piclist 1998\10\03\063651a >
Thread: 12C509 to MOSFET H-Bridge ... how to avoid shorts?
face BY : Ned Seith email (remove spam text)

> a) can I use MOSFETs of the same polarity top and bottom of the bridge?

YES, See below...

> b) are there any simple tricks I can use to reduce damage in case the
>    prototype code manages to switch MOSFETs so as to short the supply?

YES  Use optoisolators to drive the MOSFETS. A package such as an ISP817-3
or anISP847-3 both available from http://www.jameco.com will provide independant
drive of each
MOSFET in the bridge allowing the use of same polarity devices and assuring
the desired
safety isolation. Only one diagonal pair of MOSFETs in the bridge can be
"on" at any
given time. The primary control signal should be routed to the two
optoisolators that drive one diagonal pair of MOSFETs and the "inverted"
primary control signal routed to the two remaining optoisolators that drive
the second diagonal pair of MOSFETs.
For maximum MOSFET protection, reversed biased Schottky diodes should be
properly orientated from the source to the gate, from the gate to the drain
and from the source to the drain.
An alternative method is to use a transformer such as a small audio coupling
transformer available from  Radio Shack or Jameco. The control signal is
routed to the tranformer's
primary winding. The gates of one diagonal pair of MOSFETS are connected to
one end of the transformer's secondary winding and the gates of the other
diagonal pair are
connedted to the other end of the secondary winding.
The use of the optoisolaters is easiest as the biasing of the transformer's
secondary winding inconjunction with the MOSFETs is a bit tricky.
Do not add capacitors to the MOSFET gate circuitry as this will effect turn
on/turn off performance and contribute to high speed short circuit currents
that may not be readily detectable but will shorten the MOSFETs useful life.
If capacitors are added to the source or drain circuits for noise suppresion
they should be Tantulum capacitors.
Excessive capacitance in the load circuitry should be avoided as the sum of
the load impedance should be resistive and/or inductive but not capacitive.
I have used both the optoisolator and the transformer methods to control
loads up to
10 KW with supply voltages as high a +/- 800 VDC. The bridges consisted of a
array of MOSFETs mounted on a 2 inch thick aluminum heat transfer substrates
that were chilled with circulating glycol. In the past International
Rectifier had the most authoritive free literature available regarding


<3615FD20.301C70F3@nedtron.com> 7bit

See also: www.piclist.com/techref/io/motors.htm?key=h%2Dbridge
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