warm-up period.
d) Compute the stability (%change of DC output over specified period) and
ensure it is not more than the specified value.
NOTE:
To have continuous record of DC output voltage, a strip chart recorder
may be used with adequate resolution to compute stability value.
TEMPERATURE CO-EFFICIENT TEST: ( On one unit only)
Definition:
Change in output quantity following change in ambient temperature.
Equipment required: Low-High temperature test chamber
00C to 1000C
Temperature range : 150C to 550C unless specified otherwise.
Check points: 150C, 250C, 350C, 450C & 550C
Unless specified otherwise.
At each check point the temperature shall be held constant +/- 1deg.c
until the value of the output quantity has reached equilibrium. Refer
FIG.3.
For the purpose of this test it shall be considered that equilibrium is
reached when the stabilized output quantity varies by an amount less
than 5% of total change over 10min period.
Keep the following parameters constant through out the test.
a) AC input voltage.
b) Load current.
Note the output voltage at each check point after equilibrium has
reached. Calculate the temperature co-efficient i.e., % change in output
per degree centigrade over range of 15deg.c to 55 deg.c.
Ensure it is not more than specified value.
INULATION RESISTANCE TEST:
EQUIPMENT:
BPL insulation and breakdown tester or equivalent.
The insulation resistance shall be measured with the apparatus not
connected to its supply source. A DC voltage of 100 V is applied to it and
the leakage current is measured one minute after the application.
a) For output circuit(s) internally connected to the input between
1) The frame and
2) All output terminals short circuited and connected together.
b) Output circuit(s) insulated from the input between
1) The frame connected to one input terminal and
1) All output terminals short circuited and connected together.
ACCEPTANCE CRETARIA:
Ensure that the insulation resistance is not less than specified value.
VOLTAGE PROOF TEST: ( AC RMS)
TEST VOLTAGE: 2U +1000V where U is rated AC input voltage.
EQUIPMENT: BPL insulation and break down testing or equivalent.
PROCEDURE: The test voltage shall be applied gradually, starting at
50% and increase to full value is not less than 10 seconds between
metallic enclosures and terminals. (input and output).
Note:
Care shall be taken to disconnect the electronic control circuits
during the test as some electronic components may fall during the voltage
proof test.
ACCEPTANCE CRITERIA:
Ensure that there is no break down of insulation.
SWITCHING MODE POWER SUPPLY (5V,20A)
FUNCTIONAL TEST REPORTS:
"SLNO " " " "SMPS "
" " " " "UNIT "
" " " " " "
"Step 3 thru 5 of STD 200 " " " "O.K "
"Step 6 of STD 200 " " " " "
"Vol. Adjustibility (Vdc) "Min. " " "3.97 "
"Step 8 thru 9 of STD 200 " " " " "
"Load Regulation (Spec. : " " " " "
"+/- 0.7%) " " " " "
" O/P " " " " "
"(V) " " " " "
"240 V AC @FL " " " "5.002 "
"240 V AC @10%FL " " " "5.031 "
"240 V AC @FL " " " "5.002 "
" " " " " "
" % " " " "0.58 "
"Reg " " " " "
" " " " " "
"216 V AC @FL " " " "5.002 "
"216 V AC @10%FL " " " "5.031 "
"216 V AC @FL " " " "5.002 "
" % " " " "0.58 "
"Reg " " " " "
" " " " " "
" " " " " "
"264 V AC @FL " " " "5.002 "
"264 V AC @10%FL " " " "5.031 "
"264 V AC @FL " " " "5.002 "
" " " " " "
" % " " " "0.58 "
"Reg " " " " "
"240 V AC @FL " " " "60 "
"240 V AC @10%FL " " " "10 "
"240 V AC @FL " " " "60 "
" " " " " "
