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Faultfinding Guide CDR779 8. GB 59

8.2 Circuit description of the current mode
power supply

8.2.1 Blockdiagram

Lightning Rectifier 6210
Protection 5131
+5V
EMI
MAINS
FILTER
2
2643

6230
+12V
Start Overvoltage
Circuit Protection 7 5

Comp Vcc 6240
9 -8V
v
2.5V t
CLOCK
Error 6250
Amplifier PWM 7125
+ Verror comparator VFTD
- S Output
- R Q
+ latch 6220
Isense Vsense VDC2

Vfb Rsense
CONTROL
VDC1

Vcc +12V +5V

7131

0V
7201

REGULATION CL 06532151_018.eps
271100

Figure 8-10

8.2.2 Function description
7
VI

MOSFET 7125 is used as a power switch controlled by the 6V
+
controller IC 7110. When the switch is closed, energy is 5V 8
S/R REF Vref
transferred from mains to the transformer. This energy is -
16V +
supplied to the load when the switch is opened. Through 5

control of the switched-on time, the energy transferred in GND 2.5V BIAS

each cycle is regulated so that the output voltages are 4 6
independent of load or input voltage variations. The Rt/ct OSC 1 OUTPUT

controlling device UC3842 is an integrated pulse width 2.5V S
ERROR AMP I SENSE COMP
modulator. A clock signal initiates power pulses at a fixed 2 + 2R
-
frequency. The termination of each output pulse occurs when Vfb -
R 1V +
R

1
a feedback signal of the inductor current reaches a threshold COMP LATCH

set by the error signal. In this way the error signal actually I sense
3

controls the peak inductor current on cycle-by cycle basis.
Figure B : Blockdiagram UC3842 CL 06532151_019.eps
8.2.3 Description of UC3842 271100

The input voltage Vcc(pin 7) is monitored by a comparator Figure 8-11
with hysteresis, enabling the circuit at 16V and disabling the
circuit below 10V. The error amplifier compares a voltage 8.2.4 Start up sequence
Vfb(pin 2) related to the output voltage of the power supply,
with an internal 2.5V reference. The current sense t1: Charging the capacitor at Vcc
comparator compares the output of the error amplifier with C2129 wiil be charged via R3123 and R3134, C2133 and
the switch current Isense(pin 3) of the power supply. The C2111 via R3129. The output is switched off
output of the current sense comparator resets a latch, which During t1.
is set every cycle by the oscillator. The output stage is a t2: Charging of output capacitors
totem pole, capable of driving a MOSFET directly When the input voltage of the IC exceeds 14,5V, the circuit is
enabled and starts to produce output pulses. The current
consumption of the circuit increases to about 17mA,
depending on the external loads of the IC. At first, the
GB 60 8. CDR779 Faultfinding Guide

capacitor at the Vcc pin will discharge because the primairy becomes reversed. This results in a current flow through the
auxiliary voltage, coming from winding7-9 is below the Vcc tranformer's secondary winding via the diodes, electrolytic
voltage. At some moment during t2, the primary auxiliary capacitors and the load. This current is also ramp shaped but
voltages reaches the same level as Vcc. decreasing.
This primary auxiliary voltage now determines the Vcc TimeDEAD phase : when the stored energy has been
voltage supplied to the load, the voltage from the secondary windings
t3: regulation falls below the output voltage(held constant by the
The output voltage of the power supply is in regulation electrolytic capacitors) plus the threshold voltage of the
t4: overload diodes. The current in the secondary winding stops flowing.
When the output is shortened, the supply voltage of the At this point, the drain voltage of the MOSFET is not yet zero
circuit will decrease and after some time drop below the because C2609 between drain and source contains a certain
lower threshold voltage. At that moment, the output will be charge. This charge will start a sine-shaped ringing together
disabled and the process of charging the Vcc capacitor starts with the transformer's self-induction.
again. If the output is still shorted at the next t2 phase, the The oscillator will start a next cyclus which consists of the
complete start-and stop sequence will repeat. The power described three phases. The time of the different phases
supply comes in a hiccup mode. depends on the mains voltage and the load.
TimeDEAD is maximum at an input of 400VDC and minimum
load, it will be zero at an input of 100VDC and overload.
16V
Vcc V2
12V

