# Why is the current in the resonant load in my H-bridge only sinusoidal for low current amplitudes?

I am attempting to drive high-current (~1000 amps) at high frequency (10~30 kHz) through a coil. I am not an electrical engineer but a student working in a small lab where we have to build everything we need ourselves (which is pretty fun), so I may not understand some basic things that might be obvious to the EE community. Prior to several months ago, I had only worked with voltages and resistors (in year one of undergrad).

My attempt: attach a capacitor in series with the coil so that it would have a resonant frequency and then drive that load with an H-bridge, supplied by a large capacitor bank. For testing, I am currently using two 400amp and two 600amp IGBTs that we already had for the switches, but will eventually upgrade to 1200amp IGBTs if I get everything working properly. These IGBTs are controlled by gate drivers that are optically isolated from their inputs coming from an FPGA. I have configured the FPGA to create 4 square waves, 2 of which are in-phase with each other and that are out of phase with the other two. This will be a pulsed system, and a trigger signal to the FPGA controls when the system fires.

The H-bridge is basically as below. In reality, there are currently two inductors, the main coil and a second to adjust (with the capacitor) the resonant frequency. The signals from the gate drivers, which are floating, are applied across each IGBTs gate and emitter. I have not included my control circuits to avoid unnecessary clutter; they work exactly as I intended.

Using Rogowski coils I measure the current in the two branches of the H-bridge, through the coil, and I also check the signal being sent to one gate for reference. If I charge the capacitor bank to a low voltage the current waveform looks perfect. It creates a nice sinusoidal waveform just like I want but the current is low .

However, when I increase the voltage of the capacitor bank source and drive more current (> 5amps) the current waveform becomes worse and worse.

What is the reason I am not getting a sinusoidal current through my coil at max currents greater than a few amps? I need an amplitude of 1000amps, not 2! Am I misunderstanding something fundamental or doing something silly?

I realize this is a long post. Thank you for your time in reading and considering it!

• Are your inductors air core? – WhatRoughBeast Oct 28 '16 at 23:22
• You can see the resonant frequency is increasing with the higher current : in a half cycle of the gate waveform you're seeing nearly 3/4 cycle of the resonant waveform (a short half cycle and a negative going, i.e. the start of the opposite half cycle. This suggests the L or the C is decreasing : decreasing C is unlikely, but L will decrease if the inductor core saturates. At which point we need to know almost everything there is to know about the inductor : value, construction, core dimensions, core material etc. But mainly, what saturation current did you design it for? – Brian Drummond Oct 28 '16 at 23:44
• My instinctual thought was to wonder if an air core inductor was being used. My quantitative thought was to marvel at the sheer level of $\frac{\textrm{d}I}{\textrm{d}t}$ here. – jonk Oct 29 '16 at 11:21