Quote:
Originally Posted by MazdaMatt
Paul... Linky me a reference to synchronous rectification - I want to read up on it and i keep googling into the wrong stuff...
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I tried searching for it too! I couldn't find anything specific to high power motor controller design. Everything I learned about it was from people on the EVTech list.
According to evtech.org,
"To subscribe to evtech, simply write a message to
evtech-subscribe@mailman.evtech.org"
All the fancy people hang out on there. I get regular help from people like Otmar (the maker of the Zilla), Fran (chief engineer of a brushless motor control company), Lee Hart (the Godfather!), Cor van de Water (Director of Harware Systems for some major corporation), etc...! It's pretty awesome.
There are several approaches to SR. The more complicated you want to make it, the more efficient it can be, but it can be done relatively simply.
Basically, you want to remove the diodes, where most of the heat loss comes from. Each one has an approximately fixed voltage drop of like 1 v or something. So, you want to remove the diode, and replace it with a very efficient mosfet, that is carefully controlled so that it mimics the behavior of the diode. Diodes do their diode thing without even thinking about it. Mosfets need a little help.
Let's say you have a PWM duty of 75%. The freewheel diodes have current flowing through them 25% of the time. To copy that behavior, a mosfet needs to be turned on 25% of the time, so current can flow from source to drain with little loss. Current can flow from source to drain anyway, because the mosfet acts like a diode, but it has a lot of loss while acting like a diode. You want to bring the diode mimicking mosfet's gate high as soon as the regular (non-diode mimicking) mosfets' gates are brought low. You could almost use a simple inverted signal of the PWM duty, but you need to throw in a tiny bit of delay, so that the regular mosfets and diode mimicking mosfets are never both on.
I'm going to try it soon, and I'll post the schematic and stuff. If you find any information out about it. I'd love to hear it. Places like Wikipedia say SR is only good for low voltage applications, but that's not true. Because all the mosfets are in parallel, their resistance is much lower. The diodes in parallel, however, still have the same 1v drop! Terrible!
NiHaoMike: Last night I drilled a bigger indent into the bus bars so the bolt heads don't stick out at all. hehe good eye! Now I think I'll insulate the copper heat spreader from the M- bus bar, and glue them together, eventually.