Eisermann, Phil [Ridg/CO] wrote:
>
> > -----Original Message-----
> > From: Roman Black [mailto:fastvid@EZY.NET.AU]
>
> > Hey, this is a good argument! It would be cool if some
> > others added their 2c worth??
> > -Roman
>
> well, ok. Here's my 2c, but please remember to adjust for inflation:
>
> > I think once the inductor is "charged",
> > ie, it's field is established, the current fed in is
> > transferred to the output. So each time Q1 turns on,
> > the current is fed straight to the output
>
> But when Q1 is off, where does the energy go? Does it not get wasted as heat
> in the diode and feedback windings? When Q2 is off, it certainly does not
> get transferred to the output. Slow decay or not, the inductor is going to
> discharge through the diode. You charge the inductor, but then don't do
> anything with the energy you stored there.

But do we need to do anything with that energy??
Why don't we just leave that energy in the inductor?
Then when Q1 next turns on the inductor is already
charged so next energy is fed to the load?

Slow-decay seems to be a black art for people that
never work with large motor drivers or stepper motor
drivers, but it's pretty simple really. The enery
stored in the magnetic field of an inductor can be
discharged into a load, or kept within the inductor.
The rate the energy is discharged relates to the
load. With a very high load (short circuit) then the
inductor cannot release the energy, as this current
causes magnetic field in the inductor...

If you want a fast decay, dump the inductor into a
high resistance, you get a very fast very high volts
pulse. Slow decay means dumping it into a low
resistance, you get a very long, very low volts pulse.
Hence it takes longer for the field to be discharged,
so the energy is kept in the inductor. In a SMPS
circuit we can control the time the inductor has
to discharge, so we don't let it discharge. Anyone
who's made a good PWM motor driver has done this.
-Roman

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