Re: [CAD_CAM_EDM_DRO] Re: gecko's and voltage

On Sat, 2003-07-05 at 17:40, Mariss Freimanis wrote:

> Kim,
>
> It's not a matter of petitioning; it's a matter of what is possible
> for the price.
>
> 1) The MOSFETs used in the G320 are IRF540s. They are rated at 100V
> and 27A. The MOSFET "chip" is as large as the TO-220AB package can
> accomodate.

Actually, IRF540s are rated at 28 Amps and price for $1.38 at Digikey in
1s. Obviously they don't buy them in 1s, but lets use that for
comparison.

Secondly, that is not the current limit for a T0220 package. IXYS
offers a number of TO220 package MOSFETS or similar package ie TO246,
TO247 that will handle upwards of 100 Amps. Of course these devices are
more expensive, but they also handle much, much more current or voltage.

> 2) A characteristic of MOSFETs is the "on" resistance increases as
> the square of the rated voltage for a given chip size. This means to
> carry the same current at twice the voltage, the chip area would have
> to be 4 times as large.

We don't need twice the voltage, we need 10% more voltage. There is a
big difference. This situation is NOT a matter of petitioning, but
rather a matter of considering the end user requirement. Yes, using a
larger voltage transistor may increase the price of the driver by $10 or
$40, but it will decrease the transformer cost by $100-$200. Every try
to buy a 1.8KV oddball voltage transformer ?

> 3) The next higher common rated voltage for MOSFETs is 200V. At that
> voltage the drive rating would become 160V and it would be rated for
> only 5A. Higher current ratings would require the much bigger (and a
> lot more expensive) TO-247 package.

There are a lot of MOSFET manufacturers these days and many innovative,
cost effective MOSFETS. I'm sure a comprehensive search would reveal
more than 1 cost effective MOSFET rated for 25+ Amps @ 120V+.

> Other ramifications (read expensive) would be a linear voltage
> regulator would no longer be usable because of excessive dissipation.
> A switching type regulator and its attendant circuitry would have to
> be used.

Meaning that the current design uses an linear voltage regulator to drop
voltage from 80V down to say 5V ? Maxim and others build such devices
that work on half wave rectified AC, ie 85 volts DC, just like I want as
an input.

> The drives are tested at 100VDC and come apart above 108VDC. It is
> not good practice to rate equipment at 100% of the component max
> ratings. We use 80% as the de-rating value for reliability.

We ? Does that mean you work for Gecko ? If I guarantee my supply will
never exceed 100V ie use a zener protection circuit set at 90V, can I
use a G320 at 87V ?

> Mariss
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, Kim Lux <lux@d...> wrote:
> > We need to petition Gecko to make their drivers work with 90+Volts.
> > Many of the good servo motors are rated for 90V and step down
> > transformers offer very good VAs for the money.
> >
> > Kim
>
>
>
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--
Kim Lux <lux@...>