Re: Chopper circuit

Wayne,

There's no need to go nuts with the commutation diodes. Any decent hi-
speed (trr < 200 nS) silcon rectifier will do. Here's why:

The L297 has Darlington output transistors. They have a Vsat of 2 to
3VDC at 2A. Your rectifiers cannot conduct for more than 50% of the
switching duty cycle, so the contribution of their forward voltage
becomes insignificant below 1VDC.

There are two major dissipative loss components in your bridge; Vsat
and switching losses (simultaneous Icc and Vcc). There's not much you
can do about the Vsat losses but you can to some degree control
switching losses. Keep your switching frequency as low as possible;
try to keep it just above the audible range, about 13 to 15 kHz in a
noisy enviroment.

Darlington transistors are slow. You can figure the losses as being
2A times 46VDC times about 1,000 nS per edge. At 25 kHz this amounts
to 9.2W (46 * 2 * 1,000 nS * 25 kHz * 4). Conduction losses would be
about 10W (2 * 4 + 2 * 1)/2 times 2, for a total dissipation of
around 19W. Decreasing the switching frequency to 12.5 kHz would
bring the losses down to 15W.

If your rectifiers were perfect (Vfwd = 0VDC), the losses at 25 kHz
would be 17.2W instead of 19.2W.

Mariss

--- In CAD_CAM_EDM_DRO@y..., "waynegramlich" <Yahoo@G...> wrote:
> [snip]
> > What is a good value for Vf? For my 2Amp chopper board:
> >
> > http://web.gramlich.net/cnc/motion/rev_a/index.html
>
> Arrrghhh!!!
>
> The correct URL is:
>
> http://web.gramlich.net/projects/cnc/motion/index.html
>
> Sorry, 'bout that.
>
> -Wayne