Re: H bridge progress

Chris;

A few notes here.

1. The gate resistor is there to prevent oscillations. Keeping the
driving circuit VERY close to the FET helps to reduce this, as do
ferrite beads on the gate line placed very close the FET.

2. A high value resistor is nuts, as you've discovered - you just
can't get rid of the charge fast enough with any decent sized FET.

Note that the size of that gate cap does vary from device to device.

That big resistor really does eliminate most of the advantage of the
totem pole drive circuit. You might do better to just come up with a
hefty current source/sink driver to turn the FET on, and use a small
value resistor from gate to drain to turn it off.

3. The L297 turns both sides of the bridge high when turning off a
winding as this is the fastest way to get the energy out of the winding.

4. CMOS latchup, if that is what you're experiencing, may very well be
power supply related. Double-check your ground routing - for an
overkill prototype solution, run a piece of wire from EVERYTHING that's
supposed to be grounded back to the main filter cap. See what that
does.


Alan


You poor digital guys <G>

The young guys around here with their logic probes almost faint when
they see a scope trace of a digital circuit.

After that, they believe me when I tell them that digital is a SUBSET of
analog.


--

Alan Rothenbush | The Spartans do not ask the number of the
Academic Computing Services | enemy, only where they are.
Simon Fraser University |
Burnaby, B.C., Canada | Agix of Sparta