UCC28C59: Unstable output under load

Hi,

At no load, measure the waveforms of primary side MOSFET Vds, transformer AUX winding, and transformer secondary side winding, to make sure your transformer polarities are correct as the first step.

Please provide your schematics for review.

003-2086.pdfBias_Power.pdf

Polarities verified as correct.  Schematic and transformer winding info attached.  The design is 600-950VDC in, 24VDC@0.625A out.

Hi,

Please provide the waveforms of primary side MOSFET Vds, transformer AUX winding, and transformer secondary side winding at no load.

First 2 images are mosfet Vds and AUX winding.  Second 2 images are mosfet Vds and secondary winding.

Hi,

What was the load current when you measured these waveforms?

Also, what is the rated load current?

The rated load current is 625mA.  The load when the measurements were taken was around 10mA.

Hi,

can you measure the waveforms at 0mA load current and send the waveform to me? This time please also measure Vgs and Vds, and also the CS pin waveform.

First is Vgs and Vds. Second is CS and Vds. Both are at no load

Hi,

At no load, what is your Vout. COMP pin voltage ?

At no load, Vout is 24.05V and COMP pin voltage is 2.693V

Hi,

I have several suggestions to check.

First, Vds waveform does not show a typical flyback waveform. I suspect the snubber is not properly designed. To move on, usually find a TVS to substitute the R69 and R74. Since Nps = 24 x 13 = 312V, you need to find a TVS with clamping voltage about 340V to 360V to replace R69 and R74 

Second, your transformer magnetizing inductance seems very high. Can you provide your design sheet to show how this inductance is obtained.

Third, the feedback loop compensation seems not correct. Again, please show how these were obtained.

Fourth, Rcs  (R50 = 5.1-ohm) seems high, so please also provide how this value is obtained. 

480-600V_Bias.pdfBias_Transformer_15W.pdf

All values were obtained by using WEBENCH online software.  Schematic and transformer report from WEBENCH are attached.

Hi,

ok, let me have a look and reply to you. 

Hi,

Can you provide the numbers you filled in when you use the Webench so I can repeat what you did to see where can be changed?

Hi,

What is switching frequency range possible ok to your application?

Hi,

I tried to use webench to make your design but I did not find where to get this. Can you re-do your webench design and send the link so I can open your design? I am not able to input your design into the webench so need you to provide your design link for me to review.