Re: Chopper circuit

--- In CAD_CAM_EDM_DRO@y..., "paul_alz" <paul.w@a...> wrote:

>
> Hi Wayne,
>
> More than happy to help.
>
> The Vf and the reverse recovery time (trr) of the diodes does make a
> difference. We have tested enough manufacturers over the years to
> prove this. If you can get a good priced one with a Vf of 0.5 to 1V
> and trr of <100nS then you will be saving energy (in these
> environmentally conscious times !!). In our 3.5A drive we use a
> General Instruments diode rated at Vf 0.95V at 5A and trr of 35ns
> max, part # EGP50B.

Paul:

I'm currently using some 1N5822 Schottky Diodes. They are spec'ed
at a Vf of .525V and an If of 3A. Interestingly enough, the spec.
sheet I have does not list a Trr time. Presumably, since they are
Schottky, they should be fast enough.

I agree about saving power, particularly since my board is
really packed tight round the diodes. While I'm all in
favor of saving dollars due to reduced power consumption,
I'm just as interested in reducing heat generation; I
can attach a heat sink to the L298's, but not so easily to
the diodes.

> 24V PSU is great, but may limit the top speed of your motor. If you
> want higher speeds go for your 36V with a max peak of 40V, and the
> L298's should remain in one piece ;-)

Agreed. The choice of 24V is based on two issues. The
first issue is that I couldn't easily find an inexpensive
36V power supply, whereas inexpensive 24V power supplies
seem to be much more plentiful. Second, my 1N5822's have
a PIV of only 40V. At 36 volts, my PIV margin is down to
4 volts.

> 0.1uF ceramics are good across each L298, and again across the L297.
> Keep signal tracks/components short around the L297 (Osc; Vref;
> Isense etc). Use one low ESR electrolytic (100 to 470uF) across
> motor power circuit (or 2 smaller ones across each L298..... depends
> on your PCB layout).

The .1uF ceramics are already there. I'm not using an
L297 (I'm using a PIC16F628 programmed to emulate an
L297 instead.) I think I've got a fair amount of decoupling
capacitors on the 5 volt supply, so I should be OK.

> As L298's are bipolar transistors with a high Vce Sat they need a

lot

> more heatsinking than their mosfet counterpart (L6203). For our

3.5A

> drive a 2 deg C (3.6 F) per Watt heatsink keeps everything within
> spec with normal convection cooling (use a fan if you have a good
> cheap source as it should extend the MTBF of the silicon).

All the time I was designing around an L298, I kept wondering
"Why isn't there MOSFET version of the L298? MOSFET's have
lower power disappation when on and they have built in
commutation diodes". Your sentence above answers the question.
If I had known about the L6203, I don't think I would be using
the L298. Well, there is always the next revision of the board. ;-)

Fans are a prudent investment when dealing with heat removal.

> All the best with your project.
>
> Paul

Thanks for the *very* useful information. So far, the project
has been a *lot* of fun.

-Wayne