Hi @myil

I have exactly the same problem right now. Without a load, I am reading 400 V DC as I expected. But, as soon as I include the load, which suppose draw 2.1 A under 400 V, so, approximately 842 W, my output voltage goes to 140 Volt and the load takes 0.6 A.

This same problem happens to me in my push-pull and full-bridge configuration. Exactly the same problem.

As I read your solution about power supply cable, I have measured my cables (almost 1 meter) resistance which is 0.2 ohm. not as much as yours. So, do you have any another solution you may suggest me?

Mod: link to old thread.
https://forum.allaboutcircuits.com/threads/push-pull-converter-voltage-drop.172756/post-1550734

 

It "sounds like" You need a larger Transformer-Core, and heavier-Windings on that Core.

What is the "VA" rating of your Transformer ?

If the VA-rating of your Transformer is adequate, and not being exceeded by any significant amount,
then the limiting-factors will be the amount of Current your Power-Supply can deliver,
or the amount of Power that your Output-Transistors can supply without excessive Voltage-Drop.

Other factors could include an inadequate Gate-Driver-Chip, and it's Power-Supply, and Bypassing Capacitors.

A 0.2-Ohm Resistance on your Power-Supply-Wiring is totally unacceptable,
and will cause a large Voltage-Drop.
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LQC has it right!!!
Although the inverter supply voltage is not mentioned, what must always be considered is that power into an inverter will always be at least as much as power out. So if the output was 842 watts, the input must be at least 842 watts, if the efficiency is 100% If the inverter is powered from 12 volts, then the required 12 volt current will be 842/12 amps =70.19 amps. That will be a challenge.
And the inverter in the linked thread was 12 volt powered. So if this one also is 12 volt powered, you need to be able to supply a lot of input amps.