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Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
I knocked up the ZVS circuit earlier, to drive a flyback. It is using a 0.47uF capacitor, rated for 600v, and approx a 50 turn inductor wound on a ferrite rod.
At 12 volts into the ZVS, I am getting approx 1 inch output from the flyback, which I think is quite good. At this power level, the MOSFETs stay totally cool and it could run forever.
However, when I double the input voltage to 24v, the output is amazing - about 4 inches, but the MOSFETs (IRFP460s) get so hot they latch up, so I can only run it for about 10 seconds. What would cause this? Should I increase the value of the inductor to limit current, or increase the value of the capacitor, or both?
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
Your inductor is to blame. You need to use a powdered iron toroid (the normal place to get these is from a computer power supply, get the big yellow or blue one) which needs about 80 turns (to be on the safe side).
The zvs should be able to cook your flyback before the fets are dissipating enough heat to make a small heatsink warm.
Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
Thanks, I have just found a nice yellow toroid for the inductor
I take it a powered iron toroid has a lower permability then, allowing more current to pass? I've never really understood the difference between the powdered iron toroids, and the type used in GDTs
I'll let you all know how it goes anyway, I'm building a mini SGTC.
Registered Member #58
Joined: Thu Feb 09 2006, 05:40AM
Location: Tri-Cities, Washington, US
Posts: 317
One big difference would be that Iron cores saturate at lower frequency's. While ferrite can be used in a much wider and higher frequency range, like tesla coils...
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Well, MOSFETs don't latch up, you're thinking of IGBTs. Having said that, when a ZVS stops oscillating, both MOSFETs turn on at once and it drags down the supply and starts to cook, so it might as well have latched up.
The difference between ferrite and iron powder is: Ungapped ferrite cores have a much higher inductance and lower losses than iron powder, but they can't stand any DC current at all. Even a sniff of DC makes them saturate and be useless. Iron powder on the other hand, can take a large DC bias and still keep most of its inductance. If you want to make a choke that passes DC current and blocks HF, it has to be either iron powder, or ferrite with an air gap.
Your ferrite rod inductor has a huge airgap because of its shape, so I think it ought to work with DC current fine. Increasing the inductor shouldn't have any effect on the current draw.
If the ZVS stops oscillating, I believe it's because heavy loading has made the Q of the circuit too low. In that case the cure would be a larger resonant capacitor and/or a larger airgap or fewer turns on the oscillator transformer.
This is just my mental model of how it works, it could be wrong :-/ It's interesting to see that blackplasma recommended doing the exact opposite to what I did.
Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
This is starting to make sense now, ta for all the help.
So I suppose when the MOSFETs suddenly get really hot, it's because I'm overloading the flyback, both MOSFETs come on, and it can't sort itself out at this point. I have shorted the flyback output a few times, that must be what's causing it.
I have wound a 40turn (all I could fit on) onto a powdered iron toroid. It seems to work, and the FETs do stay very cool at 24VDC in, except when it 'latches up'.
Anyway, back to the garage... I'm re-building it now, I'll go and experiment with different capacitor values.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
I always used ferrite bead without problems, and was convinced (recently) about that iron powdered cores, that they are also good since they are on DC bus and will face only minor ripple from the LC.
I managed to heat powdered core by using airgap on flyback (actually I couldn't remove it from that one) or saturating it. Both situations seem to inject heavy spikes into inductor and turn it into a induction heater.
Suprisingly MOSFET's stayed cool even with heavy saturation of the inductor (but cap was begging for life and soon stunk out whole room, but continued working even after that... )
Ferrite has lower permeability and it is going to saturate more easily, so it is best to be used like a bead with higher number of turns than PI core.
So, ferrite cores have advantage of being almost losless at HF for sake of lower permeability (so they need to be used as rod),
PI have high permeability but you must take care not to mistreat/saturate them as this can lead to massive heat dissipation (yep I suceeded to do this).
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Avalanche wrote ...
So I suppose when the MOSFETs suddenly get really hot, it's because I'm overloading the flyback, both MOSFETs come on, and it can't sort itself out at this point. I have shorted the flyback output a few times, that must be what's causing it.
With this "ZVS" driver; when the flyback output is shorted, the current draw is the lowest (maybe even lower than unloaded) and it rises as you draw the arc.
Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
Avalanche wrote ...
This is starting to make sense now, ta for all the help.
So I suppose when the MOSFETs suddenly get really hot, it's because I'm overloading the flyback, both MOSFETs come on, and it can't sort itself out at this point. I have shorted the flyback output a few times, that must be what's causing it.
I have wound a 40turn (all I could fit on) onto a powdered iron toroid. It seems to work, and the FETs do stay very cool at 24VDC in, except when it 'latches up'.
Anyway, back to the garage... I'm re-building it now, I'll go and experiment with different capacitor values.
If all else fails, you could put a current detector in the circuit to detect shoot-through, and momentarily open the circuit, resetting it.
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Quick ZVS Question
