0
\$\begingroup\$

I am working on 12V 30A smps that has only 1V output. I haven't got board's schematic but I found one same like mine (Schematic Link).

  • I measured high voltage capacitors (C2 and C3 on the schematic.) and bridge rectifier. They are working.
  • Also checked power switching transistor on board they seem fine. (Q1 and Q4 on the schematic.)
  • Checked PWM transistors on board. They also fine. (Q2 and Q3 on the schematic.)
  • I measured PWM IC's Vcc pin(pin12) and it is 2.4V.

I think, there could be a problem with D12 and D11 but not checked( because they are located under transformer).

Also on this link, the author says this circuit has the self-starting capability. After I read it, I become suspicious of C10 also.

Update1: I checked D12 and D11 both working. I read 110V DC over C10(Input 220V AC)

I attached pictures both front and back.

Please help me with this issue. Thanks. enter image description here enter image description here

\$\endgroup\$
17
  • \$\begingroup\$ How is Vcc generated for startup before switching? I can not see it in the schematic. \$\endgroup\$
    – winny
    Commented Aug 5, 2020 at 11:06
  • \$\begingroup\$ @winny I am not sure but it is called as self-starting. Capacitor on the input (C10 on schematic) somehow resonates the voltage then aux winding start to generate Vcc. That is all I understand from this link. \$\endgroup\$
    – Emre Anıl TERZİOĞLU
    Commented Aug 5, 2020 at 11:22
  • \$\begingroup\$ @winny Vcc is not being generated before startup. Instead, one transistor is getting triggered (through the input's positive rail) just before the other one. So the first switching action is initiated without the need of 494 -- just like in an RCC. But one thing that caught my eye is that the circuit is not isolated and how V- is connected to the input's negative rail is not shown. \$\endgroup\$
    – Rohat Kılıç
    Commented Aug 5, 2020 at 12:20
  • \$\begingroup\$ @RohatKılıç I haven't got enough experience on hardware so probably I missed. I think that the dotted primer pin of TR2 is negative rail of the primer and isn't that isolated? \$\endgroup\$
    – Emre Anıl TERZİOĞLU
    Commented Aug 5, 2020 at 12:31
  • 1
    \$\begingroup\$ Troubleshooting an SMPS without a scope is near impossible. Please get one. Since you do have output, I would expect some kind of hick-up mode. Shorted diode somewhere? \$\endgroup\$
    – winny
    Commented Aug 5, 2020 at 13:38

1 Answer 1

Reset to default
1
\$\begingroup\$

I solved the issue. The pads of one of the switching transistor were lifted. I repair these pads and SMPS worked fine.

Thanks for the support. @winny @RohatKılıç

Lifted Pads

\$\endgroup\$
1
  • \$\begingroup\$ Thanks for suppling and accepting the answer! \$\endgroup\$
    – winny
    Commented Aug 7, 2020 at 19:18

Not the answer you're looking for? Browse other questions tagged or ask your own question.