TCL LCD TV SMPS – PWL3711C – repair procedure and the working principle

PWL3711C Switch Power Supply
Please take a printout of the SMPS schematic, and then refer the description.  It will make easy to understand the functional operation.  The working principle of almost all LCD SMPS are similar.  So, understanding about one; will help to  repair others too.

Click on the pictures to magnify

AC Input and EMI Filtering Circuit
PWL3711C power supply is composed of anti-interference circuit, bridge commutation circuit, filter circuit, PFC( Power factor Correction) circuit and switch stabilized voltage circuit and so on. The merits of switch power supply are wide operation voltage, low power consumption, high power reaching over 98%.
It has many protection functions such as OCP, OVP and over heat protection.

The switch power supply has two operation conditions:
1) Normal-Normal work when power on;
2) Standby-When TV set standby, switch power supply is in narrow pulse operation condition and change to under voltage condition, then the output voltage goes down to half value of normal condition;
The theory introduction of each part of the switch power supply as following:
 Anti-interference circuit
When switch on, the commercial AC220V/50Hz passes common-mode filter composed of fuse F1,ZV1, R1, CX1, CX2, LF1, LF2 to eliminate the all the EMI and high frequency interference pulse from power input port.  X capacity and Y capacity are both safety capacitance. The difference of them is that X capacitance is connected to AC input port to eliminate difference-mode interference, Y capacitance is connected between AC input and ground to eliminate common-mode interference. Y capacitance has two kinds, one is Y1 capacitance belonging to double insulation Y capacitance and connecting between primary coil and second coil, the other is Y2 capacitance belonging to basic single insulation Y capacitance and connecting between primary coil to protect ground.

Commutating Filter Circuit

BD1, C1, C2 compose bridge commutating circuit to convert the 220 AC to 300 DC filtered by C3.
PFC Circuit
PFC circuit is composed of IC1(NCP1650), Q1, L2 and the other external components.
NCP1650 Power Factor Controller Introduction
NCP1650 chip is an active, power factor correction controller that can operate over a wide range of input voltages, and output power level. It is designed to operate on 50/60Hz power system. This controller offers severl different protection methods to assure safe, reliable operation under any conditions.
1 Feature:

Fixed Frequency OperationAverage Current Mode PWMContinuous or Discontinuous Mode OperationFast Line/Load Transient CompensationTrue Power Limiting CircuitHigh Accuracy MultipliersUnder voltage LockoutOvervoltage Limiting ComparatorBrown Out ProtectionRamp Compensation Does Not Affect Oscillator AccuracyOperation from 25 to 250kHz

PFC circuit of PWL3711C is fixed frequency average current mode boost converter. PFC circuit operates in two stages: 90Vac ～132Vac is the low input stage for PFC output 260VDC;180Vac～ 264Vac is high input stage for PFC output 390VDC. The shift stage is 140Vac～165Vac completed by comparator IC10. When input commercial current in high stage, the level of pin 3 of IC10 is higher than pin2, then Q21 turns on to make sample voltage of IC1 FB(pin6) go down and make the output voltage of PFC go up to 390VDC.

The operation frequency is about 70KHz.The output current changing should follow input voltage changing to go to pin5/pin12 of IC and the voltage entering pin6 of IC. When the inner parameters are compared and calculated to confirm the operation duty ratio and maintain the stabile voltage. Under the stable output power, the input voltage decrease, minimum duty ratio of the output pulse width of IC1 larger. When input voltage increases, the minimum duty ratio of pulse width output will decrease; vice versa.  In an output cycle, duty ratio of IC1 pulse width output is mutative in which there is a minimum duty ratio on the highest point of Vac.

Q1 alternating with D1 produce output current of bridge commutating diode to flow continuously, and the angle of flow go to 180 so that the current of AC has the same phase with voltage of AC . The waveform is sine wave, and the power factor of system should approach to 1.

