piclist 2000\05\21\170502a >
Thread: Driving a H-bridge with PWM[EE].
picon face BY : l.allen@auckland.ac.nz

Quentin Wrote...
> Hi
> I want to control the speed of a motor (24V,10A) with one PWM with
> direction and brake on a 16c87x.

This is one of those topics where the possibilities are
numerous but none are perfect IMHO.

There are nice Motor Drive IC packages that are the
easiest to use but I have found (at least in New Zealand)
are expensive, hard to get and/or are not available reliably.

I favour using mosfets in a bridge arrangement, either
mixed Nchannel and Pchannel or all Nchannel with raised
gate drives (high side driving) on the upper devices.

After numerous cost/ benefit, reliability and noise
immunity experiences my favoured system is now 4
Nchannel devices (much cheaper) forming the bridge
driven via a quad optocoupler. The collectors of the
optocoupler transistors connecting to a suitable drive
voltage (the only real snag I think), eg at 24 volts the
upper mosfets need typ 35 volts (29 volts if Logic level)
and of course the emitters of said optocouplers
connecting to the mosfet gates.

There are a number of ways of generating the gate bias
voltage but the easiest (if you are using a mains supply
via transformer/s) is to use a small transformer (or
unused winding) to generate say 12 volts and float that
supply, connecting the 0volt to the motor +ve supply, the
bias (for the upper mosfets) will always be 12 volts above
the motor voltage).

This will still need external glue logic in youre case but
this system is very reliable.

BTW I am developing a system at this very moment
using this arrangement driving a 24 volt 2.5 amp motor
but I am using both PWM outputs of a 16F877, one for
each direction.


Lance Allen
Technical Officer
Uni of Auckland
Psych Dept
New Zealand



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