AVR-Chat

At 09:06 PM 2/19/05 -0500, Ben Matthews wrote:

>Hi again,
>
>after an evening of experimentation i think we have found something
>that may work:
>we used a test circuit with one BUZ11, a 2N2904 npn transistor the
>light bulb from before and a .3 ohm resistor in series with the bulb.
>the npn transistor drives the buz11 with about an 18V gate voltage and
>the buz11 drives the bulb. Dad drew the schematic and i put it at
>http://benlinuxisbest1.tripod.com/testcircuit.pdf  .

That looks good.  The turn on might be a little slow because of the 
resistor but as long as you don't try to run it too quickly it will work.

>I drew a
>schematic of an h bridge that i  think will work based on this
>experiment, it is at http://benlinuxisbest1.tripod.com/hbridge2.pdf
>(sorry it got a little messy). it seems like the problem was that the
>gate voltage was too low, as every one said. Do you think this new h
>bridge design will work? should we run the drill on a couple volts
>less than the gate voltage? (16V on the drill, 18V on the gate
>maybe?).


Closer but still not quite there.  Two problems.  In order of importance

Problem 1 - the high side drive.  The voltage to the gate on the high side 
needs to be 10V above the high side FET source.  Let's take the simplest 
case, 100% on (with the top left FET fully on and the bottom right FET 
fully on).  In that case the right hand side of the motor will be 0V and 
the left hand side will be 18V.  In that case the gate drive for the bottom 
right should be between 10V and 20V (if it goes over 20 you may damage the 
gate).  However, the gate drive for the top left FET should be 10-15 V 
above its source.  Since that is 18V the top left gate should be at least 
28V and no more than 38V.  As you can see there is no way for those values 
to be met by a single driver.  For an all N channel H-bridge you need four 
independent drivers.  The drivers can be driven in pairs but their outputs 
must be independent. It gets worse the voltage on the high side source can 
vary from 0 to 18V during operation (very quickly during PWM) so that 
driver must follow that voltage, it cannot simply have a fixed output 
voltage.  The easiest way to solve that (and keep your N-channel FETs) 
would be to use a high side driver such as an IR2125.  The second easy way 
to solve it would be to use a P Channel FET on the high side, P channel 
FETS are driven on by driving the gate below the drain.

Problem 2- speed.  May not be an issue, but you need to keep an eye 
open.  The high resistance you are using will slow down the MOSFET turn 
on.  Think of the gate as a capacitor, you essentially have an RC filter, 
the higher the resistance to the gate the slower the turn-on.  Not much of 
an issue if you just turn it on and leave it on.  A real problem if the PWM 
period is only a small multiple of the turn-on time.  Similar issues can 
occur on turn-off by the transistor should give it a pretty fast turn-off.

If you can get a few P channel power MOSFET that would probably be simpler 
than getting and using high side drivers but both will work.  If you had 
higher current or voltage requirements I'd say you had no choice but to use 
high side drivers but I think you should be able to find appropriate P 
channel MOSFET.  The driver for those will be similar to that for the N 
channel devices, actually the identical driver should work, except that a 
high input will turn the FET on instead of off.

You are making good progress here.

Robert

" 'Freedom' has no meaning of itself.  There are always restrictions,
be they legal, genetic, or physical.  If you don't believe me, try to
chew a radio signal. "

                         Kelvin Throop, III