"216 V AC @FL " " " "60 "
"216 V AC @10%FL " " " "10 "
"216 V AC @FL " " " "60 "
" " " " " "
"264 V AC @FL " " " "80 "
"264 V AC @10%FL " " " "60 "
"264 V AC @FL " " " "80 "
"Step 10 of STD 200 " " " " "
"Line Reg. (Spec. : +/- " " " " "
"0.1%) " " " " "
"AC I/P " " " " "
"O/P (V) " " " " "
"216 " " " "5.002 "
"@FL " " " " "
"240 " " " "5.002 "
"@FL " " " " "
"264 " " " "5.002 "
"@FL " " " " "
" " " " " "
" " " " "0 "
"%Reg " " " " "
" " " " " "
"216 " " " "5.031 "
"@10%FL " " " " "
"240 " " " "5.031 "
"@10%FL " " " " "
"264 " " " "5.031 "
"@10%FL " " " " "
" " " " " "
" " " " "0 "
"%Reg " " " " "
" " " " " "
"Step 11 of STD 200 " " " " "
"threshold current @240V " " " "22.24 "
"AC " " " " "
"(Spec. : 22 to 26 A) " " " " "
" " " " " "
"Step 12 of STD 200 " " " " "
"Short CKT. Protection " " " "O.K "
" " " " " "
"Step 18 of STD 200 " " " " "
"I.R @ 100VDC " " " " "
"I/P ( M OHM) " " " ">200 "
"O/P ( M OHM) " " " ">200 "
" " " " " "
"Step 19 of STD 200 V.P " " " " "
"Test " " " " "
"AC (I/P) @1.5KV " " " "O.K "
"AC (O/P) @500V " " " "O.K "
" " " " " "
"Step 2.1 of IP 1017 OVP " " " " "
"@HL " " " " "
"(Spec. : 110% to 120% i.e" " " " "
"5.5V to 6V) " " " "5.71 "
" " " " " "
"Step 2.2 of IP 1017 " " " " "
"FULL LOAD ON/OFF TEST " " " "O.K "
"Normal Condition " " " " "
"Cont.Bet.J1-b12&z12 " " " "O.K "
"Dis.cont.Bet.J1-b12&d12 " " " "O.K "
" " " " " "
"Alarm Condition " " " " "
"Alarm observed @ (Vdc) " " " "4.5 "
"(-5% to -15% i.e 4.75 to " " " " "
"4.25) " " " " "
"Cont.Bet.J1-b12&d12 " " " "O.K "
"Dis.cont.Bet.J1-b12&z12 " " " "O.K "
" " " " " "
"Switch off P/S & Observe " " " " "
"Cont.Bet.J1-b12&d12 " " " "O.K "
"Dis.cont.Bet.J1-b12&z12 " " " "O.K "
" " " " " "
"O/P Vol. @Test Jack (Vdc)" " " "5.002 "
TROUBLE SHOOTING
1) No D.C output, both the LED's LED1, LED2 on the front panel are not
glowing.
Sol:-
a) Check the input A.C line.
b) Check the switch, change it into ON position, if it is in OFF
position.
c) Check the voltages at the primary and the secondary of transformer
T1 ( it should be 230V A.C at primary and 17V & 6V A.C at the two
secondaries and the same 17V A.C at J13 and 6V A.C at J14). If these
voltages are o.k then fault may be in PCB-4 (ALARM CIRCUIT).
d) Check the diodes D29, D30, D31, D32 and IC 6 a regulator chip. Their
must be some problem.
2) No D.C output, LED 1 is glowing, LED2 is not glowing.
Sol:-
a) For this problem, remove the connector J-13 at PCB-4 and apply 17V
A.C through J13 connector with external transformer, and power
supply is connected to mains. Observe the voltage at J4-5, 12V DC
appears, if not check IC 6 for fault.
b) If yes, observe the perfect square wave of 10V peak to peak of 50KHz
at J5-10 w.r.t ground. The shape of the square wave is as shown in
figure 7(a).
c) If the pulse is not appearing, fault is in control section (PCB-2).
d) If yes, observe the pulse at Q5 collector. At Q5 collector, the
pulse is as shown in the figure7(b) of 25V peak to peak. If it is
not so check the diodes D10, D11, D12, Z3, transistor Q5 and
transformer T4, there may be some problem.
e) If yes, the same pulse appears at the secondary of T4 pin-7, pin-10
w.r.t ground. If not so, check the diodes D8, D9, D13, D14,
transistors Q3, Q4, there may be some problem.
f) If yes, observe the square wave at the gates of MOSFET's Q1, Q2 the
pulse is as shown in the figure 7(c), there may be some problem.
g) If yes, check for the high voltage 340V D.C at drain's of MOSFET's.
