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Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
I took some oscillographs today. The gate looks worse than I remember. The picture of the drain voltage was taken when the comparator had begun to kick in, raising the frequency, which is why it doesn't match the gate. Looks like the voltage is under 500V, and it seems to stay at about the same level as the capacitor bank. So avalanching might not be the problem.
Inductor saturation might not be far off. I just took a random inductor, measured the inductance and designed the circuit around it and the IRFP450. The core is pretty small, and I'm running it at 80% duty, 5kHz. Does B = ( V x t ) / ( N x Ae ) apply here? If so I'm screwed.... How should I measure efficiency? Like this: (Capacitor energy / charge time) / (current draw * supply voltage) If so the efficiency is an appalling 40%.
I don't think I'll use a boost converter again, a transformer charger would work better IMO.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Does B = ( V x t ) / ( N x Ae ) apply here?
Yeah, that looks right to me, make sure your units are right (weber/meter i believe). What is the voltage waveform across the inductor? Might be insightful as to whether its saturating.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Your gate waveform looks a serious mess. (what made you think it looked good?) It looks like something is unstable and the MOSFET is bouncing on and off about a dozen times, instead of switching off cleanly once. That would explain why it heats. Try experimenting with different gate resistors, drive circuits and such like.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Boost converter with such a high stepup ratio is inherently inefficient circuit.
12A is a lot of peak current, and mosfet must suffer both high conduction and switching losses. Passive snubber practically only moves heat away from the mosfet.
Using the V/t equation is somewhat impractical with boost converters; flux density can be derived directly from current as B = (uNI)/l where N is number of turns and l length of core in meters.
For 150uH of inductance on a averagemateraial 26 toroid (8mm average cross-section diameter, 20mm length) you need 27 turns on it. For 1T max flux density you can push 7,8 amps trough the coil without saturating it. Ferrite is somewhat more tricky due to high permeability, so a rod or gapped core is often used to counter this.
Still, in your case some more obivous problems are unreasonably low frequency of 5kHz, and really terrible looking gate waveform. Please use schottky diodes and decoupling capacitors.
Considering you have 5-10 volts of ringing during off period.
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Poor gate drive was the culprit all along!! I swear the gate signal looked perfect the first time I saw it. Arg... I cut out the totem pole and just drove it directly. It works well now, and the IRFP450 dissipates just around 5W or less, which can easily be taken care of with a small heatsink. Thanks for all the great help!
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Boost Converter Mosfet heating
I cut out the totem pole and just drove it directly. It works well now, and the IRFP450 dissipates just around 5W or less, which can easily be taken care of with a small heatsink. Thanks for all the great help!