Hi, i was just thinking, instaed of using a semiconductor switch to control
the motors, why not use a switch mode power supply to directly drive the
motors?

If your circuit doesn't require reverse motor drive, wouldn't it be ideal
to cut out the switches and just use the SMPS's drivers?

Battery powered controllers would benefit even more from the SMPS's ability
to step up the battery voltage and squeeze out every last drop from the
batteries, (don't over squeeze to the point of battery breakdown tho..)

Current sense in the SMPS could double as overload protection too. If
response time is somewhat more critical, use a smaller filter cap for the
outputs.

What say you folks? Viable?

Terry




At 10:24 PM 7/24/99 -0700, you wrote:
>At 01:55 AM 7/24/99 -0400, you wrote:
>>I am a bit confused here: why would the FETs require more precautions? The
>>only thing I can think of would be that the power dissipation for a FET
>>goes as the second power of current,and for a darlington/BJT, as the first
>>power of current. Granted,this makes a big difference as current goes to
>>infinity, but in certain cases, you might burn up the BJTs way before the
>>FETs if the RDSon of the FETs is really low.
>
>It's actually even worse than that. The R of the MOSFET increases with
>temperature and current, so unless you go overkill or have protection you
>have a big problem. With comparable devices, the bipolar will usually take
>a lot more abuse(assuming it has enough drive current). Generally in a
>small application it's easy enough to use a larger MOSFET, but I've
>designed 1200 amp motor controllers and it makes sense to put in good
>protection so you can use fewer MOSFETs.
>
>Cheers,
>Bob
>