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Registered Member #1143
Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721
Hello, For a long time i am thinking of developing very high power (2-4KW) H bridge for my project ( single pcb with mosfets, drivers, transformer and so on). With advances in new technology, it is possible to get Schottky diode with 600V rating and currents of 20A with "sub" ns switching time. My question would be, does blocking internal diode with high speed Schottky yield any benefit, in hard/soft high frequency switching applications ( usually with inductive load) ?
maybe other users will be interested in H bridge design on single PCB, where you only need supply HVdc and 12V, and all it is done on board
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Yes shcotkey diodes are getting better, but dissipating the heat on a single board, i think still remains an issue. Not saying its impossible. youll just need really good heat thought-outed-ness. if you look at the peak thermal transmission off the die you may find the SMT's are not as good as the stud or TO-247 type diodes.
Ive used the smallest case type thats a screw stud, but with mixed results. I put them close together but not on the same board.
Registered Member #1143
Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721
As for selecting mosfet, i think only one part can be used here, but expensive. This part is so good, that it looks it can work without blocking internal diode
Any other useful parameter overlooked ? ( Like switching energy, what a hell is that, los due to turning on in single cycle, so heat is E*f?)
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
I basically already did this- I have a >3KW 3 phase PCB.
I had a lot of trouble with it.
For my circuit. I found that the stray inductances around the switch devices, and the capacitance within them is usually the much bigger issue for these diodes than the load inductance ever normally will be.
The problem is that the switching transients include ~20MHz frequencies which challenges the diodes far more.
The problem is that these strays are inherently very low resistance and hence very underdamped and, even worse, resonant at about these frequencies, whereas inductive loads usually have significant resistance, and also usually roll off at these fairly high frequencies, and so fairly normal diodes can clamp them fine.
Basically board layout matters much more than having special diodes. Also resistors; not on the gates so much, although they are certainly valuable, but between the ground planes of the controller and the switch devices. Having resistance there helps damp out the switching transients far better than anything else I tried (and I tried reasonably magic diodes, but they didn't seem to do much on my board for whatever reason, it looked like adding them just moved the resonance around.)
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"Ultimate" H bridge design
