Re: PWM Motor Control Using MOSFETS

From: Gerrard Shaw (barneycalhoun2_at_hotmail.com)
Date: 06/07/04 

Date: 7 Jun 2004 06:20:20 -0700

Winfield Hill <Winfield_member@newsguy.com> wrote in message news:<c9v67t01sot@drn.newsguy.com>...
> Gerrard Shaw wrote...
> >
> > www.orient.fsworld.co.uk/Motors/motor3.GIF
> > www.orient.fsworld.co.uk/Motors/TD340.GIF
> >
> > Hopefully this will be the FINAL circuit with no more major
> > changes...
>
> You indicate B1 as a 12-24V supply, but the TD340 specifies
> 6.5 - 18.5V operation, and has an over-voltage lockout that
> activates at nominally 20V (see page 11).

I'll change that value to 8.5 - 18.5V. Actually very few power tools
use 24V batteries so I'm OK there.
 
> You should connect the TD340's reset line to the uP's reset.

I'm not sure how useful this function is - can you advise? The PicAXE
controller has a 4K7 resistor pulling the input pin high then a PTM
switch from the pin to ground. If I used the reset pin would I just
connect the input into the pin with the pull-up resistor or would a
different arrangement be necessary?

> You certainly will want to make use of the TD340 standby pin
> to handle the H-bridge when the uP is starting up, etc.

I'll connect this one straight up to the PicAXE controller. Am I right
in thinking that I should provide a high output to the pin on
microcontroller startup to enable the IC and then send it low when
there is no pressure on the trigger i.e. motor off?
 
> The TD340 provides a rather high fixed 2.5us deadtime during
> switching, so you may need snubber networks for all the FETs
> to handle the inductance flyback.
>
> You have 100-ohm gate resistors, but for large high-current
> FETs, a different gate-drive network may be appropriate.
>
> The FET you specified is not now and probably never was made.

Since the circuit is a theoretical idea I'll write the values for
resistors etc that I have there now and maybe state an example FET
(any ideas what one would work with the current components?) I'll then
put in the evaluation that it would be necessary to match the
components used to the requirements of the motor / FETS

Thanks for your ideas,

Gerrard

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