Re: [CAD_CAM_EDM_DRO] Wire EDM

John Archibald wrote:

>Folks,
>
>Has anyone tried to use an IGBT to drive a wire EDM process?
>
>I'm thinking about a home made unit here.
>
>
>

I think this would be a big mistake, unless a lot of effort went into
controlling the peak current each IGBT sees. The problem with
IGBTs is that they are a positive temp coefficient device, opposite
of MOSFETs. When MOSFETS are put under short-circuit conditions,
they conduct evenly across their surface, and even share current between
devices. When commanded to turn off, the do so.

When IGBTs are put under short circuit conditions, they don't share
current, even across the area of a single device. Hot spots develop,
and when commanded to shut off, they DON'T. This is called secondary
breakdown. It will occur anytime the transistor is not held in deep
saturation or full cutoff, ie. in the linear region. So, first it takes
strong
fast drivers to keep the transistor out of the linear region even under
normal circumstances. And, under short-circuit conditions, which is
basically
what the EDM process is, you have a very short window in which to turn the
device off, or it won't obey.

Unless you are an expert in power systems design using IGBTs, I would avoid
them in this application. For the most part, 150 or 200 V MOSFETs can
do about as well, in terms of conduction loss, they are faster, and they
are so much more robust in the regime we are talking about here.

Jon