Re: SLA7062 technical stuff - supply voltages

I hope nobody minds me sticking my nose into this.

For fun I took a G201 and turned it into a unipolar drive by removing
the top MOSFETs in each full bridge. The motor center-taps went to
the +supply, otherwise everything is the same. The drive actually
works that way.

Run this way (kind of like an SLA7026 with 33A-rated MOSFETs) it
avalanches on each on-to-off output transition. The avalanche voltage
is 112VDC, the current is 3A and the time is 250nS. The energy is
84uJ. This results in 8.4W dissipation due to avalanche heating.

Avalanche operation should be avoided no matter what mfg. literature
says. Monolithic power ICs usually "pop" during that condition.

I calculated an RC snubber to eliminate this behavior (I = 3A, V =
42V, 750nS time constant gives 15 Ohms and 0.05uF). The snubber was
built and applied to each output; it limited the voltage "spike" to
12V.

The SLA7062 claims 100V MOSFETs. This gives 44VDC as the absolute
maximum supply voltage (Vds = (100V - 12V) / 2). Throw in a sane
derating of 80% and you get 35VDC as the max rated voltage for the
drive.

These snubbers and 35VDC max should stop the "popped SLA7062" effect.

Mariss




--- In CAD_CAM_EDM_DRO@yahoogroups.com, "turyga1963" <TIntruder@...>
wrote:

>
> Dave,
>
> You're right...I wish I hadn't spent so much time looking for a
> transformer that would supply those extra volts!
>
> I think this is valuable information, but I also wonder what the

end

> effect is on how the steppers operate?
>
> If a board is rated to 42vdc and you supply only 34vdc or even
> 24vdc, what happens? If you still have the amperage, do the motors
> still turn as fast with as much power? If not, is the effective
> decrease linear?
>
> Tom
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "turbulatordude"
> <dave_mucha@> wrote:
> >
> > --- In CAD_CAM_EDM_DRO@yahoogroups.com, "turyga1963" <TIntruder@>
> > wrote:
> > >
> > > Dave,
> > >
> > > Good to know. Problem in my case and I suspect that of many
> others
> > > who have had similar problems is that we were expressly told by
> HCNC
> > > that we could use up to 42vdc and the circuit board is even
> > > silkscreened to show that voltage. Further we were told the
> closer
> > > to 42vdc we got, the better as that would insure maximum
> power/speed
> > > from the steppers.
> > >
> > > Oh well, live and learn...
> > >
> > > Tom
> >
> > I found that many of the parts seem to fit well with existing
> > transformers.
> >
> > a 24V transformer times 1.414 to get 34 volts fits well to a 44
> volt
> > maximum unit.
> >
> > 48 volts to the Gecko offers about 66 volts to the drive. Again a
> > nice fit without leaving too much room for improvement.
> >
> > Some time ago, Mariss posted some results of life expectancy of

his

> > drives. at 114 volts the life expectancy was something in the
> minutes
> > range. at 80 volts it was in the years, at 90 it was in the

months

> > and only got worse as the voltage increased.
> >
> > I suspect that this is the case with all drives. there is a sweet
> > spot that offers a very long life and a bleeding edge where one
> > mistake, one time results in that sinking feeling you get when you
> > smell that magic smoke.
> >
> > Dave
> >
>