Mazilli ZVS Driver Modification Problems

[davekni]

When using ferrite cores for inductors (as opposed to transformers), they always need gaps to reduce effective permeability.  Otherwise the core is almost always saturated.  45kHz is a good frequency for PC40 material - looks like it's intended for roughly 25kHz-100kHz, although could be used a ways outside that range.

Don't worry about too-hot-to-touch.  PC40 is designed for 80-100C operation, with lowest loss at 90C.  If it's getting over 100C, then it's time to back off.  Most ferrites designed for power applications are designed to run hot (designed with minimum power loss at ~100C).

Do you have a scope?  Or, how are you measuring frequency?  If you have a scope, adjust your inductor gap until the inductor has half the voltage of the work coil with nothing inside the work coil.  Then your inductor is half of the work-coil's inductance.

You mentioned 900kHz!  That's impressive for IGBTs!  They are probably not too efficient at such high frequencies.

[Andrew321]

The core gap I really should have caught, that was half my battle getting the HV ZVS driver working in the first place lol. But that's really interesting about the ferriets working best at high temperatures. I had no idea they were optimized like that. I just thought that if its getting hot that means its wasting power and while a certain amount is acceptable/unavoidable, I thought the 100 degree range was just too much.
I don't have a scope, just a basic multimeter. I connect the leads across the capacitors just after the one closest to the coil. Seems to give me an accurate reading, but I don't remember the range its capable of so maybe the 900kHz was not quite spot on. I only had it running at the high a frequency for maybe 5 seconds or so. Just long enough to get a measurement.

I have given up on the HV ZVS induction heater for now. I almost had it prefect on my last set of parts. I didn't like the heat wasted in the inductor core so for the series inductor I tried the air core. It was 200 strand litz wire, 6 turns, 3 inches dimeter. I discovered I could plop a ferrite in and out of the coil (very small in comparison to the coil itself) to control the frequency and current some what. Nothing got hot. I was ready to actually try my first melt when I realized that the crucible wasn't sitting completely in the coil and the top 1/4 of it was sticking out the top of the work coil. I didn't think it mattered to lift the work coil so that it matched the crucible but when I made that one change, it fried the whole thing. I think in doing that I increased the coupling factor too much. I gave up on it for now and I set up a low voltage one using MOSFETS and my big step down transformer.
Eventually I'll come back to this induction heater. When I do I'll use properly rated MOSFETS (IGBT's get hot at 50-90khz after 15 minutes even with cooling fans) and instead of a separate inductor in series with the work coil I'm going to make the work coil longer than the crucible to reduce the coupling factor.

Thanks for all your help Dave! Don't think I could have done it with out you. When I get the high voltage stuff up and running again I'll make better videos and share.

[davekni]

Adding extra turns is essentially the same as adding a series inductor.  It's simpler, but a bit more tricky to insure staying within limits for ZVS oscillation.  Sorry to hear of the frying event.  Encouraging that you have a likely cause.

If you can afford an inexpensive scope, that makes a dramatic difference in debugging and understanding circuit behavior.  Something like Hantek DSO5102P for $250 new on EBay from several stores.  I have the slightly more expensive version that includes a signal generator.  There are USB scopes that use a host computer for display and control, some a bit less expensive.  There are also old used scopes for yet less.

Keep learning and having fun!