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400V out of 21V-30V, 200W

Hi,

I have to deliver 5V-400V out of 21V-30V, max. 200W, but load covers a very wide range from no load to 750mA (not at 400V). I also have to use standard parts, no custom transformers!!

After lot of investigation, discussion and simulation cycles I came up with a modular design. 4 identical moduls, each of them produce 1.5V to 100V, 50W. The outputs were connected in series to get the sum.

And I used a 4-phase switching schema to get lower ripple at the input.

Now I start working with the real thing, but:

Switching frequency is about 1.8Mhz instead on 150kHz?? In the first step I used only one module free running to test the basic functionality of a module.

The first module dies within minutes, but the second module I have produced let make me some measurements, I included togethet with the schematics. The Res pin told me, that the module enters some Kind of protection mode. Ok.

My questions: Any idea to get the switching frequency down to the value I calculated with? Yes, the RT is correct and the Connection is correct, so no Problem during PCB design or manufacturing (broken wire).

How can I find out, why the chip enters protection mode? But maybe this is a result of the wrong switching frequency.

Thanks a lot for helping.

With best regrads

Gerhard

TI Forum Infos 7 3 2014.zip