If voltage is appearing at drain's, fault is in MOSFET's or
transformer T2.
h) If high D.C voltage is not appearing at drains, check the voltage at
capacitor C11, resistors R1,R2 and bridge rectifier BR1. If the
voltage is not appearing at any of the above, switch off the supply
and check the continuity of C11, R1, R2 and BR, there may be some
problem.
3. Fuse is blowing when the power supply is switched on.
Sol:- There are two possible conditions for fuse blowing i.e. (a) MOSFET's
are short circuited, (b) Bridge rectifier is short circuited.
a) To clarify where the fault is, remove the control card first and
switch on the circuit. Now, if fuse is not blown, the fault is at
MOSFET's. Switch off the supply and check the continuity of the
MOSFET's D5, D6. There may be some problem.
b) If the fuse is blown, check bridge rectifier BR1, capacitor C11. If
they are OK check the continuity between phase and neutral.
4. Voltage is decreasing with increasing load.
Sol:-
a) For this problem, observe the pulse at CT at IC1 pin4 w.r.t pin-3,
the shape of the pulse is as shown in figure 7(e) (1), (2), if it is
not so we can adjust by varying the potentiometer RV1 to the
required level. Still the problem is not solved then switch OFF the
supply and check the continuity of CT and RV1, there may be some
problem.
5. No D.C output, at transformer T2 secondary square wave of 50KHz is not
appearing as shown in figure 7(d) of voltages related to that particular
boards, as mentioned.
Sol:-
a) First switch off the supply and check the continuity of the
schottky diodes D1, D2, coils L1, L2, there must be some problem.
6. Over voltage tripping provision is not working.
Sol:-
a) Check the voltages at pin-4, pin-8 of IC-2 (i.e. ground and Vcc
respectively).
b) Check the voltages at pin-5 & pin-6 of IC-2. Reference voltage of
5.1V reduced to 2.5V by a potential divider R41, R42 is applied to
pin-6. Output voltage reduced by a potential divider R43, R44 is
applied to pin-5. When the output voltage increases over a specified
limit, the voltage applied at IC-2 pin-5 also increases above the
voltage at pin-6, then 5V generated at pin-7. If 5V is not generated
here, then the fault is at IC-2.
c) This 5V voltage is applied to the base of Q7 through Z4, thus there
will be no voltage at the base of Q6. At Q6 collector 5V appears,
which is connected to pin-16 of IC-1 through jumper, because of
this, the circuit is shut down. To switch ON the supply, decrease
the voltage by adjusting the RV3, and then switch ON the power
supply.
d) When IC is in working condition, below the over voltage limits no
voltage appears at base of Q7 and at collector 5V appears, which is
applied to the base of Q6 through Z6. At Q6 base0.7V appears, at
collector no voltage appears. If not so, check continuity of zener
diodes Z6, Z4, transistors Q6, Q7, there may be some problem.
7. Current limiting provision is not working.
Sol:-
Check the voltages at IC-2 pin-8 Vcc and pin-4 ground respectively.
a) Check the voltages of IC-2 pin-2, pin-3. Ref. Voltage of 5.1V is
reduced to 1V by a potential divider R52, R53 is applied to pin-2.
Pulses from CT (J5-1) are applied through a divider network formed
by R48, RV3 (potentiometer) to pin-3. By varying RV3, we can adjust
the voltage of the pulse to a required level. When the voltage at
pin-3 is greater than the voltage at pin-2, some voltage is
generated at pin-1, and is fed to the base of the transistor Q9, the
collector voltage is fed to pin-7 of IC-1, thus limits the output to
a required level. The shape of the pulse is as shown in the figure
7(e).
