Oscillations

Posted by: gfili on 05/12/2010

Power supply oscillates when input goes to Vmax. Not sure if it is a stability problem or the switch turns off for some reason. On-off frequency is 1Hz with 20% dutycycle. Thanks

Comments

Could you please provide me with your specification, schematic and transformer design? Without this information I cannot help you very much.

Could you please clarify what you are measuring frequency and duty-cycle of? Is it the output voltage? What load conditions do you see this behavior at?

I think you may be seeing the effects of auto-restart but I cannot be sure without more information.

-Wesley

Vin=40V ..275V
Vout1= +22V/250mA
Vout2= -22V/25mA
The on-off state or auto start up triggers when Vin=250V
Transformer EEl16, npr=38, Nsec=9

TNYPK 377

Looks like an overload shut down however this happens at 50% load. Switching frequency 500KHz

Schematic attached. Thanks

Hello, I have quite a few comments so I will itemize them so you can reply to each major point more easily. :)

1. Your design is 40 - 275 VDC input yes? We recommend not going below 70 VDC input and the datasheet for the TNY-PKS device only guarantees operation down to 50 V via the Drain Supply Voltage parameter. It is possible to design lower than this but I cannot offer advice on this because we have little experience using this device at this low an input voltage.

2. How did you design the transformer? Did you use PI Xls or PI Expert? I did this design in PI Expert 7 and it got a transformer similar to yours so I'm guessing you did use PI Expert and we can more or less eliminate the transformer as the problem.

3.Try removing the UV resistor from the EN/UV pin? I don't think this is the source of the problem but it could be interacting with the supply.

4. Also I don't believe that the switching frequency is 500 kHz. The part's maximum switching frequency is 280 kHz as stated on the datasheet.

5. Lastly does the supply operate at VMAX with no load? Or does it not operate here?

Let me know! Thanks!

-Wesley

Thanks, Wesley.
1. Yes, I need to go as low as 38V, actualy. Working out of the spec is a problem.
2. The wholde design was generated using PI XIs. I used the "user defined input voltage" to stretch to 38V
3. UV resistor removed, no change.
4. OK, I will remeasure
5. The supply operates untill the load reaches 130mA. Normaly it should go to 270mA. Could the 13kohm resistor used for BP to big?
6. Another issue is with the -22V output. It goes all the way to -30V when the load for the +22V increases from 50 to 230mA.

This is a plot of the +22V output, from what I see it is 400KHz unless I am missing something. Thanks