Another Power Supply

[OpenCircuit]

Stopped work on the other PSU since this one has a problem I can actually observe. One of the MOSFETs on the input side of the DC-to-DC main step-down transformer (red circle) released the magic smoke and had a blown fuse. Replaced the MOSFET (red circle and the adjacent MOSFET) along with the fuse and it repeated itself. I suspect now there is a short somewhere on the other side of the transformer or within the transformer itself (now removed). I did find a short on the two diodes just to the right of the main transformer (annotated in yellow)-will remove from circuit to confirm. There appears to be some thermal damage on D27 above J71. D27 and the two next to it are Low drop power Schottky rectifiers (stps30l45ct). Each  Schottky rectifier has a complimenting rectifier on the other side of the aluminum heat sink for a total of 6. No testing on them yet. Was wondering if anyone can explain the  purpose of these six?  Thanks.

Click image once then again on the next page for a full size image:

[OpenCircuit]

Black diode (D11)  read 534/1 when removed from circuit. ZD2 still shorted after being tested out of circuit. Main cap indicates thermal damage where one spot directly corresponds to Th1. D35 also shorted. Seems this PSU is a mess as well.

[BradC]

Replaced the MOSFET (red circle and the adjacent MOSFET) along with the fuse and it repeated itself.

You need to make yourself a dim bulb tester and stop needlessly torturing semiconductors.

Just get a mains lead, put a bayonet cap socket in series with the active wire and plug in a small wattage lamp somewhat larger in value than the idle current of the device under test. You'll get the lamp flash brightly as the main filter cap(s) charge up and then it'll settle down to a nice dim glow. Of course, if you have a semi-frying fault the lamp will generally continue to glow at or near full brightness, but limiting the current to a value that generally will prevent the smoke coming out.

Also good for reforming capacitors on old Valve kit that has been idle for more than a couple of years, and with the addition of a diode, making a "get me out of the poo" battery charger.

I have an assortment of lamps from about 5W up to 100W. Much nicer than hitting the power switch and ducking for cover as things explode.

[OpenCircuit]

Replaced the MOSFET (red circle and the adjacent MOSFET) along with the fuse and it repeated itself.

You need to make yourself a dim bulb tester and stop needlessly torturing semiconductors.
Much nicer than hitting the power switch and ducking for cover as things explode.

  Yes, no more tests w/o implementing this light bulb trick and everything else that is faulty.

[OpenCircuit]

What to replace Q18 with (lower-middle)? Transistor, but...?

[rmacintosh]

I believe that's part of a gate drive cct something like the attached...

So q19 which is readable looks like the pnp making up that circuit, get the npn variant for q18
Looks like the markings on the transistors could be read


Also, looks like zd1 (just to the left) also had a bad day...?

[OpenCircuit]

Q19, Q17 and Q16 read "K2F Y8"
Q14 is "K1P Y9"

Last 2 are just the date code.

Searching for "K2F diode (MMBT2907A)" I get this data sheet: http://datasheet.octopart.com/MMBT2907A-7-F-Diodes-Inc.-datasheet-7840139.pdf

Indicates that "...complementary "NPN Type Available (MMBT2222A)."

Go with that?

Not sure what happened to ZD1, but I replaced it.

[OpenCircuit]

Blown MOSFETs in a high voltage circuit almost always means blown gate driver/controller.
Not sure if this is a digital PSU or an analog one, but in case it is a digital one, then there is no practical way to fix it. If it is an analog one, then you may need to replace control IC and gate driver, if there is one.

Pic of PSU is in the first post (800watt SMPS for a desktop computer). Main controllers are the PS229 and a CM6800TX