PWL3708B-SMPS TCL LCD TV power supply – working principle and repair procedure

ON37A switch power supply.
About ON37A Power Supply
The circuit adopt PFC control IC NCP1653 and PWM control IC NCP1377,it has over voltage protection, over current protection, over hot protection function etc. IC’s Follow Boost pattern can increase whole circuit efficiency at low voltage, increase dependable stability, and it can eliminate noise on some point of load, it makes the unit’s performance steadier.

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Please take a printout of the SMPS schematic; and follow the working principle.  It will make easy to understand.  The working principle of almost all LCD SMPS is similar. So knowing the working principle will make it easy to repair most of it.

Input Circuit
When turns on the power switch, AC220V/50Hz passes common model filter consisted of fuse F801, RV801, C819, L806, LF802, LF801, R824 etc, it will restrain multiplicative electromagnetism interference from power supply circuit and eliminate high frequency interfere pulse from electricity net voltage.  After BD1 bridge commutate , about 300V direct current output.
DC 300V is processed by each control circuit.

Standby Start-up Circuit

After being commutated to output 300v voltage ,one way is through D819.R832 connecting with transformer T803’s primary coil , pin3 connect with pin5/Pin6/Pin7/Pin8 of IC U4,through U4 inner start circuit connect with pin4 VDD of U4, cause U4 ‘s Source-Drain switch on, so that it makes the T803’s primary coil work , so secondary coil output inductive 5V standby voltage.

DC 300V after Standby Start-up Circuit 1 commutating 3’s secondary winding pin4 is separated 2 ways , one way output 15V VCC voltage ,supply power to IC , the other way through diode connect with pin4 of U4 to supply power for U4

After commutating, output voltage of the secondary winding’s pin6 is sampled via R838, The sampling voltage input pin1 of U3, and the current occur between the pin3 and pin4 of photo coupler U3.The change of the current input pin4 and pin3 of U4, and the U4 adjust PWM to control transformer to output steady 5v standby voltage.

Standby Control Circuit
While connecting AC power supply, the standby circuit wait to start.
Others control circuit units can startup After the standby circuit work under normal situation .Then the main transformer start working, and output power supply 24V and 12V.
Power On
When the control unit send one high level signal via pin3 of P802 to pin B of Q808 through R842, R841, D819, Q808 turn on. In the meantime, pin1 and pin2 of photo coupler U2 is on, so inductive current pass pin3 and pin4 of U2. It makes pole B of Q807 has low level and turn on.  The 15V supply power VCC1 and VCC2 to pin6 of U5 and pin8 of U6  otherwise, when pin3 of P802 send a control signal which is low level. The signal passes R842.R841.D819 to pole B of Q808 and Q808 shut down, then pin1 and pin2 of photo coupler U2 open, therefore it doesn’t wok, pin4 and pin3 of U2 open too, then Q807 shut down, 15V VCC power supply voltage can’t supply VCC2 and VCC1, and U5 and U6 can’t get the working voltage, so U5,U6 stop working.

PFC Circuit

While VCC1 is about 15v voltage to supply pin8 of U6 (NCP1653A), U6 start working, and pin7 of U6 output pulse signal to pole B of Q801

The pulse signal control Q801 turning on and shutting down, so it makes L801 repeat save energy and discharge power, After being commutated, the voltage increase to about 390v, filtered by C807 send to PWM circuit.

PFC circuit adjustment is finished by pin1/pin3/pin4of U6 .

Bridge rectifier output voltage through resistance R860.R1.R864.R865.R867 and connect with pin3 of U6,for input voltage limit controlling.

PFC circuit output voltage passes R862, R86, R863, R866 to connect with pin1 of U6.  As feedback signal , inner circuit of U6 adjust and control pulse width signal output from pin7.In addition, it is over voltage protection function .  The negative current of PFC circuit passes R869 to connect pin4 of U6.

PWM Circuit
When VCC2 is about 15v voltage and connect with pin6 of U5,U5 start working to make pin5 of U5 export pulse control signal to pole B of Q805 and Q802.

