I am trying to repair the power supply in an LCD monitor. It is a fairly basic design around an OB2268AP in the 20-30 Watt range. The power supply had failed spectacularly because the main MOSFET short-circuited, vaporizing one pin on the regulator IC, burning 2 resistors to a crisp, damaging another plus some collateral damage.
Here's the part of the circuit after filtering and rectifying mains voltage, so there's some 300V DC between U+ and U-.
There are a few oddities on the PCB:
- R706 is not a resistor but a choke inductor (makes sense)
- ZD702 is not mounted
- R708 is not a resistor but a zener diode. I can make out '24' at the end of its designation, so it's probably a 24 Volt zener
R710 and R712 were burned to a crisp so I can't make out the original values, and I need some advice on the values. The reference design for the OB2268 does not mention R710 but I suspect it's a low ohm resistor to have some 'protection' against the gate capacity of Q701. I guess something like 2.2Ω, 4.7Ω maybe? Any higher and the rise and fall times for the gate will suffer, I guess.
The one that has me stumped is R712. Pin 6 on the IC is the SENSE input of the current limiter. It has a threshold of 0.86 volt; together with R711 of 3.3Ω that makes a limit of 0.25 Ampère. If R708 is indeed a zener diode of 24 volt it would act as a secondary limit for the IC's own power circuit (D703, 'R'706, etc). So what's your guess for R712? Maybe the value is non-critical (the input resistance of pin 6 is 40 kΩ according to the datasheet), maybe it can't be too high else zener R708 won't work reliably.
Update: R711 is actually 0.33Ω
Update 2: I repaired it with the following components:
Q701: IRFB9N60A (600 V, 9.2 Amp mosfet) R701: 2.2 ohm R712: 1 kohm I702: an optocoupler I had lying around :P
I hooked up an oscilloscoop to the gate of Q701 and the rising edge is a bit curved and there's a tiny bit of oscillation/overshoot but otherwise it seems okay; the descending edge is straight and sharp.
Note on the IRFB9N60A: in contrast to the original 7N80C this transistor not an isolated package.