Hi guys,
First time coming here, though I have been in Kaizerpowerelectronics blog before.
Since it will be pointless to make another thread just for the same induction heater, I'm borrowing kamelryttarn's thread to post about my machine. If you don't mind, that is (I'll do another thread if you do mind).
First of all I have to thanks Mads for his advice on using multiple pairs of MOSFETS to deal with high-power circuit. I have made a prototype, and it worked out well:
However, the consumption rate is bad. Even at no load, it consumed approx. 16A for 15.7V input (dropped from 18V). When I put a screwdriver in, the input current rose to 17.47A. Putting in a piece of stainless steel (3x6 cm²) it became 24A. Basically, the wasted energy into heat is just too big.
I assumed that the soldered joints and copper traces burn out a lot of energy, so I decided to redo the circuit (simpler, and wider traces for high-load). Also, getting a bigger work coil.
But for this time, it does not oscillate. I fried 3/4 MOSFETs when it failed to oscillate, and had to redo the circuit (again). Still no oscillation. Just in case, I only make the newest circuit to run under voltage for around a few seconds to observe the problem.
It seems that the input voltage has dropped very low, around 7.5V (from 18V) at 20A consumption. Something must happen badly with the power supply, so I'm posting it all here as well.
The transformer is one I wound by myself. It is very big (enough to be put inside a computer case) and personally I think it can support at least 2kW:
Little final touch:
Assembled with rectifier diodes. You can see below the two square components (the bridge rectifiers) that can deal 1kV-50A each. I used two so that it can work well to 100A (I'm using the transformer at 40A as my aim, btw). To protect the transformer I also added a 20A anti-leakage circuit breaker for the primary coil.
Basically I'm confident that my transformer can support a lot of current even for 18V secondary. I'm, however, doubting the filter I used.
I'm using capacitor-input filter (pi-filter with 1 capacitor bank + 1 coil + 1 capacitor bank) to increase the filtering efficiency. You can see the big-ass coil of 48mH underneath the capacitor bank (actually two banks of around 7000µF each I combined on one circuit board).
I assume that with bigger coil the circuit will need stronger power spike to start up, and with that it overloaded the filter. Do you think that it is the case with mine?
afk, that is one HUGE toroid transformer. I'm amazed that you wound it yourself! Nice work.
If it is drawing an insane amount of current at no load - you have too little turns on your work coil and/or your tank cap is too small/big.
Mads should be around soon to tell you about the thread thing.
If you'd like you can take a look at the troubleshooting guide at the bottom of my thread on the mazzili driver. Best of luck! and let me know how things go :)
Hmm... I'm trying to tackle with my coil a bit... Still, I'm having a 6-turn, 10-cm-diameter work coil but it is currently not running. The old coil that worked had 5 turns with 7 cm of diameter, but the consumption rate was bad.
It is hard for me to find better tank cap. The best one I can find here is MKV cap (which is, honestly, heating up fast and that worries me). I don't have much experience in finding good cap myself... There is a retailer here that sold power cap (used for actual induction heater) but apparently it was not selling well, so currently they did not stock up the cap...
First off- if anything fails in this driver - it would be a good idea to check all other components too. Your transformer is probably fine. Just make sure your Zenner diodes arent shorted and that your ultrafast diodes are ok. Next step would be mosfet replacement. Then you can test tank cap - charge it up to 30v and see if it starts to discharge quickly. If it can hold its charge (and its capacitance always reads the same) then its good.
Alright , It may be that your frequency is inappropriate for your switches or if your driver isn't working at all anymore your zenner/fast diodes have blown. If that is the case your FETs will also be damaged. a multimeter on diode test can quickly ensure that everything is working and not shorted out. If it only works with one coil, then i would check that it is not pushing the frequency up too far.
I see you have ~80Khz resonance - Thats not bad. Sometimes a bad cap can start to blow up mosfets. What size is your capacitor and how much capacitance does it have ?
Are you sure your psu choke isn't part of the problem? 48mH is a LOT! My ginormous choke has an inductance value of 1.8mH. Maybe the DC-resistance in your choke is too high causing a voltage drop when you start to load it?
