Re: [CAD_CAM_EDM_DRO] Servo braking resistor in series with motor armature

Just an additional comment:
The quoted document said:

> What determines the need of a series resistor is the current rating of
> the diode inside each MOSFET.

These diodes are parasitic, ie. they are not even desired to be
there! They are horrible diodes, and have turn-on times in the
microseconds. Therefore, they really can't be used for any good
purpose, and my (and other's) opinion is that ever letting them
become forward-biased is a big mistake.

> As I said at the beginning: I don't fully understand how the
> regenerative current can be so large so that it destroys the diodes in
> the MOSFETS but the expertise says that is a fact. So my only hope now
> is that someone jumps in and tells me my understanding of the issue is
> all wrong.

Using the first example, the motor has .24 Ohm, and can deliver
98 V open circuit. Well, I=98/.24 = 408 A, neglecting the
resistance of the cables and transistors. I'm not sure if it
makes any difference whether the transistors are in the shorted
condition, which the UHU doesn't do, or doing the usual 50% duty
cycle idling pattern of a synch-antiphase drive, the current
would still be the same, except for the PWM modulation on top of
the 408 A. Since this would be a circulating current, the
single current sense resistor wouldn't see the true nature of
it, it would see + then - 408 A at the PWM frequency, and
depending on the time constant, it might average it out to zero.

With separate sense resistors and comparators in both low-side
transistors, this condition would not be missed, and the
transistors could be turned off. An output filter inductor
could give the electronics time to respond before the current
built to a damaging level. Proper high-speed freewheel diodes
would protect the transistors from the crummy characteristics of
the body diodes and divert the braking current back to the power
supply, or an external braking resistor switched into the DC bus.

Now I've revealed almost all my secrets.

Jon