8. Under voltage indication is not working.
Sol:-
a) For this problem, check the voltages at pin-8, pin-4 of IC-5 (i.e.
Vcc, ground respectively).
b) Ref. Voltage of 5.1V is reduced to 2.5V by a divider network by R86,
R77 is applied at pin-6. Output voltage is applied to pin-5 through
a divider network R82, R85. When the voltage at pin-5 is greater
than the voltage at pin-6, 5V is generated at pin-7, which is fed to
the base of Q10 through Z8. If 5V is not generated fault is at IC-5.
c) When the voltage at pin-5 is less than the voltage at pin-6, the 5V
at pin-7 is grounded and no voltage is applied to the base of Q10,
12V appears at the collector. If no voltage is appearing at Q10
collector, then the fault is at Q10.
d) When 12V appears at Q10 collector, the green LED at the back panel
which was in the ON condition is switching OFF and red LED which was
in the OFF condition is switched ON indicating under voltage. If not
so, the fault may be in relay RL1. Still the problem is not solved,
check the connector J-6, there may be some problem.
9. Voltage adjustment potentiometer is not working.
Sol:-
a) Check the voltages of IC-3 pin-8, pin-4 (i.e. Vcc, ground
respectively).
b) Ref. voltage of 5.1V is reduced to 2.5V with a divider network
formed by R65,R68 and potentiometer RV3 (PCB-3) is applied to pin-3
of IC-3. By varying the pot, we can adjust the output voltage. If it
does not vary, calculate the value of resistance for particular
board mentioned in components list, if any difference, change
component.
c) The sensed output voltage is fed to pin-2 through R59, R62, R69.
According to the voltages present at the pin-2 & pin-3 of IC-3,
output voltage appears at pin-1. This voltage is called error
voltage, is fed to the pin-1 of IC-1, if no voltage is appear at pin-
1 fault is at IC-1.
10. Line failure alarm provision is not working.
Sol:-
a) For this fault, check the voltage at J-14 connector for 6V A.C, if
no voltage is appearing at J-14, check the transformer T1 secondary
which is connected to J14 . At there also no voltage is observed,
change the transformer T1. If 6V is appearing, check the connector J-
14 continuity after switching OFF the power supply, there may be
some problem.
b) I 6V is appearing at J-14, check the voltage at pin-4, pin-8 of IC-5
(i.e. Vcc, ground respectively).
c) Check the voltage at pin-2 of IC-5. Ref. voltage of 5.1V is reduced
to 2.5V by a divider network R74, R75 is applied at pin-3. Voltage
present at J-14 of 6V AC is rectified by D24, D25, D26, D27 and
filtered by C46, is applied to pin-3 through R70, R71. The voltage
applied is 2V, if not so, check the diodes.
d) 12V Vcc is reduced to 6V by R79 is applied at pin-1. The pin-1 of IC-
5 is fed to pin-1 of IC-4 through R78, D33, C43 .
e) At J-9, an external voltage of 5V is applied through 10K ohms
resistor, which is connected to pin-4 & pin-5 of IC-4.
f) When power supply is in OFF condition, 5V appears at J-9. When
switched ON the power supply, the voltage at pin-1 of IC-5 is
applied at pin-1 of IC-4, drives the opto coupler and the voltage at
J-19 (at switch OFF condition) disappears.
g) The time duration between the power supply OFF and the line failure
alarm circuit is decided by R78, C43. If the time duration is not
sufficient, change the capacitor. If still it does not work, fault
is at IC-4.
