Homebrew PCBs

--- In Homebrew_PCBs@yahoogroups.com, "Stefan Trethan"
<stefan_trethan@g...> wrote:
> On Tue, 21 Jun 2005 05:36:54 +0200, Phil <phil1960us@y...> wrote:
> 
> > why do most hbridges and stepper controllers use schottkys?
> 
> 
> because they are faster. all switchmode/pwm circuits use faster diodes.
> In the simple controller suggested there is no chopping, the
frequency is  
> low, a 1n400x could be ok.
> 
> ST

It was a rhetorical question, of course they are faster.  It's
completely up to the designer what he wants to do but I'd be careful
and engineer for reliability.  I wouldn't use 1N400x even though they
are practically free.  The 1N400x will probably work most of the time
but I'd worry about extreme cases like a high speed stall or direction
reverse.

Also, I wasn't aware that the kickback waveform speed was related to
the drive frequency.  I thought it was intrinsic to the driver and
windings (and mechanical design) of the motor.  To be good enough, the
diode needs to let less energy though than can damage the driver (er,
shunt it off).  The reverse recovery time of the diode needs to be
fast enough for that.  Some energy will get through so the issues are
driving voltage and switching time.  I've heard that typical
rectifiers have a Trr of about .5 uS.  Unfortunately, this isn't
spec'd.  Schottkys are in the range of 10s of nS so there is a big
gap.  Unfortunately no one seems to document Trr on the spec sheets
for barrier schottkys, either.  sigh.