parallel FETs

Hello,

I want to make a controller for a golf cart motor and am considering using
paralleled MOSfets or HEXfets. I know that there can be problems with the
individual xsistors not sharing the load equally. Would it help if I had a
seperate line driving each one rather than hooking up to some common rail to
activate them? Any info would be much appreciated.

Thanks

 

nope, won't do...
if using them digitally, you might parallel them as they are,
or
put a very small resistor in each source and actually feedback them which is
the more complex way to go

 

I'm sure you are already aware of "thermal runaway" in BJTs. But there have
been some comments about using paralleled hexfets on sci.electronics.design, a
few year ago, I believe. I gathered that safely paralleling power MOSFETs only
works well when switched completely on/off, where V(GS) is substantially above
the zero thermal coefficient point. Somewhere in the discussion about the
problems of the temperature coefficient for Id, given a fixed Vg, the Hitachi
2SJ160 and 2SK1056 family was mentioned as attractive because of their
zero/negative coefficient in this regard.

There may also be serious oscillations to worry about, particularly when
operated linearly, as well, if memory serves. I'm no expert in this, at all. I
haven't tried to analyze this problem; and never ever tried to do something like
you are going to try -- hopefully someone who has can comment. But controlling
large inductances at large currents with paralleled HEXFETs (or MOSFETs or BJTs)
and where the actual motor load/acceleration can vary in spurts and jerks, I'm
sure, will take some careful attention to get working well -- probably combining
mechanical and electrical means.

You might use google to look up some threads in sci.electronics.design and/or
otherwise post this question there.

Jon

 

Hi,
Fets .
The first requirement is to develop the gate drive pwm that has a rise time
of at least 100ns and a range of zero to 12 volts.
The fets should be of the same part number from the same mfg and connected
close together as possible on a heat sink. The source leads as short as
possible to the common connecting point. The most important requirement is
the value of the series gate resister for each fet. If you are using To-220s
use 33 ohm resisters. If you are using To-247s use 15 ohm resisters. The
above is if the number paralled is below 10. If you are paralleling more
increase the above resister value by 20% for each additional 10. The gate
drive return should connect as close as possible to the source of the fets
to the center point of the common connection. Also a large filter cap should
be placed as close to the fets and the free wheeling ( fast recovery diode
or schottky diode ) path. The motor can be located at length away but it is
best to keep the switching transistors near the battery.
Bipolars.
No one has been able to parallel bipolars successfully without careful
matching. They can be purchased in modules that work great but they use
large die and are very close together.

I hope this is of some help.

Ray

 

Have you considered using the FETs as on/off switches and controlling their
duty cycle to control the speed of the motor? Much more efficient. Fewer
heat problems, among other things. If driven hard enough, load sharing
should be a minor issue. Switching frequencies of a few hundred hertz should
be adequate.