Flyback with TNY274 issues

Posted by: dado73 on 06/27/2013

I have designed and built a flyback with the TNY274 (input from main and triple output).

The attached picture shows the switch drain voltage and the +5V output (the regulated one) monitored with a scope.

(+5V channel is monitored in AC mode for simplicity)

I have three questions:

1 - is there a way to damp the ringing produced on the +5V output when the mosfet turns on & off ? (I have already tried cutting the overshoot on the mosfet drain with a lower bd voltage TVS but nothing changed)

2 - why the switching frequency is not stable ? the data sheet says that there's a peak-to-peak jitter of 8kHz around the standard 132 kHz, but what aI see is totally different

3 - monitoring the +5V output for several seconds I see that the voltage steps up and back down of exactly 10 mV in absolutely random moments: what does it mean ?

Thank you for supporting

Comments

Hi,

Thanks for contacting PI. Regarding you questions, please see my comments:

1. You could damp the ring by optimizing the snubber circuits on primary side and secondary diodes. Or you could add a small ceramic capacitor to filer the noise on each out put.

2. TNY switch is a on/off control switch, so the switching will be kept or skipped with different load and variable input voltage (voltage ripple). More over, as what you said there is also a jitter switching frequency added. So the sw frequency will be not stable.

3. Sorry, I did not really understood your question. Do you have some waveforms to show? What is the frequency of this 10mV change?

Thank you Terry.

1. The snubber circuit is currently made with a TVS: may an RCD snubber perform better for low noise purposes ?

2. The switching behaves in the same way at any input voltage, even at lower input voltage (when Ton approximates 50%). How is it possible ?

3. Filtering away HF noise the outpu voltage results very stable, but once every 2-3 seconds (frequency "lower" than 1 Hz) goes up and down of 10 mV (note: it could also be a problem of scope resolution...)

Just one more suggestione from you:

considering the power of the converter (less than 10W) what is the most performant solution to filter from low frequency disturbance coming from the main ?

Thanks a lot

Davide

1. Would be possible to send me your snubber circuits schematic?

2. Do you have waveforms to share with me at low input voltage and high input voltage?

3. Looks like the power supply is not stable if the ripple is not line voltage ripple. Possible to share the waveforms?

Basically it does not require specific filter for low frequency disturbance, the bulk capacitor after the bridge plays a role as the low frequency filter.