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Registered Member #120
Joined: Fri Feb 10 2006, 07:07AM
Location: Westchester New York
Posts: 83
Hi all,
I'm building a simple LC induction heater with a half bridge.
At the moment I have a half bridge outputting to a 1uf dc blocking capacitor in series with a parallel LC circuit that is connected to ground. My tank capacitor is 0.4uF and my inductor is 1.5uH about. I can't seem to tune my circuit and I'm unable to get a clean sinewave across the inductor and tank capacitor. Could my dc blocking capacitor be affecting the circuit? Also is it laid out properly? res freq should be around 205k I checked 150k to 280k and found nothing.
Oh one last thing I basically have this circuit without the matching inductor I don't see why I can't build a less efficient induction heater without a matching inductor , but why I can't get a sinewave is baffling me.
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
The matching inductor is a must in this circuit. It prevents the mosfets from having to switch hard on, meaning switching while there is heavy currents or voltage across them. Generally, you want to stay within ZVS or ZCS (zero voltage/zero current switching), and the matching inductor helps achieve this. Without it, you'll be frying FETS all the time. Plus, as a result of zero anything switching, your mosfets will be a LOT cooler.
Registered Member #120
Joined: Fri Feb 10 2006, 07:07AM
Location: Westchester New York
Posts: 83
Inducktion wrote ...
The matching inductor is a must in this circuit. It prevents the mosfets from having to switch hard on, meaning switching while there is heavy currents or voltage across them. Generally, you want to stay within ZVS or ZCS (zero voltage/zero current switching), and the matching inductor helps achieve this. Without it, you'll be frying FETS all the time. Plus, as a result of zero anything switching, your mosfets will be a LOT cooler.
'
Switching hard. Yes that sounds about right. My bridge does have a lot of ringing with only 17Vdc in and my MOSFETs get very hot. Okay i know what I must do now.
I can't seem to be able to model ZVS however, I'm curious where I could find this information. Not a single text book has this information.
How do I size the matching inductor? should the frequency of Lm and Cm equal the frequency of Lr and Cr?
Oh I have a flyback core. One of the Oval shaped ones. I should be able to use this right? I just wrap the inductor coiling on one size. I wonder what the permittivity is though. I good guessing numbers? I guess I could find it, but I don't have a resistance decade box.
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
Inducktion wrote ...
The matching inductor is a must in this circuit. It prevents the mosfets from having to switch hard on, meaning switching while there is heavy currents or voltage across them.
This is not right. The tank circuit is what prevents hard switching. The matching inductor prevents the resonant capacitor from shorting out the harmonics of the switching waveform and does impedance matching.
Registered Member #120
Joined: Fri Feb 10 2006, 07:07AM
Location: Westchester New York
Posts: 83
Anders M. wrote ...
Inducktion wrote ...
The matching inductor is a must in this circuit. It prevents the mosfets from having to switch hard on, meaning switching while there is heavy currents or voltage across them.
This is not right. The tank circuit is what prevents hard switching. The matching inductor prevents the resonant capacitor from shorting out the harmonics of the switching waveform and does impedance matching.
Okay so mine is just miss tuned. I was trying to tune to the nearest 1k, and I found it impossible to get a sinewave across the output. without a matching inductor I should be able to tune the circuit then. Do you think the DC blocking capacitor is messing things up? unless I have damaged high speed IGBTs?
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
The problem is the lack of a matching inductor. Without it, your bridge tries to make the tank voltage a square wave. If you add a matching inductor, you should get something more like a sine wave.
Registered Member #120
Joined: Fri Feb 10 2006, 07:07AM
Location: Westchester New York
Posts: 83
Anders M. wrote ...
The problem is the lack of a matching inductor. Without it, your bridge tries to make the tank voltage a square wave. If you add a matching inductor, you should get something more like a sine wave.
Alright. I'm going to get a 50uH 7.6A inductor from mouser since I have to get some addition equipment. It's a little expensive, but it seems to be the only decent one I can find that's not a choke.
I read all about induction heaters, but I can't never find a mathematical explanation on the matching network. There's also little information on the LCLR topology. I heard that the matching network is Q based? I have the equation for the entire circuit, both in differential form and in phasor form. But I am still confused on the interaction between the matching network and work network. Is it a series resonant relationship with the parallel resonant?
Registered Member #3610
Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506
That's probably a bit small as far as current capacity goes. Why don't you wind one yourself? Seems like I saw someone using a spool of wire as an air core inductor for something like this. You could also make one with a salvaged ferrite core. Choke is just another name for inductor.
Registered Member #120
Joined: Fri Feb 10 2006, 07:07AM
Location: Westchester New York
Posts: 83
James wrote ...
That's probably a bit small as far as current capacity goes. Why don't you wind one yourself? Seems like I saw someone using a spool of wire as an air core inductor for something like this. You could also make one with a salvaged ferrite core. Choke is just another name for inductor.
Oh my capacitance is small because I'm not using a matching network at the moment. Right now its resonating at 205kHz.
Once I add a matching inductor I'll increase the capacitance. Not sure how much though. I guess for now I'll copy Rich's design in terms of values. I could make a air core inductor, but I don't have enough cabling. I prolly need like 60 ft or so of 16awg.
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LC Parallel induction heater question