When the pulse signal is high level,Q805 cut off and Q802 turn on. VCC2 passes Q802/R813 to charge up C820. when the pulse signal is low level, Q802 cut off and Q805 turn on. C820 discharge from Q805. As C820 repeats charging and discharging , that make pin1 and pin2 of transformer T802 primary coil discharging or charging, so there is the inductive current in pin6 &pin5 and pin4 & pin3of secondary coil. The drive signal control Q803 and Q806 to close and open.
For Q803 and Q806 close and open to make output voltage of PFC circuit which connect with primary coil of the transformer to repeat saving energy and release energy. So the inductive output voltage of secondary coil export 24V and 12V to main board.

Regulated Voltage Circuit

Regulated voltage Circuit is controlled by U5 and export 24V through R803 connecting with pin1 and pin2 of U1.  Because of photo coupler , inductive current occur between pin3 and pin4 of U1, and pin4 of U1 connects with pin2 of U5. The feedback signal will be sent to inner of U5 to adjust the pulse output from pin5 of U5 which keep transformer T801 output steady voltage.

Over Voltage Protection

Output circuit of +24V and +12V connect with rectifier diode D804 and D806, then incorporate the them into one way then pass D809/R856 to connect with pole B of Q812.  While work normally, pole B of Q812 is low level, then Q812 and Q810 cut off.  When main voltage 24V and 12V is over voltage , pole B of Q812 is high level, then pole B of Q810 is high level, so that Q812 turn on to makes Q810 turn on, through diode D820 connecting with resistance R842 to pull the voltage down. That makes the power on high level signal not work, which equal to turn off signal. So the power shuts down; and stops working.

Over Current Protection Circuit

In 12V output circuit, pin13 of secondary winding of transformer connects with R908 and R902 to pin2 of U7A.  While over current occur, pin1 of U7A exports negative voltage and pin7 of U7B exports negative voltage too. The negative voltage connects with the pole B of Q801 to make the Q810 turn on, then passes D820 and connect with R842. That makes the voltage of this point go down quickly.  So high level of turning on signal doesn’t work equaling to turning off signal. The power shuts down and stop working.

Repairing
1 A 40W or 60W filament type [Do not use CFL or similar type]  bulb is necessary to be connected between the AC lines and lest the power board damaged.  The bulb will twinkle if the PWB is good. The input power is about several decades Watt. You can measure 12V and 24V can be measured when P-ON voltage is given or connected with 12V;If the bulb is lightening and no output is detected, the PWB still need further repairing.
2 Possible defective causes: 

* defective Design

* defective Material

* Producing engineering problem:  short circuit(soldering-connected, pin-shorted),open-circuit(false soldering, pin-unconnected)

3 Troubleshooting steps:

* If the fuse is OK and no output, IC is possibly damaged or shut down under protective status. The other side, if the fuse is broken, you should check more parts like IC, MOSFET, Diodes, Zener diode etc.

* You should know the pin functions of IC
Checking IC2 and IC6, 13V DC supply to VCC with no AC input. Test the voltage of PIN2 if the voltage is 4.2VDC or not, and test if the output pulse width decrease or not by an oscilloscope. If not, replace the IC.

4 Holes of double-face board , small copper wire will easy open when the power board pass oven ,it’s not easy to be found, the places where is easy open are MOSFET S pole to grounding, drive circuit and ISENCE circuit.

5 Component check: transformer /inductance /photo coupler /TL431 on the power board aren’t easy to damage , most MOSFET damage is like G-S short circuit ,G-D-S short circuit, even crack, most transistors damage are as B-E short circuit, C-B-E short circuit even crack., diode damage is as short circuit. Zener diode damage is open circuit even crack. IC2 damage is possible ground to output(D1) short circuit or less than 0.5; ground to VCC(D1)short circuit or less than 0.5 even crack.

Resistor damage is the resistance value increase even crack. SMD Capacitor damage is crack or capability too large ; electrolytic capacitance damage is crack, plump , electrolyte leakage