Re: [CAD_CAM_EDM_DRO] Re: H bridge progress

beer@... wrote:

> 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.

A resistor from gate to DRAIN will turn it ON! To turn it off, you want
a resistor from gate to SOURCE! But, the 2A or so drivers from IR do
a fine job of turning fets on and off, and also handle driving the high-side
fet, whose source is swinging through the full DC voltage supply.

> 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.

No, that actually allows current to recirculate (through the reverse diode
on one of the transistors) indefinitely, reduced mostly by the IR drop of
the coil itself.

> 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.

These can be tough to track down. But, I think this is a hand-wired
prototype. You can have a lot of inductance in small wires that allows
large voltages to develop when FETs turn on and off in 40 nS or so.
A wide circuit board trace helps to eliminate these di/dt induced spikes.

Jon