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Registered Member #494
Joined: Thu Nov 09 2006, 02:42AM
Location: Udine, Italy
Posts: 31
Some time ago, looking at Richie Burnett IH pages, I've seen this:
and an idea started bouncing in my mind, what about buildina one of these things?
So I've started to design a simple driver, using a 4093 as oscillator, and a couple of UCC amps as gate drivers, the schematic can be found HERE
Today, during a raining day, I've decided to go down to the garage and start building it on a breadboard, due to the extreme simplicity of the little circuit. After some work, I've got out this: (300Kb Jpeg) the driver allows easy change of the capacitor and the resistor, to choose the right frequency at any time.
Then I've build a quick half bridge on a little aluminium bar using a pair of FDH44N50 from fairchild, 15ohm gate resistors and 15V protection zeners, plus a good 4,7µF 700V bypass cap for any current source need. Photo: near the bridge you can see the inductance matching choke, about 20µH
And finally, the work coil, wound from a thick 6mm copper tubing, 7 turns wound on a 2.3" former. in parallel with the coil is placed the resonance cap, rated at 0,45µF (30 x 0,015µF) 1500V:
As result, I've been able to heat up a steel M10 bolt at over 250°C in some seconds powering up the system at 65V, current drawn was about 1,2A. Tomorrow I'll change the fets with some proper IGBTs for higher working voltages (my scope says 170V PP at this imput, rectified mains would give me about 950V... I need igbts!)
Some more photos coming tomorrow, some probe shoots and maybe some red metal :D
coming soon: probe shots on all the circuit, and maybe some photos of red things!
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
The switching devices cant see more than the rail voltage (except for short spikes). You may be correct that there is resonant rise across the parallel LC, but there is also just as much voltage drop across the Lmatch. If this werent true, then you would charge up your DC supply to pi*input, and then it would be pi*pi*input, and so on (i hope you see my poorly worded logic here).
You may also need to be careful about the tuning. I cant recall, but it may be possible to get zero current switching out of the bridge, though i have a feeling that its tricky with a simple oscillator control. When i talked to Richie about making one of these heaters, he mentioned that i'd probably need a micro controller to do the feedback properly (to guarantee ZCS).
Registered Member #494
Joined: Thu Nov 09 2006, 02:42AM
Location: Udine, Italy
Posts: 31
Steve Ward wrote ...
The switching devices cant see more than the rail voltage (except for short spikes). You may be correct that there is resonant rise across the parallel LC, but there is also just as much voltage drop across the Lmatch. If this werent true, then you would charge up your DC supply to pi*input, and then it would be pi*pi*input, and so on (i hope you see my poorly worded logic here).
You may also need to be careful about the tuning. I cant recall, but it may be possible to get zero current switching out of the bridge, though i have a feeling that its tricky with a simple oscillator control. When i talked to Richie about making one of these heaters, he mentioned that i'd probably need a micro controller to do the feedback properly (to guarantee ZCS).
yea, I've done some probing on the fets and I have about 70 volts, right.
I was thinking to use a self tuning system for resonance, with maybe phase control using a feedback transformer (like drsstcs). By now, I'll see how far this will go!
Registered Member #76
Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458
wouldnt it be possible to get the feedback signal with the right resonant frequency like in a sstc or drsstc via a feedback transformer wound on a ferrite toroid placed on the lc circuit? i thought about this but havent had the time to try it out
Registered Member #494
Joined: Thu Nov 09 2006, 02:42AM
Location: Udine, Italy
Posts: 31
Reaching wrote ...
wouldnt it be possible to get the feedback signal with the right resonant frequency like in a sstc or drsstc via a feedback transformer wound on a ferrite toroid placed on the lc circuit? i thought about this but havent had the time to try it out
I was thinking about a rogowsky coil on the copper bar, and then some feedback.
tomorrow btw I'll try to power up the inverter at 170V, let's see if it goes over the 130W range of today!
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Ok, I might not be having enough sleep, but;
- If you get some 1200V IGBT's, and are planning to change the circuit in time: simplest way to get zero voltage switching I can think off could be famous royer/mazzili oscilator, directly. It has already driven SSTC's under same circumstances, and since it is practically a paralell LC you'l get high enough currents in your coils.
This would require a center-tapped coil, 1200V devices to run on mains, a big input choke and a separate supply for gates. Once you set it up, it would practically be plug-and-play, and I believe it could give some serious power with proper switching devices.
I know, this may be 'a waste on resources' with IGBT's, but if I ever get some more serious 1200V devices, I'l surely want to try it!
Registered Member #494
Joined: Thu Nov 09 2006, 02:42AM
Location: Udine, Italy
Posts: 31
Firkragg wrote ...
Ok, I might not be having enough sleep, but;
- If you get some 1200V IGBT's, and are planning to change the circuit in time: simplest way to get zero voltage switching I can think off could be famous royer/mazzili oscilator, directly. It has already driven SSTC's under same circumstances, and since it is practically a paralell LC you'l get high enough currents in your coils.
yea, maybe I'll put a center tap on the inductor soldering some wire directly on the coil (I know it'll be hard but I will get it :D) and then try a royer config. I have there some 1700V fast igbts (FGL60N170D) that should be really good for this job.
tomorrow btw I'll try to run the circuit off 170V dc (rectified 230 - 55+55 transformer), let see what happens.
a royer design like this with just 2.5µH of primary should be a bomb to play with, at 300V imput I've estimated an LC current of 125A RMS!
EDIT
done some new test today, I've got it working well at about 170V DC getting in (rectified 120AC + cap) with about 600W of power drained, and pretty good results:
first photo, the assembly of the whole system: driver, h-bridge, impedance matching inductor and the work LC system
second photo, the system running for some seconds on a M10 bolt, heating it to a bare red
after some more time... the bolt gets really hot, red-orange, let's say over 700C:
after that I've deciced to try acting on the tuning pot (the little trimmer on the board) but someting gone wrong, I was running the circuit at the maxium freq, supposely a gate driver blown out and now I have the two fets completely shorted... I suppose that the detuned system shutted into the fets some extra voltage over 500V and blown them.
due to hard tuning (when the bolt gets hot the system is detuned due to the curie point and magnetical proprieties) and fet blowing risk I've decided to build up asap a gooood ZVS / royer with the 1700V IGBTs, solder or hook in any way a wire at the 3,5rd turn of the inductor and run the system just in a zvs way, like something I've done some time ago with muuuuch less power.
Registered Member #494
Joined: Thu Nov 09 2006, 02:42AM
Location: Udine, Italy
Posts: 31
Some news about Royer circuit and direct induction heating use
I've done some test today with a royer built up with 1700V IGBTs, but it looked like to work not that way well for a powerful heater, I've got much better results with the hand-tuned 4093 + bridge LCLR driver. At 70V DC imput I was drawing just 2 Amps, heated well but not much like the same power level of the other system, and with more than 100-110V the current driven was getting really high, I've got over 8A with a 130µH choke, and strangely, just a lil more heat than before. So, from what I've seen, I'm not going to suggest the royer / zvs oscillator for big heaters, just maybe little ones, or at least not direct driven.
At now, I'm gonna rebuild the driver of the half bridge with a 4046 ant feedback, to get the right resonance frequency at any time (with the "core" and with the core at curie point) and for better switching (and happier fets!). I've seen Steve Conner's DWSSTC self tuning driver here --> , I'm going to use it with the little fixes he posted on his page, let's see what happens!
My main idea is to be able to use the indheat to melt small amounts of aluminium / low melting alloys, if will come out good I'll post everything there and maybe on a site with better detail.
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induction heating!
