On Thu, May 29, 2008 at 02:21:35AM -0400, Sean Breheny wrote:
> 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)

I haven't. I'm still in the design phase.

> In order of preference for "gotchas" and ease of use I'd say:
> 1) Single FET
> 2) Single IGBT
> 3) Parallel FETs

OK. Will at least at first glance single FETs are out. So it looks like
back to IGBTs.

> 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.

Understood. It was one of the reasons I was looking to these single IGBT
modules. I understand they come with a basketfull of issues too.

> 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.

That's the game plan. I was looking at 200V parts. I don't plan to run them
over 144V.

> 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.

Got it.

> 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.

Hmm. So that means that a separate diode is required.

> 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 plan to throw the entire array of safety circuitry at it. I'm still
trying to wrap my head around the essential components necessary to get
even a basic controller going.

> 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.

That was my plan. Does the voltage significantly impact the issue? By this
would testing at 12V give a good sense of the issues that are going on?

Thanks for the advice.

BAJ
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