Re: [CAD_CAM_EDM_DRO] Stepper Motor Driver

ew...that is bad...
i just checked a data sheet which said that reverse
diode max was 4v, though. and that delay was
apparently so small they couldn't give it a
number...is this cuz of improved versions perhaps?
have i not had a problem cuz of only using unipolar
steppers? (and optoisolated driver circuitry?)
thanks for this lesson, btw - can i squeeze a bit more
out of you?
i'll definitely set the scope on a plain vanilla
stepper circuit to see it for myself.
--- Jon Elson <elson@...> wrote:

> juan gelt wrote:
>
> >the ir series are hexfets - very special fets.
> >i have the international rectifier designers
> handbook
> >here, so i'll quote it:
> >"The integral body-drain diode is a real circuit
> >element, and it's current handling capability is
> >typically as high as that of the transistor
> itself."
> >
> Yes, but the POLARITY of the pulse will be wrong to
> make the
> diode conduct. The drain voltage will go very
> positive when
> the other transistor turns on, due to the
> center-tapped transformer
> action of the motor winding.
>
> The best circuit puts a diode with the anode to each
> FET drain and a
> power resistor connected to both diode cathodes and
> back to the
> motor power supply + terminal. This allows the
> inductive spike to
> be dissipated safely, at a voltage acceptable by the
> transistor,
> by selecting the resistor value. The transistor has
> to be able to
> handle twice the DC supply voltage at an absolute
> minimum.
>
> >i admit i have never scoped it, so i don't know
> first
> >hand just how fast it is. i can say i've never had
> a
> >hexfet powerfet fail on me from driving steppers
> with
> >it.
> >
> The body diodes on the standard hexfets are quite
> slow. On my
> PWM servo amplifiers, I saw them fail to conduct
> with as much as -12
> V across the FET for over 250 nS. That was enough
> to pop the
> driver chips. I put in a separate MUR820 fast
> diode, and it cut
> the undershoot to less than 4 V, which the driver
> chip could
> accept.
>
> Jon
>
>
>
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