View Single Post
Old 02-22-2010, 01:48 PM   #2989 (permalink)
MPaulHolmes's Avatar
Join Date: Feb 2008
Location: Maricopa, AZ (sort of. Actually outside of town)
Posts: 3,832

Michael's Electric Beetle - '71 Volkswagen Superbeetle 500000
Thanks: 1,368
Thanked 1,202 Times in 765 Posts
Hi Sawickm, the problem with the bms schematic is that it got way out of sync with my layout, and my crappy software won't let me update my schematic from the layout. I'd be happy to post it later. There's also the software, which I'm still working on, but sure! The charger is nothing special. It's just a buck converter with an 80amp rated chassis mount mosfet (I might try for a 50kHz switching speed). I'd be happy to post that too. I'm still working on changing it though (to a dspic30f4011 chip) so it will have a serial interface and CAN interface (like the BRUSA chargers). Right now, with the dspic30f4012 it only has a CAN interface. So that stuff is coming, don't worry. (same with SR controller).

Yo Jack! If there was a short circuit, it would be super bad, so it's really really really touchy. The dsPICs have 6 pwm channels. There's PWM1H, PWM1L, PWM2H, PWM2L, PWM3H, PWM3L. They can be configured as all running independently, or in complementary mode, so that (for example) PWM1H and PWM1L are complements of each other, with the added bonus that you can program the amount of dead-time so that PWM1L and PWM1H are NEVER EVER EVER EVER EVER on at the same time. The beauty of it is, if you want to be really conservative, the only bad thing is that the mosfet body diode will conduct longer. If you want to be all wild and crazy, the dead time can be made as small as possible to minimize heat generated. Here's the benefit:

A body diode has an aproximately fixed voltage drop of around 1v or something. The mosfet has a fixed ON resistance of 0.0075 Ohm (yes,they are that good. hehe)! So, let's say you are doing gentle town driving, and are using 100amps at 144v. Well, that's 10 amps per mosfet/"freewheel mosfet" pair, and the most that the freewheel mosfet could see is at 50% duty, which would be 5 amps (I think that's right). So, the voltage drop of this new fancy "freewheel mosfet" would be 0.0375v. Oh heck ya. Better than 1v.

Now, that was the conservative case. What about doing 1000 amps?! The voltage drop under the worst case across the freewheel mosfet will be 0.375v, which is still better than those stupid diodes! hahaha.

The 3rd and final benefit is, there are no diodes out there that are rated for 230amp and cost $6 new in modest bulk. The legs on these suckers are rated for 160 amp according to the data sheet. Also, their package can dissipate 1600 watts of heat I think (according to the datasheet).

Real life will of course be worse than this, but whatever it's all good! hehe. The dsPICs are designed for just this sort of thing (and also for 3 phase AC drives and such) where high and low side transistors being on at the same time would be catastrophic, so it should be good! I can't wait to find out.
kits and boards
  Reply With Quote
The Following User Says Thank You to MPaulHolmes For This Useful Post:
deodeo (03-23-2012)