WEBENCH® Tools/UCC28740: Flyback converter doesn't work

Andrea,

First of all, I would recommend checking out this getting started debug guide for TI PSR controllers/regulators

www.ti.com/.../slua783.pdf

That should be helpful.

From an initial look at your sch, I do see a small error, the Rcs resistor R1 is 330-ohm, but it should be 300 m-ohm per the Webench design.


If you are seeing very high current flow with low input voltage, it would suggest a possible short on your board, or a reversed diode (maybe D2?).

I would recommend checking the board carefully for shorts, reversed components, etc.


I will review your design files in more detail later and try to get back to you with any further inputs in a couple of days.

Thanks,
Bernard

Dear Bernard,
Thank you for your reply.
Yes, I found that the resistor is wrong and the MOSFET has source and drain inverted.
I change components and now the short is disappeared.
Now I have the same problem described in the attachement you left to me: the flyback goes to VDD UVLO protection.
I increased the capacitor between VDD and GND, but nothing happens, only the times to protection increase.
There is no load to the output.

do you have any suggestion? I'd like to leave the same transformer.

best regards,
Andrea

Andrea,

What input voltage are you using for testing, is it still 60 V? If so this is too low, this should be expalined in the debug guide.

What is your transformer spec (mag inductance, Ae, Np, Ns, Naux)?

If you used the Webench values, the ratio of Np/Na = 1/0.455 = 2.2.

For the upper VS resistor = 255 k, then the IC will only start when input voltage peak is > 225 uA * 255 k * 2.2 = 126 V, or ~ 90 V AC.


Thanks,
Bernard

Dear Bernard,
You're right.
Now I connect the Flyback directly to the UCC28070EVM (I have this board at the office) after the rectifier bridge.
I turn on the board with the 230Vac and I measure the input voltage on the flyback with a tester. I see 330Vdc, but the flyback doesn't start. It seems that the UCC28740 doesn't start to switch. The output voltage fluctuate between 900mV and 1.5V; I measured it with a differential probe.

Best regards,

Andrea,

I would recommend following the steps one by one in the section 2 of the debug guide (pages 3-8) to see which of these is the cause of the failed startup.

By observing VDD, DRV, Vout and Aux winding, you should be able to figure out the cause.

Are you testing with load connected at the output before/during startup? Maybe the load is too high and is preventing the output caps from charging due to the Iocc limit?

Can you check the waveforms and post to the forum?

When VDD reaches the 21-V start level, capture the waveforms of the first 3 switching cycles, looking at VDD, DRV, CS & AUX winding.

Does the IC stop switching after the first 3 cycles, or does it continue to switch until VDD falls to the UV off level? If switching continues, capture a plot over a longer time scale until VDD falls to UV, and show VDD, DRV, AUX winding and Vout.


Thanks,
Bernard