L298 Commutation Diodes (was Re: Chopper circuit)

--- In CAD_CAM_EDM_DRO@y..., "mariss92705" <mariss92705@y...> wrote:

> 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 muchyou 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).

I'm having a little trouble figuring out what the 4 is about.
My best guess is below:

46V = Vmax = Maximum voltage for L298
2A = Imax = Maximum current per coil.
1uS = Tedge = Width of current spike
25kHz = Fmax = Maximum chop frequency
4 = ?? = 2 spikes per chop x 2 coils per L298??

> Conduction losses would be about
> 10W (2 * 4 + 2 * 1)/2 times 2,

I am totally baffled by this equation. I can't get
the units to work out. Any help explaining it would
be greatly appreciated.

> 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

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