COMPONENTS USED IN SMPS
Resistors:
"Sno "Description "Identity "Value "
" " " "(OHM) "
"1 "Thermistor "R1 "2.5, 8.4A "
"2 "WWR "R3,R4 "100,2.5W "
" " "R88,R89,R90 "1.5K,6W "
"3 "MFR "R66, R67 "10,1/2W "
" " "R8,R11 "22 "
" " "R14,R33 "100 "
" " "R78(for 5V only) "220 "
" " "R17 "470 "
" " "R7 "1K "
" " "R15 "2.2K "
" " "R2 "470K "
" " "R5,R6,R9,R10,R12,R13,R18,R21 "100,1/4W "
" " " R19,R32,R47,R51,R60 "1K "
" " " R22,R62 "2.2K "
" " "R29,R30,R36,R55,R79,R87 "3.3K "
" " " R23 "4.7K "
" " " R52 "6.8K "
" " "R20,R24,R25,R31,R34,R37,R38,R39 "10K "
" " "R40,R41,R45.R46,R48,R50,R56,R58, "10K "
" " "R59,R63,R64,R65,R68,R70,R71,R72, "10K "
" " "R73,R74,R75,R77,R80,R81,R84,R86 "10K "
" " "R28 "20K "
" " "R53 "22K "
" " "R27 "39K "
" " "R61,R85 "100K "
" " "R26 "150K "
" " "R54,R57 "1M "
" " "R83(FOR5V),FOR(12V,15V,24V) " 2.7M,1M "
" " "R36(FOR 5V) "15K "
CAPACITORS:
"Sno "Description "Identity "Value "
"1 "MPC "C1,C8 "0.15MF, 630V "
" " "C12 "0.22MF, 400V "
"2 "Al Electro "C33 "10MF, 25V "
" " "C43(FOR 5V ONLY) "33MF "
" " "C21 "100MF "
" " "C4,C5,C6,C7(FOR 5V/100W) "220MF "
" " "C49,C50 "470MF,40V "
" " "C11(FOR 100W) "330MF,400V "
" " " (FOR 60W) "220MF,400V "
"3 "CER CAP "C23 "0.47MF,63V "
" " "C22,C16,C17,C38,C40 "0.001MF 100V "
" " "C39,C47 "0.01MF "
" " "C41,C42,C43,C44,C48,C51,C19 " "
" " "C20,C25, C26,C27,C28,C29,C30 " "
" " "C31,C32,C36 "0.1MF "
" " "C24,C34 "100PF "
" " "C13,C14(FOR 100W) "470PF 1KV "
" " "(FOR 60W) "220PF "
" " "C9,C10 "0.01MF 2KV "
" " "C2,C3 "10nF 1KV "
TRANSISTORS:
"Sno "Description "Identity "Value "
"1 "POWER MOSFET "Q1,Q2 "IRF840 (IR) "
"2 "PNP TRANSISTOR "Q3,Q4 "BC177A(BEL) "
"3 "PNP TRANSISTOR "Q5 "TIP42C (BEL) "
"4 "NPN TRANSISTOR "Q6,Q7,Q8,Q9 "BC107(BEL) "
"5 "PNP TRANSISTOR "Q10 "2N2219A(BEL) "
DIODES:
"Sno "Description "Identity "Value "
"1 "DUAL SCHOTTKY RECTIFIER "D1((5,12)/100) "MBR4045PT(MOTOROLA) "
" " "15/100,(12,15,24)/60 "MBR1545PT(MOTOROLA) "
"2 "DUAL SCHOTTKY RECTIFIER "D2(25/100) "FST5045 (MOTOROLA) "
" " "(12,15,24)/(60/100) "UES2404 (UNITRODE) "
"3 "ULTRA FAST RECTIFIER "D3,D4,D5,D6,D7 " "
" " "FOR 60W "UF4006 (GI) "
" " "FOR 100W "MUR480 (MOTOROLA) "
" " "D8,D9,D13,D14 "UF4006 (GI) "
"4 "SWITCHING DIODE "D10,D11,D12, D15, " "
" " "D16,D17,D18,D19, " "