A choke that has 26 times the inductance of the one I got but is one tenth in size has got to have a lower current rating, thinner wires and therefore much higher voltage drop at high current. (I think... I'm no expert)
Are you actually using 3 different power supplies?
Do you plan on running the device on only 24 volts?
What maximum amperage/wattage are you expecting?
What Fres are you designing for, and why?
Pete Stanaitis
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"but I'm aiming the freq to fall between 70-200kHz. It is used to melt copper and silver for metallurgy. Since I'm working with powdered metal, that frequency should be enough.":
This will be interesting to me, since the non ferrous metals that I have attempted to heat have done VERY POORLY so far with my Chinese 1000 watt device at anywhere from 43KHz up to about 113 Khz. I can only get about 1 amp of NET current into a piece of 1/2" OD copper tube. Same for a 1/2" X 5/8" solid tin bar.
So, at 48 volts input, that's only about 50 watts.
I do see that people are melting small quantities of solder and aluminum, but they are using graphite crucibles. In that case, I think it's the crucible that gets hot, imparting its thermal energy to the work. There is one guy, "The Radio Mechanic", who seems to be successfully heating a small pot of solder using a ceramic crucible. He even has a cheap PID controller and thermocouple setup to control the induction heater's output in an on/off mode.
From what little I have read, non ferrous metals need much higher frequencies and a LOT of power.
High frequency increases heating power, but the skin effect prevents eddy current from penetrating deeper for heating purpose.That's true for a fixed coil inductance and current. A higher frequency will imply a higher voltage on the coil in this case, though. Often the constraints are different, e.g. the power supply will have a certain voltage and current capability or the transistors. For a given voltage and current rating, doubling the frequency e.g. implies halving the tanks inductance and capacitance. That will keep tank voltage and current unchanged.
QuoteHigh frequency increases heating power, but the skin effect prevents eddy current from penetrating deeper for heating purpose.That's true for a fixed coil inductance and current. A higher frequency will imply a higher voltage on the coil in this case, though. Often the constraints are different, e.g. the power supply will have a certain voltage and current capability or the transistors. For a given voltage and current rating, doubling the frequency e.g. implies halving the tanks inductance and capacitance. That will keep tank voltage and current unchanged.
Under this constraint, i.e. keeping f*L constant, there is a sweet spot for the frequency for the max power transfer. It's basically an impedance matching issue between the samples inductance and resistance. The frequency is lower for more conductive materials and for larger objects.
I believe most of us already know this, our circuit is in fact a LC oscillator with a charger that supplies the oscillator energy. In ideal situation, the LC circuit will keep oscillating without losing energy. In such case, the charger will not supply any energy at all, so the current consumption is zero.
QuoteI believe most of us already know this, our circuit is in fact a LC oscillator with a charger that supplies the oscillator energy. In ideal situation, the LC circuit will keep oscillating without losing energy. In such case, the charger will not supply any energy at all, so the current consumption is zero.
I'm not familiar with this particular circuit. I believe, that the transistor will still carry current if there is no dissipation. But this current does not seem directly related to the tank current but only to the max expected power supply load. So in principle one could have an arbitrarily high current in the tank without overloading the transistors. So how would you optimize? By maximizing the ratio between power in the sample and power loss in the working coil itself? That seems to favor large frequencies. The effect flattens off at some frequency.
Edit: I thought a little about this circuit. It seems, that transistor current in the unloaded case just depends on the choke inductance (and voltage and frequency). So it is also not directly related to the tank current. There is one thing I don't understand: The max voltage on the transistors seems to be pi*Supplyvoltage in the steady state case, i.e. a constant load. But in a transient state, e.g. supply voltage is applied suddenly, there seems to be a large overshoot of up to twice the steady state value.
| Frequency (kHz) | Workpiece type |
| 5–30 | Thick materials (e.g. steel at 815 °C with diameter 50mm or greater). |
| 100–400 | Small workpieces or shallow penetration (e.g. steel at 815 °C with diameter of 5-10mm or steel at 25 °C with a diameter around 0.1mm) |
| 480 | Microscopic pieces |
The best way to optimize is to cut off the parasitic resistance as much as possible (to reduce unneeded heat loss and also current drop), and choose the frequency at the right value depending on the material and the shape of your workpiece.
