Push-Pull question

[roomi]

I want to make a sine wave inverter of capacity 500W.
I have decided to use push pull topology for the 12V-320V boost converter and wanted to use sg3525 for this purpose. the feedback will be from the 320V output to keep the voltage constant.

Is push-pull appropriate for this purpose ?

I looked at the app note of microchip for sine wave inverter. they used a second stage chopper after dc-dc push-pull stage. why they did that ?

[dtproff]

You'll need to list the operating frequency.

If you are wanting something in the 50-100KHz range I would probably use a series/parallel (or even a LLC) resonant full bridge. And before you ask, I will admit to being biased against push pull topologies and I really like resonant converters. At least this is what I would use if I wanted a SIN wave at 70KHz and no intermediate stage boost votlage.

Please list the Microchip spec to which you refer and we'll take a look at answering the question as to why there are two stages. But to hazard a guess it is because of the large discrepancy between the 12V and the 320Vac voltage you want. Usually you would boost to 320V*sqrt(2) and then use a HB LLC to convertert to a sin wave from there.

I hope that helps.

Tony

[Eva]

At these current levels (over 40A) and frequencies (below 100Khz), sine like current waveforms due to fully resonant operation are not a good idea. Switching losses are already much smaller than conduction losses.

The traditional push-pull becomes ZVS by adding enough dead time.

The second switching stage makes it easier to produce the sine or quasi-sine wave. It's required anyway in order to produce both halves of the waveform, since the rectified output from the transformer can only be either positive or negative.

[roomi]

I was mentioning abt the microchip app note "PICREF-1"(Uninterruptible Power Supply Reference Design). in this they used 2 stage dc-dc.

Infact I have already made a sg3525 based push-pull 12-330V dc-dc converter. But waveshape across the two end of the tranfrmr is not like as it should be. and sometimes the fet blew up during load test (i am using irf3205). and even at 15A drain current the temp. of the FETs are really high as it shoud't be.

[roomi]

The switching freq I am using is 40Khz

[dtproff]

Roomi;

Can you pst what you have (schematic and transformer construction)? Maybe we can help with what you already have.

Tony