DC-DC Shift:
12V Section: IC6-NCP1377
NCP1377 PWM Current-Mode Power Supply Controller
NCP1377 combines a true current mode modulator and a demagnetization detector which ensures full borderline/critical Conduction Mode in any load/line conditions together with minimum drain voltage switching. Due to its inherent skip cycle capability, the controller enters burst mode as soon as the power demand falls below a predetermined level. As this happens at low peak current, no audible noise can be heard.
The transformer core reset detection is done through an auxiliary winding which, brought via a dedicated pin, also enables fast Over Voltage Protection. Once an OVP has been detected, the IC permanently latches-off. The 1377 features a sampling time of 4.5us whereas it is 1.5us for the B
version.
The NCP1377 also features efficient protective circuitries which, in presence of an over current condition, disables the output pulse and enters a safe burst mode, trying to restart.  Once the default has gone, the device auto-recovers. Finally an internal 1.0 ms soft-start eliminates the traditional startup stress.

Features Free-running borderline/critical mode
Quasi-resonant operation
Lockout OVP
Auto-recovery short-circuit protection via
UVLO crossover
External latch triggering, e.g. via overtemperature signal
Current-mode with adjustable skip-cycle capability
Internal 1.0 ms soft-start
Internal temperature shutdown
Internal leading edge blanking
500mA peak current source/sink capability
Under voltage lockout level of 12.5V (On) and 7.5V(Min) Direct optocoupler connection.

Pin8 of IC6 connects R101 to “+”end of C16 via internal 4mA constant-current source to charge C34. When reaches about 12.5V, output of pin5 of IC6 is pulse width waveform and 4mA constant-current sourse will shut off, then auxiliary winding of transformer T2 supplies C34.  Because of PFC circuit, voltage of C16 is constant, and the changing of duty ratio of IC6 is only with output load. At the moment of output loads enhance, 12V drops and the current of IC8 reduces, then level of pin12 of IC6 goes up, so largens pulse width duty ratio of IC6 and makes the turn-on time longer to make transformer transmit more energy. Output of 12V goes up to larger turn-on current of IC8.

In fact, under output load changeless, IC6 also performs the above adjustment, but the adjustment amplitude is slight. Output constant voltage relies to above closed loop adjustment, so that forms output voltage ripple form.

24V Section

24V branch section is basic sameness to 12V section, but only little difference in function of IC NCP1217(IC2) is current mode PWM controller, applying to offline switch power supply.

Feature

1 Current-mode with adjustable skip-cycle capability

2 Built-in internal ramp compensation

3 Auto-recovery internal output short-circuit protection

4 Internal 1.0ms soft-start

5 Fixed frequency versions at 65kHz, 100kHz and 133kHz

Switch Voltage Regulated Circuit
Power Supply DC Output
24V/4A power supply branch: One way goes directly to primary winding passing primary winding 1-3 to pin D of Q2/ Q17; the other way passing D3 and goes directly to pin8 of IC2 connecting internal circuit of IC to pin6 to charge C21. When the voltage set point reaches 12.5V, IC2 operates normally and internal circuit latches off between pin6 and pin8, then C21 discharges through internal circuit of IC2. At the same time, output of pin5 is driving signal to Q2/Q17 which turns on, so the primary winding of T1 turns on to store the energy.
When C21 discharge to 5.6V, the discharge circuit shuts off and driving signal of pin5 latches off , so each secondary winding of T1 induces output voltage via to energy shift.  At the same time, secondary winding pin6 charges C21 through L5/D7, when reaching 12.5V, pin5 of IC2 sends out drive signal to Q2 and Q17 which turns on, so primary winding of T1 turns on to enter new save energy loop process and ensure inducing constant output voltage from each secondary winding.

