Re: [CAD_CAM_EDM_DRO] Stepper Motor Driver

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