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Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Okay. The top trace at least proves that you can introduce plenty of deadtime into the drive waveforms. The flat tops and bottoms of the inverter output are where the MOSFETs are conducting, and the curved sections in between are where the free-wheeling load current is charging and discharging the MOSFETs output capacitance during the deadtime.
The deadtime in this picture seems to be a rather large percentage of the total switching period, and is probably a little excessive. You only need sufficent to prevent conduction overlap and horrendous ringing on the inverter output voltage. (Notice how the voltage waveform now has no overshoot or ringing.) Do you have snubber capacitors across the MOSFETs??? The reason I ask is that I would have expected the voltage to slew more quickly from rail to rail. If not, then I would guess the tests were done at reduced DC supply voltage or with a very light load current??? At full supply voltage and with plenty of amps flowing the voltage will slew more quickly between rails and you don't need so much deadtime.
As for the 2nd scope trace. I don't have a clue what is happening there! It looks like the conduction time of the MOSFETs is very small again compared with the total period and the deadtime is too large a % of the period.
As for the 3rd scope trace, make sure you have a 10uF tantalum cap and 100nF ceramic capacitor across each gate drive IC right at the pins of the chip. Also make sure that you keep the control electronics well away from the power bits that are carrying high current, and in particular away from the work-coil/capacitor tank circuit. That goes for scope leads too!
I wouldn't worry at all about tiny little wiggles on signal lines at the switching instants, because these can appear just because of the fast gate drive signals. It is big wiggles, overshoot and ringing on the power side you need to erradicate to keep the power semi's happy. It's worth noting that this control scheme you're using is very robust - It takes a whole lot of noise to knock it out of frequency lock. The neat thing about it is that during normal operation the two inputs to the phase comparator from the tank circuit and the inverter output are 90 degrees out of phase. So any noise that appears on one signal resulting from a transition in the other signal does not adversely affect the operation!
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Yeah, the dead time was a bit excessive and nothing I plan to keep. I just wanted to eliminate the shoot-through suspicion. As you guessed I'm running about 1/10th of full voltage and current. (Just one 1.8µF decoupling cap across the supply, no individual snubbing)
But I guess the ripple is nothing to worry about then. The voltage regulation did work so I'll just keep moving forward as planned. (After some vacation! See you guys in a few weeks)
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Why aren't air cored inductors used instead of ferrite cored inductors for the matching inductor? You wouldn't have to worry about saturation, or core heating. More wire is required for a given inductance increasing the resistive losses, but that's the only drawback I can see. What am I missing?
Registered Member #941
Joined: Sun Aug 05 2007, 10:09AM
Location: in a swedish junk pile
Posts: 497
Because a air core matching inductor cannot be put inside the chassi of the ih, cuz it radiated fields everywhere and can itself induction heat things as well, also it´ll be much larger than a cored one.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
That's basically it. A properly optimised design with gapped ferrite and litz wire will have the lowest total losses (when Cu and core losses are added together) and be more compact than an air cored coil. The leakage field of the gapped ferrite core set will also be more localised to the non-magnetic gap than a large air core inductor which will induce currents into everything around.
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Induction heater as an induction launcher (now with control circuit discussion!)