" " "D20,D21,D22,D23, " "
" " "D24,D25,D26,D27, " "
" " "D33 "1N4148 (CDIL) "
"5 "RECTIFIER DIODE "D28,D29,D30,D31,D32 "1N5614 JAN(PHILIPS) "
"6 "ZENER DIODE "Z1,Z2,Z3 "1N4746A(18V/1W) CDIL "
" " "Z4,Z5,Z6,Z7,Z8 "CT 4.7A(4.7V/0.5W) (CDIL)"
"7 "BRIDGE RECTIFIER "BR1 FOR 60W "DC608 (ATE) "
" " "FOR 100W "DC108 (ATE) "
RELAYS:
"Sno "Description"Identity "Value "
"1 "RELAY "RL1 FOR 5V "V23042-A2001-B101(OEN) "
" " "FOR 12V "V23042-A2003-B101(OEN) "
" " "FOR 15V "V23042-A2004-B101(OEN) "
" " "FOR 24V "V23042-A2005-B101(OEN) "
INDUCTORS:
"Sno "Description "Identity"Value "
"1 "OUTPUT INDUCTOR "L1 "MA-19-443723-A3 "
"2 "CHOKE FOR 5V "L2 "L05100 "
" "FOR 12V "L2 "L12100 "
" "FOR 15V "L2 "L15100 "
" "FOR 24V "L2 "L24100 "
" "LINE FILTER "LF1 "(EMD) "
INTEGRATED CIRCUITS (IC's):
"Sno "Description "Identity"Value "
"1 "CURRENT MODE PWM IC "IC1 "UC1846J (UNITRODE) "
"2 "DUAL COMPARATOR "IC2 "LM193J (TI/NSC) "
"3 "DUAL OP-AMPILIFIER "IC3,IC5 "LM158J (TI/NSC) "
"4 "OPTO COUPLER "IC4 "4N37(SIEMENS) "
"5 "+VE VOLTAGE REGULATOR "IC6 "LM7812CT (TI/NSC) "
TRIM POTS:
"Sno "Description "Identity "Value "
"1 "25 TURN MINATURE TRIMPOT "RV1,RV3 "176S-10K OR "
" "10K " "BOURNS-RJR24F-X103R(OEN"
" " " ") "
"2 "25 TURN MINATURE TRIMPOT "RV2 "151S-1K(OEN) "
" "1K " " "
"3 "25 TURN MINATURE TRIMPOT "RV3 "176S-5K(OEN) "
" "5K "(PCB-3) " "
TRANSFORMERS:
"Sno "Description "Identity "Value "
"1 "STEP DOWN TRANSFORMER "T1 "MA-19-443719-A3"
"2 "HF SWITCHING POWER TRANSFORMER "T2 FOR5/100 "T05100(RACE) "
" " "FOR 12/60 "T120609(RACE) "
" " "FOR 12/100 "T12100 (RACE) "
" " "FOR 15/60 "T15060 "
" " "FOR 15/100 "T15100 "
" " "FOR 24/60 "T24060 "
" " "FOR 42/100 "T24100 "
"3 "CURRENT TRANSFORMER "T3 "MA-19-443721-A3"
"4 "PULSE TRANSFORMER "T4 "MA-19-443722-A3"
-----------------------
FEEDBACK
AND
CONTROL
RECTIFIER
AND
FILTER
SWITCHING
ELEMENT
ISOLATION
POWER
TRANSFORMER
OUTPUT
RECTIFIER
AND
FILTER
FEEDBACK
AND
CONTROL
ISOLATION
POWER TRANSFORMER
RECTIFIERS
AND
FILTERS
SERIES
PASS
ELEMENT
FEEDBACK
AND
CONTROL
INPUT AND
OUTPUT
ISOLATION
INPUT
RECTIFIER
AND
FILTER
SWITCHING
ELEMENT
ISOLATION
POWER
TRANSFORMER
OUTPUT
RECTIFIER
AND
FILTER
RFI
FILTER
CURRENT
LIMIT
CIRCUIT
OUTPUT
SUPERVISORY
CIRCUITS
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