Re: DC Motor Control: H-Bridge +5A, 48v

Winfield Hill wrote:
> Ethan wrote...
> >
> > Several years ago I was using the HIP4082 in a couple motor
controllers
> > and had a lot of trouble with EMC radiated susceptability. When
the
> > controller was exposed to a big enough field it would start
switching
> > incorrectly and turn on upper and lower MOSFETs at the same time,
> > resulting in a spectacular failure. This was several years ago so
I
> > don't really remember the field strengths to get it to do this, it
was
> > large but not too unreasonable, maybe 10V/m at around 100-300MHz,
> > maybe. Eventually with carefull board layout we were able to get it
to
> > pass our internal testing requirements. It is not easy to improve
EMC
> > performace on a high power device, the usual band aids such as
ferrites
> > just saturate all the time, and don't really work, so you have to
be
> > much more clever about how you filter noise. The product was never
> > particularly reliable, we had a lot of failures in the field, and
the
> > HIP4082 was the scape goat. Eventually we replaced it with a
couple
> > IR2110's and some extra dead time circuitry, which was not nearly
as
> > clean of a design as the HIP4082, however the reliability was
infinitly
> > better.
> >
> > I always suspected part of the problem was the single ground pin on
the
> > device for both the high power side and the logic inputs. Keeping
the
> > input logic signals clean is always a chalange when they are
referenced
> > to a high power ground plane.
> >
> > Has anyone else experienced similar problems? Does anyone know if
> > Intersil has done anything to address EMC susceptability? If this
> > problem has been fixed in the IC, I would consider it again. It
really
> > was a clean design.
>
> This may be a possibility. But I have always used the HIP4081A
> instead, which has separated logic-ground and MOSFET-source pins,
> with an allowed +/-2V transient voltage difference. End of the
> problem, or at least well on the way to solving it. But if I was
> forced to live with an HIP4082 instead, I'd add to or increase the
> gate series resistors to slow down the FET and thereby reduce the
> FET's source-lead bounce, and isolate the driver at the same time.
>
>
> --
> Thanks,
> - Win

My problem was more due to the EMC susceptability that the ground
bounce from switching transients. The HIP4082 has logic internal that
is supposed to make it impossible for the top and bottom FETs from
turning on at the same time. The example circuit in the data ***
shows the input logic holding the high side FETs on all the time and
relying on the guts of the IC to take care of everything.
Unfortunately the internal logic would screw up when exposed to just
the right (wrong) RF signal, and the fireworks begin.

Keeping the input logic signals clean during the switching transients
is difficult with only one ground pin, but not insurmountable. Gate
resistors are always a good idea for driving power MOSFETs. I would
want a pretty good justification before not using one. You can get
some ugly oscillations on the gate without it. Not to mention radiated
emmisions from uncontrolled switching speeds.

Another good trick for controlling switching speeds is to use a gate
resistor and add a cap from gate to drain to increase the Miller
capacitance. This makes the FET look like an integrator and you get a
very clean slope on the drain voltage.

This works for each leg of a bridge if the current is always flowing
through the load the same direction. Then it is always one FET
controlling the switching and the other is just there for synchronous
rectification. If current starts to flow in the other direction the
wrong FET will start to control the switching, and the miller cap will
cause some ugly transients on the gate of the FET that is suppose to be
off.


Ethan Petersen

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