When output of secondary reaches over 24V, the current of photocoupler IC3 enhances as R24 sample voltage largens, so that photo coupler shines strongly to feedback to pin2 of IC2, the point voltage of this pin going low. Output signal of pin5 of IC2 shuts off to adjust duty ratio.  12V/4A power supply branch: One way to primary winding 1-3 of T2, then to D pin of Q5; the other way via D11 to pin8 of IC6(NCP1377), connecting internal circuit of IC6 to charge C34, when voltage of C34 reaches 12.5V, IC6 starts up to operation, and the internal circuit disconnects from pin6 and pin8. C21 discharges via internal circuit of IC6, and pin5 exports drive signal to make Q5 turns on, so connects primary winding of T2, and T2 starts saving energy.  When C21 discharges to 5.6V, the discharge circuit will shut off, and output drive signal will stop from pin5. For energy shift, inducing output voltage produces from each secondary winding of T2.
At the meantime, secondary winding pin6 charges C34 via D13/R37, when the voltage reaches 12.5V to drive pin5 to export drive signal, Q5 turns on and connects primary winding of T2, then renews to save energy to ensure constant inducing output voltage from each secondary winding.  When output voltage of secondary winding reaches 12V, the current of passing photocoupler IC8 enhance, so the photocoupler shines strongly to feedback to pin2 of IC6, the level of this pin goes down to output signal of pin5 of IC6 shuts off to adjust duty ratio to ensure constant output voltage 12V.

Over-Current Protection

24V/4A supply power branch

The sample current passes R20 to get the sample voltage which on R17 to connect internal circuit of pin3 of IC2. When the sample current reaches a certain rated value, IC2 stops to work and protects the circuit.

12V/4A supply power branch

The sample current passes R39 to get sample voltage which is on R42 to connect internal circuit of IC6’s pin3. When the sample current reaches a certain rated value, IC6 would stop to work to protect the circuit.

OVP Circuit 12V/4A Supply Power Branch
Secondary winding pin5 of T2 connects IC6’s pin1 via R35, when testing circuit of IC6 test the voltage over the rated value, IC6’s pin5 stops to export drive signal to make Q5 shut off.  When canceling the test signal, IC6’s pin5 exports drive signal again to make Q5 turn on and continue to operate 24V/6A Supply Power Branch
when the output is higher than 31V, so ZD2 and ZD3 work normally and signal 24V-OVP is high level. Q3  pinC connects to ground. The current between pin1and pin2 of IC3, so inner diode shines strongly, the current larger between pin3 and pin4. This signal is sent to pin2 of IC2 to latch off the drive signal. The power is off.When the high level signal 24V-OVP is sent to pinB of Q12, Q12 turns on to the ground. The photo coupler is shines strongly and the current of receiver increases. So the low level is sent to pin8 of IC2 to shut off the power.

Power Standby
When plug in AC commercial current, the 12V of power supply continues to operate, power supply standby circuit control is only for 24V supply.
Remote control signal connect P2’s pin1 via J2’s pin4 to passing R33 to B pin of Q4. When receives power on signal, the B pin of Q4 is low level, Q4 turns on and Q12 shuts off

Power off 

When receives the remote signal low, Q4 shuts off and Q12 turns on. Because of induction action of photocoupler, the level of IC2’s pin2 is pulled low, so IC2 stops to operate Power on: when receives power on high level signal, Q4 turns on and B pin of Q12 equals to connect ground. For photocoupler doesn’t work, IC2’s pin2 releases and this set point voltage is high level. IC2 renews to operate to export drive signal from pin5, so the circuit exports 24V power supply.

There are two supply outputs: +12V and + 24V by different transformer in this power supply solution. When TV switch on, +12V output is ok to all the boards; then send standby signal to power on, +24V output to the panel.  If HV is higher than130V, but High and Low Voltage Converter Circuit don’t work, +12V and +24V output will be still ok.

Repairing

1 A 40W or 60W filament type (Do not use CFL or other 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 cause 

a) defective Design 

b) defective Material 

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

3 Troubleshooting steps

 a) 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.

b) 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