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

Henrik Olsson wrote:

> Nope, I don't see any separate freewheel diodes in the new design either.
>
> How come recirculating the current thru the MOSFETS "internal" diodes could
> be a problem while doing it thru external diodes is OK? I don't get the
> difference.
>

The difference is that external diodes are usually chosen for
fast turn-on, while the body diodes are not optimized for any
purpose at all, and will sit with 5-12 V in the forward-bias
direction for microseconds before they start to conduct. A good
high speed diode can turn on in tens of ns, and never allow more
than 1.5 V in the forward-bias direction.

> I know you said the internal diodes are slow but yet the datasheet for the
> IRFP264N states that Ton for the diode is negligible and depends on the
> internal inductance of the Drain and Source leads, which is 5 and 13nH.
>

Well, I can't speak for that specific transistor, but I have
seen -7 V on the low-side transistor when it switched off while
sinking -20 A. This was with an IR IRFB31N20D transistor, which
is what I used to use in my servo amps. -7 V is about the
absolute limit of what the IR 2113 FET driver could take before
popping. I observed this reverse voltage to last for several us
right across the transistor leads. The dv/dt was not so fast
that the lead inductance was a major factor. The IRFP264N may
have a "FRED" diode on a separate substrate inside the package.
These diodes are quite fast, but at least in the quantities I
buy them, using an external diode is cheaper.

> How many drives do actually have external freewheel diodes? I thought
> everybody relied on the MOSFETS internal ones.....but this whole issue goes
> to show how much I actually know... ;-)

I have external diodes with extremely fast turn-on time on my
LOW-side transistors, only, to save the driver chips from too
much reverse voltage. The high-side transistors don't need the
protection.

Jon