CX600W PSU bad SOT23 PFC gate driver transistor

[lordvader88]

After replacing the CM600 PFC/PWM chip that did seem dead (maybe not), I still got nothing from the outputs. And nothing at all on a scope from that looked like any mosfets trying to startup.

And then I noticed a bad B-C junction of a PNP transistor, 1 of the gate drivers of the PFC mosfets. So the B-C was shorting it do GND, along with the PFC-out of CM6800. I'm sure I checked that before a few times but, but I didn't even know the pinout of sot23 last week.

Its Q2 on page 15
http://www.champion-micro.com/datasheet/Analog%20Device/CM6800.pdf
Mine is K2F PNP 0.6A
https://www.diodes.com/assets/Datasheets/ds30040.pdf
So far looking at random PCBs I have with SOT23 stuff, I haven't found a high enough rated 1 for V or I, I bet I have suitable TO-92 tho. I'll just trim the leads real close.

So IDK for sure if it just fried or what??? If those 2 bases where short to GND before, I should have been alerted, even tho IDK 1/2 as much back then. I noticed right away this time.

When I applied 10.5V to Vcc of the original CM6800 It wasn't outputting any Vref, neither is this new chip, but I never checked before I put it back in its card, and back in the PSU.
The data sheet says "Buffered output for the internal 7.5V reference", shouldn't that be outputting 7.5V even if nothing else is connected ?

[lordvader88]

I resoldered it all back together, and just finished cleaning it up with alcohol. And I attached a DVD drive as a load, and put it in series with a 100W lightblub limiter. And the light light right up and flashed about 3-5 times in under 1 second. So I shut it off immediately.

Does that sound like it's trying to start The DVD player didn't, and I don't have it's fan hooked up.

Nothing blew-up and I didn't notice smoke, but I've never tested a working ATX PSU before.

It reminds me of when I tried a UPS with a 40W limiter, and that wasn't enough for it to turn on and it just buzzed a bit and I think the blub flickered. 100W blub was enough for it draw enough current.

Now I'm 1/2 afraid to try again with the DMM and scope attached.


Before I replaced the CM6800 PWM/PFC chip, the highside would power up, and the 12V stby, and it would not light the 100W at all.

[lordvader88]

I've turned it on for ~1sec with a manual range set on my BM869, and I saw +220V on the main cap which should mean the PFC section is active, otherwise it can't go over the rectified 169V. And why else would it be flashing. I think the IC does a slow start-up. Not a twitch from the cold side, but I'm thinking the PWM mosfets don't start until the main cap goes over 300V.

I'm just worried tho because if the lightbulb is maxing out the current, that's 0.83A AC rms. That sounds like an awful lot just to boost a cap.


If there's a problem, I'll know pretty quickly if I don't use that current limiter.

[lordvader88]

Ok I was forgetting that it's just a mosfet turning on/off across the rectified 169V. So the average current should be pretty high regardless. I haven't really done any maths on any SMPS, (even tho I have ton's of electronics pdf's)

But it's quite possible I fried that PFC gate transistor with my soldering iron cranked up for the non-leaded solder. Again if it was fired before, I should have noticed. So I can't say for sure that was the problem.


Does any one here have an big SMPS, and a lightblub current limiter ? what happens ? I'll have an auto-transformer in 2-3 weeks


I've been busy with work so I never tried anything since. In the chips datasheet for the slow startup/checks/etc, they point to a cap that is for the timing, but it's some SMD on the PCB anyways so I never checked anyways.