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Impedance matching woes

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Bjørn

Wed May 02 2007, 12:48AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Many digital cameras interpret IR as violet. If you add a high quality IR blocking filter in front of your camera it will register the correct orange colour.

uzzors2k

Sun Sept 23 2007, 03:01PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
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I'm having impedance matching woes too. This weekend I thought I would slap together an induction heater. It's a simple TL494 based mains-powered half-bridge ATM. I've tried both LCLR and matching impedance matching. In both cases I'm using a parallel resonant tank circuit with a 1.75ÂµH work coil and 940nF of tank capacitance.

In LCLR mode the work piece remains cool while the matching inductor is induction heated. Smaller inductors heat the quickest. The wire is thick enough, it's the actual core that is heating. I used a 680nF DC blocking cap. Why is it heating the inductor and not the work piece?

When I use a matching transformer, the secondary and wires to the tank circuit heat excessively. It seems to me that they have become part of the tank circuit, aka another inductor in parallel with the work coil. I've only taken the matching transformer up to a 15:1 ratio so far, at which the power is 100W. The work piece heats normally. Do I need to go series resonance when using a matching transformer, or is there another way to stop the secondary from melting? I would prefer that the matching transformer didn't heat at all.

Marko

Sun Sept 23 2007, 04:20PM

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

You just need to tune the right frequency and it should be fine, no need for any kind of transformer.

You should make your matching inductor air core or powdered iron; it doesn't need to be really big after all, like 50-100uH. You regulate the power by the matching inductor.

Experimentonomen

Sun Sept 23 2007, 04:44PM

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

A matching inductior should be ferrite, NOT iron powder. Iron powder is meant for dc filtering chokes.

for matching inductor, i reckon a large flyback core should work nicely, that is any big core from a flyback style smps.
Thoi have never managed to get LCLR to work, i can only make it work with impedance matching transformer.

Marko

Sun Sept 23 2007, 05:36PM

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

Actually all good power converters use powdered iron as their energy storage inductor. In PC power supplies, the powdered iron choke is main buck inductor, not ripple filter.

They have superior flux handling than ferrite (about 1T vs about 0.3). Their low permeability (like a sort of distributed airgap) makes them uselles for transformers but great for inductor applications.

Ferrite flyback core would do exactly opposite from what we want, it would be able to store very little energy, saturate and cook very quickly. We would need a gapped or rod core, but still it wouldn't match powdered iron in max B.

Experimentonomen

Sun Sept 23 2007, 08:50PM

Registered Member #941 Joined: Sun Aug 05 2007, 10:09AM
Location: in a swedish junk pile
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You claim that if i make LCLR with lets say a red/black or yellow/white toroid, the circuit will work ?

When i tried LCLR i used either ETD45 or something (overheated and smelled bad) and air coils, in both cases there was no work piece heating, only Lmatch heating.

Steve Conner

Sun Sept 23 2007, 09:48PM

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

According to Richie, iron powder is too lossy. It's only for DC with relatively small ripple. The matching inductor current is pure HF and would cook an iron powder toroid in seconds.

Gapped ferrite or air core is where it's at.

Experimentonomen

Sun Sept 23 2007, 09:54PM

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

I said gapped ferrite such in flyback core or any gapped ferrite core. But marko sez diff as you see.

Who is right ?

Sulaiman

Sun Sept 23 2007, 10:49PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Due to my new interest if radio I've been doing a little research on toroids for rf.

Have a look at the bottom two documents here

and here is some other info.

and the FC-L file under 'High frequency power transformers' here

It seems to me that with the correct choice of material good "Q" inductors and low-loss transformers are viable.
After all, how are rf power amplifiers made?

Steve Conner

Sun Sept 23 2007, 10:57PM

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

There are two different kinds of iron powder: hydrogen reduced, which is the stuff in your ATX power supply, and iron carbonyl

which is the stuff used in RF power amps. Carbonyl has much lower permeability than the low frequency stuff, sometimes as little as 3.

I've never compared iron carbonyl to gapped ferrite at induction heating frequencies, but I expect gapped ferrite would win, if only because the larger carbonyl cores are expensive. I also think Marko is wrong.

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