UCC256302: Dual output Unstable waveform

Ben will reply to you.

HI Doi-san,

For the test conditions where you see the peak resonant current is oscillating, can you measure the current coming out of the FB pin? You can do this by removing R21, install a 10kΩ resistor at R32 and measure the voltage across R32. Also check if the VCR pin peak to peak ripple is changing cycle to cycle when at steady state.

The changes you listed in 3.2 of the PDF are reasonable and I do not anticipate any negative side effects from the changes you listed.

The waveform in 4.1 looks like burst mode. The burst pattern is a little uneven but is otherwise acceptable

If you do see oscillation in the FB pin current, this would indicate some adjustment to the compensation network is necessary.

Best Regards,
Ben Lough

Hi Ben-san,

We measured the FB pin current waveform of the instruction and the ripple waveform of the VCR terminal and attach them.
 
Please check the data and tell us the problems, countermeasures and so on.

UCC256302 Unatable 2.pdf

Hi Doi-san,

I think some adjustment to the compensation will help the oscillation. Try increasing C28 to 82nF and add 470pF at C29. If you are able to take a loop measurement, it would also be helpful for suggesting compensation values.

Best Regards,

Ben Lough