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Registered Member #538
Joined: Sun Feb 18 2007, 08:33PM
Location: Finland
Posts: 181
I'm building a a LCLR induction heater with feeback taken from a CT and fed to a comparator (you know the very basic SSTC circuit). The problem with this circuit will be that when the workpiece heats up over the curie point (or the workpiece is too small to start with) the tank circuits impedance drops dramatically causing very high current flow and most likely burning my FETs at higher power level. I'd need to design a circuit that would prevent this. I have been thinking about putting a CT before the bridge rectifier to measure input current and start cutting the gate drive when the current exceeds a set limit.
A PLL would be perfect for this application and I have tried it but it has always been working a tad odd for me (using Conner's "basic" PLL circuit). It kinda locks but on a very narrow frequency range and my FETs still heat up a bit (I think theres some phase shift) so its not working well enough.
Any suggestions ideas regarding either way of doing it?
Heres a pic of the watercooled work coil + MMC cap arrangement: The T-connecters stick to a magnet (couldnt find any brass ones) so I dont think they're good for higher powers but the current workcoil is a tad too small anyway :)
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
My basic PLL circuit did work "a tad odd" when I tried it with an induction heater. That was one of the reasons I redesigned it. I was also thinking of induction heaters when I added the current limiter to the new version.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
PS. this part of post was removed for common good since it contained unaffirmed speculations.
I used a small SCR as overcurrent protection in a halfbridge controlled by SG3525. I used a current transformer and burden resistor to sample the current, and a diode in series with gate. When voltage on burden gets higher than diode and gate forward drop the SCR triggers, shuts down the SG3525, and lights up a signal led.
I think you could do something similar if overcurrent is killing your mosfets.
Surely, if you need accuracy go for a comparator instead of SCR.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
As far as i know, basic feedback and even using a PLL will not work properly for an LCLR heater. When i discussed the control system for this design with Richie awhile back, he gave a thumbs down on the PLL (after i already tried it and asked if he had any thoughts as to why it didnt work properly) and said the best way is using a microcontroller for more advanced control algorithms.
As i recall, my PLL controller worked "OK" but could never get true ZCS from the H-bridge.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
As far as i know, basic feedback and even using a PLL will not work properly for an LCLR heater. When i discussed the control system for this design with Richie awhile back, he gave a thumbs down on the PLL (after i already tried it and asked if he had any thoughts as to why it didnt work properly) and said the best way is using a microcontroller for more advanced control algorithms.
Hmmm... can anyone explain what happens there? It really makes no ligical sense why direct current-transformer feedback wouldn't work, at least as it does with SSTC's.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
The problem is the ~90 degree phase shift in tank circuit current vs inverter output current.
The CT *must* be placed in the tank circuit since that is the resonance we want to excite. You cant put the CT on the output of the inverter, because then you would resonate the Lmatch with the DC blocking C, and the tank LC which results in the wrong driving frequency for maximum energy storage in the tank circuit.
I might offer more explanations if i didnt do this experiment over a year ago, my memory of the behavior of this thing is very incomplete. But as i said, the PLL works "OK" but still isnt perfect.
Registered Member #538
Joined: Sun Feb 18 2007, 08:33PM
Location: Finland
Posts: 181
What is the reason why basic feedback wont work properly?
Working pretty well now, driven with the basic feedback circuit but I have not exceeded the curie temperature yet so not 100% sure what will happen there.
Whole setup: (not using the isolation tranny anymore). Bridge stays cool with no cooling.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
It looks really ontroversal to me..
You cant put the CT on the output of the inverter, because then you would resonate the Lmatch with the DC blocking C, and the tank LC which results in the wrong driving frequency for maximum energy storage in the tank circuit.
All the time I tought the 'right' driving frequency would be just that, the one at wich you would set a VCO anyway; it's a necessary evil when one wants to drive the circuit with squarewave..?
That frequency would be tight mixture of matching network and main LC's resonant frequencies and that's what I would expect feedback to lock onto. And I tought it to be only way to acheive soft switching in any case.
The problem is the ~90 degree phase shift in tank circuit current vs inverter output current.
The CT *must* be placed in the tank circuit since that is the resonance we want to excite.
But, 90 degree phase shift would happen even if you driven the circuit with a VCO at f0 of 'work LC'? For that reason I didn't see what would be different with feedback. My understanding of resonant converters is obivously poor. I don't want to speculate anymore about these things
Dago: Are you using feedback from work LC or bridge output? How does the circuit behave without load?
I think it would be best for you to ditch the feedback scheme and use SG3525 for simplicity's sake.
Registered Member #538
Joined: Sun Feb 18 2007, 08:33PM
Location: Finland
Posts: 181
Firkragg wrote ...
Dago: Are you using feedback from work LC or bridge output? How does the circuit behave without load?
I think it would be best for you to ditch the feedback scheme and use SG3525 for simplicity's sake.
My feedback is taken from the LC circuit (you can see the CT in this picture: ).
I have not tested the behaviour without a workpiece but I'll try that. The basic feedback circuit seems to behave perfectly until the curiepoint at which it starts to... do something. The workpiece heats past 660C atleast (aluminum melts on the workpiece) and when it gets hot enough it seems to stop heating more and the waveform on my scope starts to jump around (looks like the phase of the drive signal starts to jump 180 degrees or something) and the whole circuit starts to make some noise (something like a 500Hz buzz or something). I'm not 100% sure but I think the bridge starts to heat up a bit faster at this point. Any ideas/solution?
I'd like to try to refrain from using a non self-resonant topology because of the lower switching losses and I'd like the heater to work ok without fiddling some pot all the time :)
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Self-resonant LCLR induction heater current limiter
