H bridge progress

Made better progress with my H bridge circuit today. The idea is to
learn how to get the IR2104 high/low side driver working with some
IRF530/540 MOSFETs and then to use it in both a PWM servo amp and a
bipolar chopper with an L297.

Some of the stuff I've learned:

Things seem to work better with small or no resistors between the 2104
and the mosfet gates. The gates are capacitive loads, adding series
resistance slows down the transition times and increases the chance
that both the top and bottom are briefly on at the same time, shorting
out the power supply(!) Why there are resistors in the 2104 data
sheet is a mystery to me.

The L297 chopper/step converter is an interesting beast as well.
Yesterday I was using it with only a single half bridge connected,
connecting the other side of the winding to the positive supply or
negative via the sense resistor. Turns out if you put the 297 is
phase chopping mode, the way it turns the winding off is to send both
halves of the bridge high. When I only had one half of the bridge
built and was testing the high side, this meant there was no way to
turn off the current! Temporarily switching to enable chopping fixed
that as the L297 then activates the inhibit line, which causes the
2104 to turn off both MOSFET's on each side of the bridge.

Alas one problem I'm still fighting with is a tendancy of the L297 to
go into CMOS latchup (stops working, starts getting alarmingly hot).
Power cycling seems to make it recover unless it's permanently
toasted, but I need to figure out what to add to suppress this
problem. I've already got a series resistor on the sense input with
clamp to the L297's 5v supply.

Now I have to figure out if I want to breadboard the whole stepper
driver, mill two more half bridge boards, or just make a PC board for
the whole thing...

Chris

--
Christopher C. Stratton, stratton@...
Instrument Maker, Horn Player & Engineer
22 Adrian Street, Somerville, MA 02143
http://www.mdc.net/~stratton
NEW PHONE NUMBER: (617) 628-1062 home, 253-2606 MIT