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Help - TNY280PN based 5VSB - ATX Power Supply standby 5VDC

Posted by: thrust73 on

Hi PI Experts,

I have a Dell computer with ATX power supply which stopped working on a hot day.
Heard a pop sound before it went dead.

Opened up the PSU and found the TNY280PN ic top portion plastic adrift & exposing
the internal metal surface.
Checked for any burnt visually and by sniffing found none.

The 5VSB which is for standby is not working. I put in a replacement
TNY280PN and still it's not working.

I looked at your datasheet/reference design/design examples and observed that pin BP/M
is connected via a cap to Source line.

On my PSU it's the same but it has 30 ohms when measured from BP/M pin to the + lead
of the electrolytic cap. [10uF 50v]

So based on my nooby knowledge I put a jumper wire from BP/M to the + lead of the cap.
Now no more 30 ohms.

Tested it still not working but now the clamping circuit for Drain [D]
has burnt. The SMD capacitor is burnt on the soldered side to the power
connection. The SMD resistor [marking 470] in series with the capacitor
is open circuit. The zener diode [P6KE160A] is visually ok but when I removed and
measured it, I found it's shorted [continuity both direction].
I also removed the other diode [STTH110] and checked it, found it's good.

The clamping circuit looked similar to your design example/reference design with
some small differences.
I attached a hand written schematic. Transformer secondary windings that are isolated
from Drain & Source are not shown.

I would like to ask a couple of noob questions,

1. Is it correct to have a resistance of 30 ohms between the BP/M pin to the + lead
   of the 10uF 50V cap in the first place?

2. Apart from the clamping circuit, what are the other possible failure causes?

3. I'm going to replace the defective parts, can you recommend better alternative
   to the P6KE160A zener diode & STTH110 diode or shall I stick to the same part numbers?
  
4. I'm planning to replace the SMD cap & resistor with through hole type probably
   mounted on a daughter board, can you recommend the spec of the capacitor & resistor?

5. I will replace the TNY280PN again but this time with ic socket, can I use the newer
   TinySwitch -4 version or stick to the same ic?

Many thanks from a Noob.

Files

Attachment Size
Schematic 493.95 KB
Broken plastic 560.8 KB
PCB pic 587.26 KB

Comments

Submitted by PI-NANO on 05/24/2016

Hi,

I would strongly recommend not to repair for yourself, if something goes wrong you do not want to damage the computer. The schematic that you have drawn is very basic and some other details are missing like transformer design.

It is not clear why the part has got damaged, Generally most of the PI parts has over temperature protection. So I am not sure if the snubber design is marginal. Clamp design changes with the leakage inductance of the transformer and reflected voltage from the output.

I would recommend you to return the power supply to the manufacturer in order to get the power supply repaired.

Regards,

PI-NANO

I would recommend you to return the 

Submitted by babatunde on 06/14/2017

can i replace, tny280pn by 276pn

Submitted by PI-NANO on 06/19/2017

Hi,

If the design is done using TNY280PN then You should use only TNY280PN. Any changes in the design should be fully validated before using it in an application.

Regards,

PI-NANO