I’m currently working on a Flyback design & using UCC28742. I’m now testing for a particular condition called open feedback response.
Basically, this is a test where I would be checking the output overvoltage limits. System is running and I would be opening the feedback to the controller at runtime and measure the outputs. I’ve two outputs being 15V & 24V.
Supply voltage range is 0-450Vdc. 15V bus rating 1.5W, 24V bus rating 20W. Based on my set resistors at Vs pin (60k4-top, 24k9- bottom), output is ON at inputs 92Vdc & OFF at 39Vdc.
While testing open fb, things were all ok until 66Vdc, but below this point and until 39Vdc, the output is starting to drive and I’m getting voltage on V15 bus as max as 25V (very little load 10mA - ~0.15W) and relatively high load on V24 bus (~10-20W).
Since I’ve a Zener on V24 (27V Zener), the voltage is limited to 30V max (I’m safe here). But power on V24 goes up to 30W in this condition(const resistance load).
From this there are two dangers,
- I can’t let 15V to jump >20V but this 25V
- Max power on SMPS I’ve designed is 22W, can’t let 30W on it which will eventually heat up the components on the bus.
Few questions:
- I would like to understand why this happens at first place
- Threshold on input voltage is 92-39 = 53Vdc. It seems to be high. Start is 210uA & stop is 75uA
Why input under voltage isn’t configurable?
- Or there any solution to this problem?