Liteon PA-4291 SMPS repair gone wrong after a complete reverse engineering

[Zanot92]

Hi all, I'm Fabio from Italy. I take this opportunity to wish you all a happy Christmas ad a happy New Year.

I've got a faulty Toshiba TDP S8 projector laying around for a while so I've decided to try troubleshooting it during these winter holidays.
Before I've taken apart, the projector was not powering up, even the led indicators on the fort panel were off.  Since at a first visual inspection the lamp seemed ok and also it's driver, I focused on the PSU, a Liteon PA-4291.
It's suppoded to deliver 385V for the lamp ballast, +13V, +5V and +3.3V for the rest of the projector circuit. I've tested it apart and there were no output voltages.

Since I'm (unfortunatly) plenty of time I've reverse-engenieered the board, see attachment (I named only the components that have a silkscreen on the PCB). I think it's quite comprehensive.

I've deeply tested apart all the major components, the two mosfets with a basic switching circuit, measured inductors and their saturation current (not needed, just for fun!), checked the ML4800 control IC feeding the right voltages with external supplies to see if it was still good and it was so.

The only things I found wrong were two:

• a faulty electrolytic cap C206 rated at 3.3mF but measuring only about 26uF (I didn't measured ESR)
• R125 PFC current sensing resistor shorted.

Reading R125 color, red-green-white-gold-green, I think It was a 25.9Ω 2W BUT I've ignored, and here I think I've made a mistake, trusting silkscreen on PCB that says 385V @ 0.65A max. So according to ML4800 datasheet page 8 I've calculaded the new sensing resistor value as 1V/0.65A chosing 1.5Ω. After replacing those two component the PSU was running fine at no load but when I've connected the lamp ballast I saw the magic smoke..

As result, the new R125 was shorted, the 10Ω SMD 1206 resistor marked on the schematic completly carbonized, also the 3 1n4148 clamping diodes protecting pin 3 I_Sense of the control IC were shorted. Also the ML4200 has been damaged because now it's draining over 60mA at 12V Vcc becoming very hot. Mosfets instead are still fine.

I suppose, and here I'm asking your kind advice, that only a dead short between the 385V connector and GND could had cause such a failure, making the voltage on ML4200's pin 3 too negative blowing the resistors and clamping diodes and IC too.

What it's confusing me is that I've also started the reverse engineering of the ballast, an Osram PT VIP4AC 380, and for what I can see now It's power stage is basically a buck converter feeding a H bridge for switching the lamp. All mosfet here are good (tested outside pcb) and there are no short at the input 385V connector!