# H bridge inverter

I am trying out an H-Bridge inverter using GaN FETs at 140 kHz. The schematic is as follows. (In the experimental prototype, I used TI's LMG5200 and the load is connected via a full-bridge rectifier.

The switching waveforms are

The circuit works fine when it is not loaded. The output of the inverter:

But when I load the inverter with the resonance LC circuit and the receiver, the output waveform becomes like this. The output voltage is not stable.

even in the LTSpice simulation, I can observe a similar waveform:

But if I simulate at a higher frequency (by changing the value of the resonance capacitor), the output looks stable. See the below waveform at 640 KHz

What am I missing here?

• Why do you have output cap? Also why leg A and B are not 180 phased? – jalaffo Jul 6 '19 at 8:36

## 1 Answer

You are asking for trouble; the drive frequency is 140 kHz and your primary series resonant tuning is precisely 140 kHz therefore, the L and C act as a short circuit at the switching frequency. You need to run the primary circuit either from an output stage designed to handle the series resonance or use a different approach - maybe add a current limit resistor in series with C4(C2).

• thanks, In fact, it will not be fully short-circuited. There will be a reflected impedance $(\omega M)^2/R_L$ in series with the ESR of the inductor. But at 140 kHz, it is extremely small. I understand that at a higher frequency, the reflected impedance will be higher and it makes the current lower. I may have to use larger L1 and L2 (so as the M) so that I can limit the current. The use of a series resistor is not an option due to high losses. What do you think? – Pojj Jul 3 '19 at 15:01
• I have done a similar job. I filtered the square wave to a sine and parallel tuned the primary. I don't know if this is possible for you. I was operating at 600 kHz FYI. – Andy aka Jul 3 '19 at 15:45