Re: Stepper Dissasembly

Chris,

Hi. The fuse belongs in the + power supply lead. MOSFETs are much
better than bipolar transistors in their failure mechanism due to
over-dissipation.

The difference is MOSFET "on" resistance increases with temperature
while bipolar transistor "on" resistance (using the term very loosely
on purpose) does the opposite.

MOSFETs are indeed millions of individual little feild-effect
transistors connected in parallel. If any one of them does a better
job of conducting current than its neighbors, then it heats up a
little more than the others and the increased temp lowers its ability
to carry current. The result is inheirent current sharing.

A bipolar transistor on the otherhand has a built-in self-destruct
feature. If any particular spot on the transistor carries current
just a little bit better, then that spot will get hotter and carry
current even better. This will result in even more heat and the
current will pinch off in a localized spot until the device fails.

This is positive feedback and it occurs very quickly, sometimes in 10
uS or less. This is what safe area of operation (SOA) on a bipolar
transistor is all about and no fuse is quick enough to protect it.

MOSFETs fail in bulk due to generalized overheating under the same
conditions. This takes tenths to several seconds. A fuse will protect
the part from failure.

The only delicate part of a MOSFET is the gate terminal. It is
insulated by a quartz glass (SiO2) layer only a few molecules thick.
The insulation is nearly perfect until it arcs thru at about 30 to 40
volts. Then the part is destroyed.

It is safe once in a circuit though. No voltages are applied by
design that come even close to this rupture voltage.

Mariss

--- In CAD_CAM_EDM_DRO@y..., Chris Stratton <stratton@m...> wrote:
> I keep trying to figure out what would happen if one put a fuse in
the
> output of a chopper drive.
>
> I'm thinking perhaps it wouldn't accompish anything, because the
fuse
> blows (melts) due to heat, and the chopper would limit the average
> current even if it couldn't prevent extremely brief spikes of
> extremely large currents when shorted. So I'm thinking the fuse
would
> not blow.
>
> So the question is, what happens to the power MOSFET? I recall
being
> told these devices are actually a bunch of tiny MOSFETs on the same
> die, and perhaps this makes them delicate (in more than the gate-
oxide
> sense) so that they get damaged by brief overcurrent spikes that
would
> not affect a fuse.
>
> Which raises the question of some sort of fast acting 'silicon
fuse'.
> But at that point, I'm tempted to say, power MOSFETs are actually
> pretty cheap, just use them as a fuse and replace them if they
blow...
>
> Chris
>
> --
> Christopher C. Stratton, stratton@m...
> Instrument Maker, Horn Player & Engineer
> 22 Adrian Street, Somerville, MA 02143
> http://www.mdc.net/~stratton
> NEW PHONE NUMBER: (617) 628-1062 home, 253-2606 MIT