As for the transient state, you have to charge up the LC circuit. There will be a great power spike seen at the PSU, which is typical, but in the actual LC tank, the voltage should be rising up gradually.
re: "I'm thinking of coating the heatsink with a bit of paint":
Isn't it the job of the heatsink to take heat away from the Mosfets? If so, then the LAST thing I'd do is to cover the heatsink with some coating. You will probably want to fan-cool the Mosfets anyway, so just place the fan over the heatsinks so you can't get to them to short them out.
You have some good surface area on the MMC to take away some heat from the capacitors and also low losses due to minimized skin effect. Good that current sharing is also taken care of by insuring a almost even current path from connection points to all capacitors.
It is better for the heat transfer from MOSFET to heat sink that it is mounted directly onto the heat sink without insulating pads, so I just mounted the heat sinks with acryllic or other plastic material to keep them floating with each MOSFET. I would also advise against painting the heat sinks, unless its with a special heat conducting paint, I have seen that used in some motor drives, but I doubt it has any insulating properties.
Hello, afk.
You said that "The machine also heated up immensely." Tell us more about this.
-Mosfet or IGBT heatsink temperatures
-Tank capacitor temperatures, and, did the temperature vary much between capacitors
-Temperature of the cooling water. Talk about your method of moving the cooling water through the system and what you are doing to cool the water
-temperature of anything else that got hot.
-Did any of these temperatures stabilize, or did they keep climbing?
Tell us about current flow changes as the metal heated toward the curie point.
Pete Stanaitis
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Thank you for sharing all the oscilloscope measurements, it all looks very clean, stiff and stable. I might just be wrong or my own Royer was much worse, but I seem to recall that the gate waveform was not as nice as yours.
Have you tried heating a large diameter water pipe? Just to try something with a larger diameter to get a better coupling to the work coil. I think that bolt is simply too small to be effectively heated in such a big coil.
I suggest that you be very careful when you insert a graphite crucible or a large iron tube for the first time. The current will increase dramatically and you might instantly blow your output Mosfets. I think I'd prefer the graphite crucible over a steel crucible. I have tried a couple of graphite crucibles and they do couple pretty well, and, of course, aren't subject to the Curie point issue. Get some fan cooling on the electronic components before you proceed. So far you haven't shorted any turns on your work coil, so I suggest you insulate it electrically before you put larger work pieces in it.
That setup is basically my end goal :)
Wouldn't it also be possible to control the gates via some kind of logic gate circuit and mosfet driver circuit?
That setup is basically my end goal :)
Wouldn't it also be possible to control the gates via some kind of logic gate circuit and mosfet driver circuit?
I think that it should be possible. The second video in petespaco's post was using a temperature controller to control the gate via a general purpose relay and a contactor. It is just I don't know if I can separate the supply for the gate and the supply for the LC circuit, with LC circuit being on voltage all time as I switch on/off the gate. Theoretically it shouldn't pose any problem if the latching resistors just work out well. There is also the problem with the gate being switched on/off too frequent, and the surge at every ON.
That setup is basically my end goal :)
Wouldn't it also be possible to control the gates via some kind of logic gate circuit and mosfet driver circuit?
I think that it should be possible. The second video in petespaco's post was using a temperature controller to control the gate via a general purpose relay and a contactor. It is just I don't know if I can separate the supply for the gate and the supply for the LC circuit, with LC circuit being on voltage all time as I switch on/off the gate. Theoretically it shouldn't pose any problem if the latching resistors just work out well. There is also the problem with the gate being switched on/off too frequent, and the surge at every ON.
I have used separate supply for gates and power electronics, with shared ground, that can be done.
However I would not turn off gate drive, unless by turning off you mean shutting it down, but still powered up to keep it at zero Volt, only then would I dare having the power electronics supply on all the time, if you turn the driver supply off, you do not know the state of the switches.
Regarding Radio mechanic's solder pot:
As far as I can tell, he is NOT controlling the Mosfet gates with his relay. He is turning the DC power to the induction heater on and off.
Pete Stanaitis
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