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Registered Member #2063
Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352
When I hook up the driver to a induction coil to heat up small metal rods it works perfectly, a 4mm steel rod turns red hot in 3 seconds. But, when I hook it up to a flyback transformerand to draw an arc, the driver stopped working in 3 seconds. After it stopped working, it drains huge amount of current from the power supply, causing a short circuit. A good IRFP250 should have around 11 mega-ohms between it's drain and source (because theres a built in diode in it). After I take out the IRFP250 for a test, theres almost 0 ohms between the drain and source, indicating a damaged internal diode. I've already killed 6 MOSFETs and i dont wan't to destroy anymore. It only happens when I hook it up to a flyback transformer. Anyone know whats wrong with my ZVS driver?
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
The zvs oscillator has an alternative mode of oscillation that draws much more current than normal. More inductance in the series inductor helps in preventing it (if it doesn't saturate), but too low load will cause the alternative oscillation anyway. Put a fast fuse or circuit breaker in series with the power supply... Use large heat sinks in the transistors. If anything is heating significantly, something is wrong.
Registered Member #2063
Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352
the 2 MOSFETs are screwed on a big piece of aluminum heatsink (they're isolated from each other of course). Would a 15A 125V fuse form a old mrcrowave help? The inductor's core was found in a ATX power supply, I coiled it my self, it has approximately 40 turns of 19 AWG copper wire from an AC motor
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
A toroidal core? It may be saturating. The inductance on the series inductor shall be similar or somewhat greater than the inductances of the coils wrapped in the flyback core. So, assuming similar core materials the number of turns in the flyback coils and in the series inductor shall be similar.
Registered Member #2063
Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352
i found my problem, the 2 diodes that goes to the flyback's primary are bad, the HV from the flyback transformer kicks it back to defective diodes, since they serve no protection, it killed the MOSFETs
so, now it works fine, the only thing that Im wondering about is the capacitor, it's rated at 1uF while the ZVS dirver circuit recommends a 0.68uF cap. does it efftect it's performance significantly? Also does using a cap with less capacitance increase the switcing frequency? or is it the complete opposite?
Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
It's fine to use a 1uF cap. Decreasing the capacitance would increase the frequency. Also decreasing the inductance of the primary increases the frequency.
How would the saturation of the series inductor affect the performance of the driver? Many people have used toroidal cores from ATX power supplies with no problem.
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
I really don't know what would be the effect of core saturation in the series inductor. I imagine that it can cause the circuit to enter the alternative destructive oscillation mode, because small inductance certainly causes. I will try a simulation to see.
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problems with the ZVS driver
