How To Repair Lcd Tv Smps

PWL3708B-SMPS TCL LCD Tv power supply – working principle and repair process

ON37A switch ability 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'southward Follow Boost design can increase whole circuit efficiency at low voltage, increase undecayed stability, and it can eliminate dissonance on some point of load, information technology makes the unit's operation steadier.

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Please have a printout of the SMPS schematic; and follow the working principle.  It will make piece of cake to understand.  The working principle of nearly all LCD SMPS is similar. And so knowing the working principle volition arrive easy to repair about 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 , almost 300V direct current output.
DC 300V is processed by each control excursion.

Standby Kickoff-upwards Circuit

After being commutated to output 300v voltage ,one way is through D819.R832 connecting with transformer T803'southward primary curlicue , 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, then that it makes the T803's primary coil work , then secondary coil output inductive 5V standby voltage.

DC 300V after Standby Start-up Circuit i commutating 3's secondary winding pin4 is separated ii 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

Later on commutating, output voltage of the secondary winding'south pin6 is sampled via R838, The sampling voltage input pin1 of U3, and the current occur betwixt the pin3 and pin4 of photo coupler U3.The modify of the current input pin4 and pin3 of U4, and the U4 adjust PWM to command transformer to output steady 5v standby voltage.

Standby Command Circuit
While connecting AC power supply, the standby excursion wait to first.
Others control circuit units can startup After the standby excursion piece of work under normal state of affairs .Then the main transformer commencement working, and output ability supply 24V and 12V.
Power On
When the command unit of measurement ship 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 electric current laissez passer pin3 and pin4 of U2. It makes pole B of Q807 has low level and plow on.  The 15V supply ability 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 close downward, then pin1 and pin2 of photo coupler U2 open, therefore it doesn't wok, pin4 and pin3 of U2 open besides, then Q807 close down, 15V VCC ability supply voltage can't supply VCC2 and VCC1, and U5 and U6 tin't get the working voltage, and so U5,U6 finish 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 point control Q801 turning on and shutting down, so information technology makes L801 repeat relieve free energy and discharge power, After being commutated, the voltage increase to about 390v, filtered by C807 send to PWM circuit.

PFC circuit aligning is finished past 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.  Every bit feedback signal , inner circuit of U6 adapt and control pulse width signal output from pin7.In improver, it is over voltage protection part .  The negative current of PFC circuit passes R869 to connect pin4 of U6.

PWM Circuit
When VCC2 is well-nigh 15v voltage and connect with pin6 of U5,U5 get-go working to make pin5 of U5 export pulse control bespeak to pole B of Q805 and Q802.

When the pulse point is high level,Q805 cut off and Q802 turn on. VCC2 passes Q802/R813 to charge upward C820. when the pulse bespeak 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, and so in that location is the inductive current in pin6 &pin5 and pin4 & pin3of secondary whorl. The drive betoken command Q803 and Q806 to close and open.
For Q803 and Q806 shut and open to brand output voltage of PFC circuit which connect with primary coil of the transformer to echo saving energy and release energy. And then the inductive output voltage of secondary curl 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.  Considering of photo coupler , inductive electric current occur between pin3 and pin4 of U1, and pin4 of U1 connects with pin2 of U5. The feedback signal volition be sent to inner of U5 to accommodate 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, so contain the them into ane way and so laissez passer D809/R856 to connect with pole B of Q812.  While work unremarkably, pole B of Q812 is low level, then Q812 and Q810 cut off.  When chief voltage 24V and 12V is over voltage , pole B of Q812 is high level, and then pole B of Q810 is high level, and so that Q812 turn on to makes Q810 plow on, through diode D820 connecting with resistance R842 to pull the voltage downwards. That makes the ability on high level bespeak non work, which equal to turn off signal. Then 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 plough on, then passes D820 and connect with R842. That makes the voltage of this point go downwards quickly.  So high level of turning on signal doesn't work equaling to turning off indicate. The power shuts downward and finish working.

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

* lacking Design

* lacking Material

* Producing technology problem:  curt circuit(soldering-connected, pin-shorted),open-excursion(false soldering, pin-unconnected)

3 Troubleshooting steps:

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

* You should know the pivot 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 iv.2VDC or not, and test if the output pulse width subtract or not by an oscilloscope. If not, supplant the IC.

4 Holes of double-face board , small copper wire will easy open when the power board pass oven ,it's non like shooting fish in a barrel to be constitute, the places where is easy open up are MOSFET S pole to grounding, drive circuit and ISENCE circuit.

5 Component bank check: transformer /inductance /photo coupler /TL431 on the power board aren't easy to damage , near MOSFET damage is like Thou-Southward short circuit ,G-D-South brusk circuit, even crack, most transistors harm are every bit B-E short excursion, C-B-E short circuit fifty-fifty crack., diode harm is equally curt circuit. Zener diode damage is open circuit even crack. IC2 damage is possible ground to output(D1) short circuit or less than 0.v; ground to VCC(D1)curt circuit or less than 0.five fifty-fifty scissure.

Resistor harm is the resistance value increment even crack. SMD Capacitor damage is crack or adequacy likewise big ; electrolytic capacitance impairment is crack, plump , electrolyte leakage

How To Repair Lcd Tv Smps,

Source: https://masterelectronicsrepair.blogspot.com/2019/02/pwl3708b-smps-tcl-lcd-tv-power-supply.html

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