I've been working on an induction heater over the last few months and have had varied success. I can drive about 600W through it but as I begin to push past this point it isn't long until one of the FETs explodes. Below is the circuit schematic I based my driver on and the physical setup itself.
One of the issues that seems to plague the circuit is that each arm of the H bridge appears to switch on prematurely; the voltage at either node A or B will rise to the level of the DC bus prior to the high side gate being driven high. I've included a couple of waveform captures to help illustrate this. The first image shows the input to the high side gate (yellow) versus the output of node A (red). The second shows the 'precharge' that occurs between node A and node B.
Currently the circuit is driven with a PWM waveform at 15.34KHz, 48% duty cycle. Q1 & Q4 are switched on simultaneously followed by a short dwell period to prevent shoot through and then Q2 & Q3 are driven simultaneously followed by another dwell period, repeatedly. The dwell period isn't apparent due to the 'precharge' that appears on either node. I'm trying to understand why this precharge is occurring and what I can do to resolve it. I suspect whatever may be causing it is creating other issues such as ringing etc.
I had thought of increasing the duty cycle of the low side drivers to 50% while retaining the original duty cycle of the high side drivers in the hope of draining any residual charge, but this requires four PWM inputs which I don't currently have available and I'm not sure this is the solution to the problem. I've also tried using a 1uF capacitor as a snubber between one of the nodes and ground, I've found it does partially correct the issue but realistically just delays the effect until that arm discharges itself. You can see the effect below. The yellow signal is the logic reference.
Here you can see the effect 'pushed' onto the end of the waveform.
Apologies for the direct image links, SE is quite restrictive. Can anyone help me identify what is causing this 'precharge' effect?