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Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
Hmm... I recharged the batteries and I am now getting parasitic oscillations at 36V. It killed the IRFP250N . What resistance for the gate resistors is too much? Is there any better MOSFET than the IRFP250N (besides the IRFP250)?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
The only fix I'm aware of is increasing gate pullup resistors, or better and more advanced solution is active gate driver (Its not that complicated, my design invloves a 74HC14, inverting and non-inverting gate drive chips. If you want I can publish it here). How high voltage zeners do you use? 12V could theoretically decrease the self-oscillations, try different value zeners (~12-20V) and watch the effect.
The funny thing that "lousy" design with long/thin wires flying everywhere on the table seems to be actually more immune to the self-oscillation.
Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
Dr. Kilovolt wrote ...
The only fix I'm aware of is increasing gate pullup resistors, or better and more advanced solution is active gate driver (Its not that complicated, my design invloves a 74HC14, inverting and non-inverting gate drive chips. If you want I can publish it here). How high voltage zeners do you use? 12V could theoretically decrease the self-oscillations, try different value zeners (~12-20V) and watch the effect.
Sure, you can publish it here. Does 560ohms seem too little for 36V? The MOSFET died by the gate being shorted to everything... Maybe from too high Vds voltages? 36*pi= ~110V but they are 200V devices... Are there voltage spikes when it goes into parasitic oscillations? On power up? I use 12V zeners. Should I use higher voltage ones?
Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
Ok, so I got it all to work fine for a bit. Replaced the resistors with 560ohms and it worked fine at 24V but still had parasitic oscillations at 36V. I then changed the resistors to 620ohms and got arcs like this from my best flyback.
I changed the flyback to one with an internal capacitor and was having fun discharging it when the MOSFET died. Is there any way to clamp the voltage so that the fets don't die from exceeding the Vds rating? Maybe some neons in series with a current limiting resistor, would that work.
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
No, you're thinking of varistors, TVSes should last pretty much forever if you don't overcurrent them. TVSes are avalanche diodes and can be used where you would normally use a zener diode ("zener diodes" of over a few tens of volts are usually avalanche diodes). The standard 1.5KE series seem to be able to handle about 5W dissipation, and they are available in voltages up to 440V, and are often much cheaper than equivalent "zeners". Just keep in mind that the voltage is not as accurate.
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ZVS flyback driver problems
. What resistance for the gate resistors is too much? Is there any better MOSFET than the IRFP250N (besides the IRFP250)?
