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Induction heater theory question

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LAN_403

IamSmooth

Sat Sept 05 2009, 10:58PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

If so many amps are flowing through the work coil, how come the induced magnetic field in the object does not get it thrown out of the coil or crushed? Is it because of the high frequency, or the joules per pulse not high enough?

Also, for this schematic

what is the purpose for the diode 1N914?\

would one IGBT capable of handling 500A be used instead of the 4 75A IGBTs?

Sulaiman

Sat Sept 05 2009, 11:23PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

The 1N914's are to turn off the igbts as quickly as possible.
Maybe a UF400x would be better?

IamSmooth

Sat Sept 05 2009, 11:52PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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So, if the diode speeds up turning it off, doesn't the addition of the resistor slow turning it on? It seems like to gain with one you are losing with the other.

Steve Conner

Sun Sept 06 2009, 09:49AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
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Posts: 6706

It does try to throw the workpiece out of the coil. Aluminum workpieces will float in mid-air with enough power and the right coil configuration.

No, one IGBT will not do. Think about it.

The designer of the circuit wanted slow turn-on and fast turn-off to avoid shoot-through.

uzzors2k

Sun Sept 06 2009, 02:37PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
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I've got a little video of the effect in this thread.

IamSmooth

Sun Sept 06 2009, 03:48PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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The topology I reference uses a step down transformer. I have also see designs using an LC tank. Will both work well? The latter is more efficient?

What is the 1uf cap for in this design, or are these what make up the LC tank?

GeordieBoy

Sun Sept 06 2009, 04:57PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
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> The 1N914's are to turn off the igbts as quickly as possible.
> Maybe a UF400x would be better?

Yes, small-signal diodes are not suitable for carrying peak gate currents in the ampare range. Schottky diodes are the best choice because they have no Trr and minimal Vf.

The zener clamps should also go from gate to emitter on the IGBTs NOT across the secondaries of the GDTs! Otherwise the clamping circuit is undamped and likely to making ringing worse.

You can only use a single IGBT if you change the circuit into a single-ended RF source like a Class-C amplifier. For push-pull you need at least two active switches.

-Richie,

IamSmooth

Sun Sept 06 2009, 08:12PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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In this schematic

what is the purpose for the 1uf caps? Are these the tank capacitors that complete the LC circuit? If so, why are there two. It would see that placing one directly in series with the coil would be fine.

Is the 2500uf cap to reduce spikes on the switch?

uzzors2k

Sun Sept 06 2009, 09:08PM

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

That driver doesn't use resonance switching.

To quote myself from another post:

They form a return point for the half-bridge by voltage division. The voltage between the caps is near V/2, and the mosfets switch the primary between V and ground, which causes current to flow in opposite directions through the primary. The capacitance of the capacitors affects the impedance of the bridge, or how much current it will pass.

Basically they form a half-bridge. The large 2500ÂµF capacitor is for filtering the assumed rectified mains input. Usually smaller foil capacitors of ~1ÂµF are used to shunt any high frequency component on the DC bus, as large electrolytics have significant ESR. The half-bridge capacitors will preform that job here however.

I think you need to read about basic switching topologies. This poster will give a you a general idea for further google searches.

IamSmooth

Mon Sept 07 2009, 01:21AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Uzzors wrote ...

That driver doesn't use resonance switching.

Is the advantage of resonant switching that one can generate higher currents with less input current, and the advantage of one that does not use resonant switching is that it is simpler to implement?

I am trying to figure out if I were to build a small unit which topology I would pursue.

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