Re: [CAD_CAM_EDM_DRO] H bridge progress

Andrew,

Thanks for the hints. I've been trying to use the 2104 in a stepper
driver first (I've got some underpowered amps I can use on my servos
for now) which adds the complexity of chopper current control. I've
got it mostly working in principal, but keep blowing up parts, either
due to obvious mistakes (leaving the current sense unconnected during
a test) or spike/noise/whatever problems that are yet to be
understood.

As for board layout, I'm contemplating a single layer design, which
probably means running the gate signals with wires on top. I hope
that works out - at least it means there is the board thickness
separating them from the high current traces, and they tend to cross
the traces right angles, which in theory helps as well. Of course
once I give up my preocupation with the idea of milling the board,
most PCB houses give you 2 sides for the same price as one.

As for sounding like I know what I'm doing... well, I do have an EE
degree, but that was 99% digital stuff... Getting power electronics
to work right is a whole different ballgame, one in which I have
little experience to date.

Chris

> I built a pwm servo drive with 2104's and 8-irf540 mosfets that runs a 110
> volt, 2 hp brush dc motor with a buss voltage of 75 for the spindle motor on
> a minimill.
>
> The data sheet was not much help to me on a few points. The gate resistor
> in the data sheet gave me much more of a problem than it sounds like you
> had. I messed around for a long time before I found out a gate resistance
> less than 20 ohms worked best. I also put reverse polarity clamping diodes
> from each gate drive pin to ground to keep the 2104 from latching up. I
> still don't have things perfectly designed because I blew a mosfet and a
> 2104 yesterday, and the thing has been working perfectly for 3 months, so I
> am confused by this. I need to keep working on it.
>
> I have this same design working on all three axis of a small minimill.
> They run 24 volt 33 watt servos with a buss voltage of 40 volts. I have not
> had any problems with these axis drives, but they operate at a much lower
> voltage and current level than does the spindle drive. All the axis drives
> run cool heat sink temps too.
>
> I am driving all the pwm servo drives with pic-servo's. I can tell the
> pic-servo not to run the servo drive at 100% full on, thus I did not need to
> deal with putting a charge pump on the high side to help the bootstrap caps
> maintain gate drive voltage. Keep this in mind if your circuit will be
> operating at 100%.
>
> Also be carefull with the circuit layout. Keep the gate circuit away from
> large current areas of the board. I fear I may not have done this part
> correctly.
>
> I am no circuit designer, I just poke around until I find something that
> works, so I could learn a lot from some one who knows what they are doing,
> and from your rapid progress it sounds like you do.
>
>
> Thanks
>
> Andrew
>
>
>
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--
Christopher C. Stratton, stratton@...
Instrument Maker, Horn Player & Engineer
22 Adrian Street, Somerville, MA 02143
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