Hi Byron, I think you are going to have to do some testing and/or simulation to get a very good handle on the current involved (if you haven't already done so) In order of preference for "gotchas" and ease of use I'd say: 1) Single FET 2) Single IGBT 3) Parallel FETs Parallel FETs have their own weird issues. For example, they might not both turn on at exactly the same time so you have some fraction of the time when (at worst) only one is carrying full current. Also, even though they have a positive temp coeff on Rds_on (so that they automatically share current well when fully on), that ONLY applies to the fully on state. At least some FETs have a positive temp coeff on CURRENT in the partially-on state. This means that the hottest one will snap on first and take all the current (AND experience ALL of the turn-on transient power dissipation). These are not insurmountable but not at all trivial. I would only advocate a single FET (or even parallel ones) if you could be well within (say at least 20% below) their max Vds spec. Also, bear in mind that motor current will not in general equal battery current. This is because your motor inductance is keeping the motor current fairly steady while the battery only sees that current during the "on" portion of the duty cycle. This is the buck converter action going on. So, as a rough example, if you are running at 50% duty and the motor winding current is 200A, your battery will only see about 100A. As for the diode across the motor - I looked at that page and it looks to me as if it is the same as what I said - the diode is in a different direction (as seen by the motor inductance) than the freewheeling diode inside the IGBT. Think of the inductor current during the PWM "on" time. Now think of the path it has to take to stay flowing in the same direction when the IGBT switches off. You will see that the internal diode in the IGBT (or FET) tries to block the current, but one right across the coil does not block the current. The diodes are in the same direction as far as the supply rails are concerned, but from the point of view of the motor winding current, one is a stop sign and the other is a GO. You will ultimately need quite a bit of protection circuitry in order to prevent something like a stalled motor or shorted wires from frying your expensive controller. I would HIGHLY recommend that you trying lashing up a quick drive for say a 100W motor or even a 1000W motor as a learning exercise. You could do it with a PIC or a function generator, gate drive IC, and either a FET or an IGBT (or try both). You would be able to see the kinds of things we are talking about. Sean On Wed, May 28, 2008 at 5:38 PM, Byron Jeff wrote: > On Wed, May 28, 2008 at 08:07:10PM -0400, Apptech wrote: >> >> Do you have the IGBT module already? >> > PowerEx CM600HA-24H >> > http://tinyurl.com/648p67 >> > >> > These are 600A 1200V modules. They are way overengineered >> > for the >> > application which is going to be 144V with a max amperage >> > of 350-400A in >> > short pulses. >> >> Ah. Better than mine. >> I just checked what I have and they are "Trilingtons" - 3 >> transistors in a Darlington cascade. 1000V, 400A. >> IGBT will be much nicer to drive and the cost is low enough >> to not matter wrt the rest of the project. >> >> The 400A is close to their 400A max rating (for very very >> small values of close). Most things get grumpy near the >> edges. Using two may be a very good idea [tm] if you can >> current share OK. Also, as they are BJT output second >> breakdown SOA becomes an issue. I'd check data sheets VERY >> carefully. Best guide is what others have reliably achieved >> with them already. > > Actually in my searches today I found a couple of extremely useful > resources. The first is that the Electric Vehicle Discussion List has a > wiki with everything from Donor car selection to DC motor controller design > tips. You can find it here: > > http://wiki.saymoo.org/EvdlGems/CategoryEvdlGems > > One of the pages echoed a post in this thread that stated that IGBTs are a > poor choice because of the awful voltage drop. So poking around Digikey I > happened upon some 200V 120A MOSFETS from Ixys that are running about $42 > each. You can find the datasheet here: > > http://ixdev.ixys.com/DataSheet/96538.pdf > > I figure that parallelling 4 of them should get me in the right ballpark. > I plan to current limit in software anyway. > > I'm going to finish reading through the build your own controller page > here: > > http://wiki.saymoo.org/EvdlGems/CategoryMotorControllers?highlight=%28EvdlGems%5C/CategoryEvdlGems%29 > > Lots of useful tips. > > BAJ > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist