Re: H bridge progress

Thanks Jon,

I wondered what those inductors were on the picture of
your servo amp. I thought they might be a chopping/pulse sensing
arrangement for the current limiting circuit. IGBTs are fine
for this application and yes I'm driving them hard and controlled.

-Lee

--- In CAD_CAM_EDM_DRO@y..., Jon Elson <elson@p...> wrote:
> Lee Studley wrote:
>
> > Another important feature that is overlooked initially is the
> > fast acting over-current protection circuit. The constant
> > off-time or a syncrounous-off time circuit is described in the
> > application notes of the L2xx bridge part. Its a good
> > starting point. Also at the voltages you guys are playing with,
> > IGBT's may need come into play.
>
> IGBTs are very tricky. They need to be driven HARD, because any
> time you let the IGBT go into the linear mode, current hogging will
> occur, and the device will be destroyed. You need a very clean and
> 'firm' drive of the gate waveforms to get the transistor through the
> linear region faster than emitter current can build. (You also need
> the reverse, cutting the transistor off quickly, and allowing the
parasitic
> inductances to be supplied by ultrafast recovery diodes.) At 80
volts
> or so, the IGBT is not a good choice. Over 200 V it starts to look
> very good, and above 400 V it really shines. An IGBT running at
rated
> current will have an emitter-collector drop of 2 to 2.5 V. A low
voltage
> FET can come pretty close to that, and beat it on switching losses.
> A 600 V FET can't beat the IGBT unless you use a 30 A FET at 5 A.
> Typical 600 V FETs have a drop of 10 - 15 V at rated current, and
the
> power dissipation becomes real significant.
>
> >
> > Discontinuities in the brush commutation on bigger DC servo motors
> > can add a impulse noise that can do strange things to a H-bridge.
I
> > made a servo drive that I was testing to 180vdc. It worked fine at
> > voltages up to 80-90vdc, but then as the motor got loaded, you
> > could see massive ringing spikes(that varied with motor RPM that
> > seemed to rise quickly in proportion to the current especially
> > if the motor was near being stalled and above 90vdc. Since
> > this was my first drive design, I had overlooked the current
foldback
> > features needed to keep the MosFETs in the safe operation region.
> > Needless to say I popped a few. The international rectifier
appnotes
> > have some good tips on reducing the overshoots and protecting the
> > bridge drivers. It awfully dull reading however.
>
> I use a very expensive 2-pole output filter on my PWM servo amp.
> The inductors are some of the most expensive parts on the board.
> But, it has a number of advantages, not just the reduction of
radiated
> RF from the PWM carrier. It slows the rise time of fault currents
so
> that the fault logic can respond in time, even to a fault to ground.
> It also prevents brush noise and armature inductance from getting
back
> into the FETs and causing excessive transients. It must be
working, as
> this (my 3rd generation, at least) hasn't popped a transistor in 2
years!
>
> Having nearly 90 V dropped across a stalled motor sure develops a
lot
> of inductive energy in the armature windings. I can't imagine the
motor
> can take that for long!
>
> Jon