Hi Neil,
Another thing that may be happening is you could be getting RF
oscillation (even at VHF frrequencies) around the switching points. A
spectrum analyser would be ideal, but a good scope will show you if
it's happening. Make sure everything - especially the driver IC  - is
properly decoupled and look at placing a ferrite bead in the gate
drive line. A low value resistor (as per the ringing fix above) may be
enough though.

Using a standard rectifier diode for the snubber may not be too
effective - the reverse  recovery time is likely to be quite long. A
Schottky or fast recovery diode is really what's required. In some
cases a RC snubber across the diode may also be required to reduce RF
emissions, if you're also worried by that.

As Ruben wrote, keep critical lines short and fat, 1mm length of line
will add up to 1nH of stray inductance. Make sure the "fast" current
return paths (from snubbers, decoupling caps etc.) are also short to
minimise loop area for the highest frequency components.

RP

On 25 November 2015 at 20:45, IVP <joecolquitt@clear.net.nz> wrote:
>> for the inductive snubber I was using a 30V 5W zener across
>> the drain & source
>
> In motor drives and for SMPS inductors I've always seen, and used,
> a power Schottky, as it's meant to conduct before the body diode.
> A large zener is comparitively slow because of its capacitance.
>
> The place I'd put a zener (generally 12 or 15V 0.5W in a 12V system)
> is g to s
>
> Typically the Schottky might be something like an STPS1545, where
> '15' is the amperage and '45' is the voltage. Manufacturer prefix varies,
> eg MBR, MBRB etc
>
> STSP1545
>
> http://www.st.com/web/en/resource/technical/document/datasheet/CD00000819=
..pdf
>
> On a couple of occassions I've had to add an ultrafast power diode,
> (eg MURxxx, BYVxx), and an RC snubber at the motor terminals
> to clean up some of the hash
>
> Joe
>
>
>
> -----
> No virus found in this message.
> Checked by AVG - www.avg.com
> Version: 2016.0.7227 / Virus Database: 4460/11059 - Release Date: 11/24/1=
5
>
> --
> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
--=20
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.