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Induction heater as an induction launcher (now with control circuit discussion!)

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GeordieBoy

Sat Jun 14 2008, 04:46PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Agreed. They are supposed to look like two squarewaves at 90 degrees to eachother at f going into the XOR phase comparator. The output of the phase comparator should be a squarewave with 50:50 duty at 2f. And the output of the loop filter should be half the supply voltage plus a small residual triangular ripple at 2f.

uzzors2k

Sun Jun 15 2008, 06:45PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Alright! Prefect and automatic 90 degree phase lockage! After improving the input waveforms I found it acted strangely. When sweeping the frequency down, the XOR output would start at around V/2 and rise to V way before resonance. It would stay at V until I got close to fres, at which point it would decrease to V/2 at fres, and when dropping the frequency further below fres the output dropped to around V/4 or so. Which is exactly backwards of what I need. Hence the additional 180 degree phase shift I planted in. Worked like a charm.

Scoped Inverter output VS Tank voltage again. There is ringing at the inverter squarewave's edges regardless of being in tune or not. It could be caused by numerous things I don't even want to think about. Should I worry about it?

1213555093 95 FT46031 Perfect Autotuned Phase Lock

I'm quite satisfied with this circuit now. All that remains is testing the the voltage and current limiting circuits.

GeordieBoy

Sun Jun 15 2008, 07:09PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Waveforms look good.

It seems strange to add a common-emitter amplifier to invert the phase when you could have just swapped the inputs over to one of the LM393 comparators?

The HF ringing is probably caused by cross-conduction of the MOSFETs because there should be relatively little load current flowing at the switching instants. How much deadtime do you have in the gate-drive signals?

uzzors2k

Sun Jun 15 2008, 08:23PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

GeordieBoy wrote ...

It seems strange to add a common-emitter amplifier to invert the phase when you could have just swapped the inputs over to one of the LM393 comparators?

I tried that first but for some reason the diode clipper behaved strangely, giving a high frequency pulsed output. I narrowed it down to the inverting input being the cause of the problem. Non-ideal components are a pain.

GeordieBoy wrote ...

How much deadtime do you have in the gate-drive signals?

None. The only simple way I can think of to introduce some deadtime is increasing the GDT damping resistance. I'll watch the waveform and if problems occur at higher powers I'll know what may be the cause. I thought GDTs made for shoot-through proof designs?

Oh, and the ringing has become less pronounced since last time I checked. Being in the lash-up phase with long, curly ground leads everywhere makes even the best waveforms look messy.

GeordieBoy

Sun Jun 15 2008, 11:01PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

In a bridge leg you always want devices to turn-off quicker than they turn-on. Otherwise you get cross-conduction and a shoot-through current flows down the bridge leg at the switching instant. It is this intense but brief current spike that shock excites stray inductance in the supply wiring loop into HF ringing. Once this takes place, that HF ringing will appear on practically every signal in your circuit that you choose to look at with your scope.

You ideally want a small dead-time of 200ns or so where both devices are guaranteed to be off. You can introduce this into the gate-drive signals using RCD circuits before the gate driver ICs. Or using RD slowdown circuits at the MOSFET's gates to slow down turn-on should also be effective.

When operating correctly an LCLR induction heater's current from the inverter is either in phase or very slightly lagging the inverter output voltage. This means that the current at the switching instants is small and slightly inductive. This is the ideal situation for a MOSFET bridge. Under these conditions the output voltage waveform should be relatively free of overshoot and ringing. The rise and fall times can be quite leisurely and the waveform can have noticeably sloping sides when viewed on a scope.

-Richie,

Marko

Mon Jun 16 2008, 02:00PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I don't think shoot through is a possible problem, as so many people have built working SSTC's for power levels in kW range without really having any means of providing deadtime, even without the reverse diodes on gate drive circuit which are sometimes used.

mosfets are apparently just too damn fast.

I myself used pre-programmed deadtime but never really saw a benefit. I never observed anything like shoot-through and devices would be cool and ZVS even then (which implies there was still 'enough' deadtime happening).

uzzors2k

Wed Jun 18 2008, 04:22PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

While working on tank voltage feedback I've become increasingly aware of the ringing, which like Richie said is on every signal I scope. Even the 12V line! Although I can regulate the tank voltage it seems to be oscillating, ie the scope trace becomes fuzzy so I can only estimate what the tank voltage is. Right now I'm not sure if it's the ringing messing with the feedback or if it's just my control loop.

So I tried to add some dead time to the gate drive signals with a RD circuit at each gate. I think it worked, but the ringing didn't go away.

Inverter output

1213805931 95 FT46031 Inverteroutput Noload

Inverter output and tank voltage

1213805931 95 FT46031 Deadtime Tankcircuit Inverter

Tank Voltage and 12V line. 12V is AC coupled and at 0.5V div.

So does anything come to mind? Or is it even worth worrying about? It stays locked in phase well, so I assume the ripple on the supply line isn't affecting the 4046 in anyway.

Experimentonomen

Wed Jun 18 2008, 06:02PM

Registered Member #941 Joined: Sun Aug 05 2007, 10:09AM
Location: in a swedish junk pile
Posts: 497

Shorten the ground wire on your scope probe!!!

Experimentonomen

Wed Jun 18 2008, 06:05PM

Registered Member #941 Joined: Sun Aug 05 2007, 10:09AM
Location: in a swedish junk pile
Posts: 497

use a porcelean/steel/graphite crucible and a propane/mapp gas torch, as simple as that.

uzzors2k

Wed Jun 18 2008, 06:14PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

That halved the amplitude, but the ripple is still there. I think your other post was meant for a different thread.

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