Help with a SMPS (Lytec LP5422

[mark hodsman]

This LP5422 keeps blowing the mains fuses. (3Amp) I have checked the BR and can see no obvious short circuits on the board.  However it keeps blowing the fuses in a dramatic fashion...

Where should I look first off please? Im loathe to get stuck into this with 240V floating around. I'd be interested if I could inject 12 or 24V AC and still get meaningful test results.

Thanks in advance!
Mark H

[goaty]

Funny schematic, there´s no DC connected to the primary switching side ;-)

Check Q1 for short first. Or output from U1 is constant ON...

[mark hodsman]

I can confirm there is no connection to CN10 which is odd as you say...

I can also reveal that the ZD2 is shorted and QN1 its shorted. I have an IRF840 kicking around -0 would that work in place do you think? Not sure about that Zener though

[goaty]

Not sure about the transistor specs, but make sure the UC3842 is not dead before trying out another Q1.
The zener limits the gate voltage, but wouldn´t try it without it.

[goaty]

To avoid destroying fuses and components, you can put a lightbulb (60W) in series to mains. If it lights up bright, you know it isn´t working...

[mark hodsman]

An update if anyone is interested...
I put a 15V zener in place of the 18V and a IFR840 MOSFET and it lit up (in a good way) but it didn't work completely. Various aspects not working...then it gave up the ghost. Good news was that it didn't explode like a firework (this is progress)

I guess it really needs an 18V zener - which I didn't have to hand.

Do you think inserting exactly the right components would fix this? Why does CN10 not have anything connected? Any ideas?

Thanks for all the help btw

[goaty]

The schematic is not complete, probably CN10 is for separating input circuit from switching circuit or as a test connector ?