Re: L297/L298 And a "correction"

Thanks for the comment. That circuit works much better than any half-
bridge driver from IR or anyone else. Why?

It does not permit the MOSFET intrinsic diode to conduct. When it
does, the reverse-recovery time of said diode results in truly
prodigious "shoot-thru" currents. These currents are simultaneous
with full supply voltage imposed so the power content of the pulse is
enourmous; 50A @ 80VDC is 4 kiloWatts. Thank goodness they last only
100nS (400 uJ per transition). Still, at 20kHz they amount to 8W of
switching losses.

Why don't I use what I designed?

Simple economics. 4 PCB parts beats 20 PCB parts per half-bridge
every day of the week. The MOSFETs eat the extra 8W of losses, you
use a slightly bigger heatsink to accomodate. The board using the
IR2104 or whatever is less expensive to build so you pay less for the
product.

Moral: The very best engineering solution oftentimes is not the very
best product solution. Shame though; it is a very good driver. Parts
count kills it though.

Mariss

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Wayne C. Gramlich"
<Wayne@...> wrote:

>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "ballendo" <ballendo@>

wrote:

> >
> > Phil,
> >
> > Snips, inserts follow...
> >
> > >In CCED, "Phil Mattison" <mattison20@> wrote:
> > >I've looked at the IR2101 but it's fairly expensive and drives

up the

> > >component count since you need 4 for a bipolar stepper driver

(or 2

> > >of the dual version).
> >
> > Well, you asked for a solution to driving FET's easily and
> > inexpensively; and that one is IMO one of the best...
> >
> > FWIW (since others will be reading these messages), Mariss has

posted

> > a circuit for FET driving that he says works well, and while it

does

> > have higher component count, it IS buildable by folks who don't

want

> > to use the IR210x chip(s). AIRC, it's in the files section here

at

> > CCED or in my DIY-CNC group.
>
> Mariss' circuit can be found in the CAD_CAM_EDM_DRO files
> section:
>
> Files->CIRCUITS->Miscellaneous Circuits->mosfet bridge drive.pdf
>
> [snip]
>
> > >I understood what Wayne meant by "bipolar" (as opposed to MOS).
> >
> > I am sure that YOU did. But I'll bet that literally hundreds of

folks

> > here did not; the distinction between bipolar JUNCTION

transistors

> > and BIPOLAR stepper drive topology is non-obvious to someone new

to

> > all this. And "bipolar" to most of these CNC folks will refer to
> > drives, NOT transistor type. Which IMO makes it reasonable to let
> > THEM know the difference...
>
> I agree with ballendo many people on this list do not the
> difference between PNP/NPN bipolar junction transistors
> and power MOSFET transistors. Thus, it is reasonable to
> mention the differences between them.
>
> The following text snippet is from the "H Bridge Selection"
> section of my design notes at:
>
> <http://gramlich.net/projects/cnc/axis4/rev_a/design_notes.html>
>
>
> H-Bridge Selection
>
> When it comes to choosing H-Bridge IC's, there are some
> hard choices to make. It is fairly easy to take 8 IRF640
> MOSFET's and build a couple of very powerful H-Bridges
> that work in the 18 amp range. Since IRF64's are available
> for about ~$.70/each, it is hard to justify spending much
> more than $6-$7 per axis. There are very few H-Bridge
> solutions at that price point. All I found were the A3977
> and the good old L298. While I like the A3977, they do not
> have a very good selection of application notes. In
> particular, I could not really figure out how they wanted
> to deal with excess heat generation. Conversely, the L298's
> are quite easy, just slap a heat sink on. Maybe in some
> future revsion of this controller I will switch over the
> the A3977, but not for this revision. L298's are available
> from Jameco for $2.98. A heat sink specifically designed
> for the L298 Multiwatt-15 package is the 532-566010B34
> available from Mouser.
>
> This was written about 2 years ago, so it is possible that
> some new <$10/axis integrated MOSFET solution has been
> deployed. I certainly am not aware of any, but I haven't
> beeen looking recently.
>
> Hopefully the text above will be useful to some people.
>
> -Wayne
>