# H-Bridge Class D Amplifier with transformer

I'm working on a H bridge amplifier capable to deliver 200 W into a 50 ohm load. Supply is limited to 24/28 V, unipolar. So I need a transformer in the output. You don't see a filter because I only want to simulate with squarewaves not with a modulated PWM, but the final design should amplify a 100 kHz sinewave PWM modulated. However with this design the current in the primary side increases without limit and is not tolerable. Maybe there is a DC component on the primary but I know that with H bridge this effect should be negligible. So, is there a way to fix it? Also with a bipolar supply the problem persist.

• What is V4 setting? Why not drive 50 ohms direct, what's the transformer for? Notice that the transformer must couple the lowest frequency content of the PWM signal, not just the carrier; this makes transformer coupling prohibitive for most purposes. If all you're doing is 100kHz, that's still plenty feasible of course, but why PWM at presumably some massively higher frequency (MHz?) burning excess switching loss instead of direct driving a resonant circuit? (Is this for a resonant output after all, by any chance?) Commented Oct 18, 2023 at 21:06
• V4 acually is only a square wave with 500 kHz frequency, just to understand how works the circuit. The transformer is the solution to get 200W into 50 ohm with 24V supply, I can't drive it directly, because I also need a matching impedance with the load that is capacitive. Anyway this is not the concern of the question. The transformer should be there, but the current should no increase so. THe circuit is balanced, I don't understand.
– mtx4
Commented Oct 19, 2023 at 7:06
• Well, I count ~17 and ~20 pixels for the high and low portions, so balance is a lie. Not knowing V4's setting I can only guess at the cause. Commented Oct 19, 2023 at 8:15
• V4 is a square wave, 500 kHz, 0-5V peak. There is a drift in the negative peak of the current of 3 A in about 100 us. So after few second this circuit will burn. I often see so many converters with H bridge, closed in a transformer and should be square waves too, so I don't understand where is the fault.
– mtx4
Commented Oct 19, 2023 at 16:00