10V Vosc
Vc2134
V1
0V

0

20mA
Vcomp

Vsense
Icc

1mA

Vgate

OUTPUT

Vdrain

short

Vo Idrain

0
t1 t2 t3 t4
Idiodes

Figure C : Start-up sequence
CL 06532151_020.eps
271100 Ton Tdiode Tdead CL 06532151_021.eps
271100

Figure 8-12 Figure 8-13

8.2.5 Regulation

Figure 4 shows the most relevant signals during the
regulation phase of the power supply.
The oscillator voltage ramps up and down between V1 and
V2. The voltage at the current sense terminal is compared
every cycle with the output of the error amplifier Vcomp. The
output is switched off when the current sense level exceeds
the level at the output of the error amplifier.
TimeON phase : A drain current will flow from the positive
supply at pin 1 through the transformer's primary winding, the
MOSFET and Rsense to ground. As the positive voltage at
pin 1 of the transformer is constant, the current will increase
linearly and create a ramp dependent on the mains voltage
and the inductance of the primary winding. A certain amount
of energy is stored in the transformer in the form of a
magnetic field. The polarity of the voltages at the secundary
windings is such that the diodes are non-conducting.
TimeDIODE phase : When the MOSFET is switched off,
energy is no longer supplied to the tranformer. The
inductance of the tranformer now tries to maintain the current
which has been flowing through it at a constant level. The
polarity of the voltage from the transformer therefore
Faultfinding Guide CDR779 8. GB 61

8.2.6 Oscillograms 8.2.7 Circuit description

Input circuit
Oscillograms The input circuit consists of a lightning protection circuit and
an EMI filter.
The lightning protection comprises R3120, gasarrestor 1125
CH1 2 and R3124.
CH2 The EMI filter is formed by C2120, L5120, C2125 and R3124.
PM3394B CH3 2 V~ ALT MTB5.00us- 0.90dv ch1- It prevents inflow of noise into/from the mains.
ch1
1 Primary rectifier/smoothing circuit
ch3 The AC input is rectified by rectifier bridge 6102 and
smoothed into C2121. The voltage over C2121 is
T
ch2 approximately 300V. It can vary from 100V to 390V.

3 Start circuit and Vcc supply
This circuit is formed by R3123, R3134, C2129, D6129,
R3129, R3111, C2133 and C2111.
When the power plug is connected to the mains voltage, the
stabilised voltage over D6129(24V) will charge C2133 via
R3129. When the voltage reaches 14,5V across C2111, the
control circuit of IC7110 is turned on and the regulation
2 starts. During regulation, Vcc of IC7110 will be supplied by
the rectified voltage from winding 7-9 via L5132, D6132 and
C2133.
ch1 : Drain voltage
ch2 : Drain current Control circuit
ch3 : Gate voltage The control circuit exists of IC7110, C2102, C2104, C2107,
C2109, C2110, R3102, R3103, R3104, R3107, R3108,
CH1 1 R3109 and R3110. C2102 and R3110 define the frequency
PM3394B CH3 50mV~ ALT MTB5.00us- 0.90dv ch1- of the oscillator.
ch1
Power switch circuit
1
ch3 This circuit comprises MOSFET 7125, Rsense 3126, 3127
and 3128, R3125, C2127, L5125, R3112 and R3113. R3125
T
is a pull-down resistor to remove static charges from the gate
of the MOSFET.

Regulation circuit
3 The regulation circuit comprises opto-coupler 7200 which
isolates the error signal from the control IC on the primary
side and a reference component 7201. The TL431(7201) can
be represented by two components: