Re: Was Chopper circuit - Now ver 3.2

Jon,

I appreciate your thougtful comments. I would hate to be the poor
bastard that designed the HIP4080. Probably the most miserable half-
bridge driver ever built. Probably looking for a job now somewhere.

About IGBTs. You have to keep in mind what they are; essentialy a
Darlignton transistor that uses a MOSFET as the "low current"
Darlington driver into a bipolar pass transistor.

Where they fit into the scheme of things is for circuits that operate
at voltages above 440VDC. The advantage goes to bipolar transistors
in that range due to their lower Vsat versus Rds for MOSFETs. Below
those rarified voltages MOSFETs rule. Simply their losses are lower.

Four layer boards versus two layer boards. Printed circuit board
price matrices primarily are based on square inches of board size.
The number of holes, the number of different hole sizes and to a
great degree the number of layers are irrelevant. What matters is the
size of the board.

If you make a big board, it will be expensive; if you design a small
board, it will be cheap. That's all there is to it. 4 layers or 2, it
makes no difference.

The market that addresses the 440V range multi-HP motor drives is
very small relative to sub 200V drives. That market requires IGBT
devices. MOSFETs are the more efficient choice in the lattter.

I have designed 440V IGBT dives on a contract baisis for major drive
manufacturers. IGBTs are slow, lossy but rugged. They are not
economical below 440V, not 100V.

Mariss

--- In CAD_CAM_EDM_DRO@y..., Jon Elson <elson@p...> wrote:
> mariss92705 wrote:
>
> > Jon,
> >
> > Couple of good questions, let me address them in order.
> >
> > MOSFET circuits require RF layout techniques because of their
> > extremely fast switching times. This means a pair of board layers
> > dedicated to ground plane and power distribution. That leaves two
> > circuit interconnect layers for a total of 4 layers. Multi-layer
> > boards are more expensive than double-sided boards by about 50%.
If
> > the boards are very small, this is not much of a factor.
>
> Well, I built a 10 A, 80 V full-bridge PWM servo amp that runs at
100
> KHz. Definitely RF! And, it is all on a 2-layer board, with high
gain
> differential amps about 2" away. I would have liked extra layers
to make
> the traces even wider, but I made it work with the 2 layers. It
might
> be more compact if 4 layers were available. But, the prototyping
> costs make that an expensive proposition. As it was, with a number
> of other aspects of the design being developed at the same time,
> I got it fully implemented and working on the 3rd revision (and one
> of those was based on the awful Harris HIP4080 chip).
>
> >
> > The IR2104s are rated at 600VDC making them suitable for 440 volts
> > RMS circuits, so 80VDC is pretty much under-utilizing them. There
are
> > other factors that get in the way for higher voltages though.
> > Primarily it is the MOSFET drain to source resistance increasing
as
> > the square of the Vds avalanche rating.
>
> If you can live with 25 KHz swithcing frequency, fast IGBTs are
amazing!
> Where $18 TO-247 HEXFETs ran so hot they had to be paralleled, a
single
> $6 T)-220 IGBT ran almost cold! You have to drive them hard, and
may need to
> have the control chip powered on before power is fed to the bridge,
> but they are vast improvements over 400+ V FETs. (The trick is you
> must NEVER let them enter the linear mode, as they have the reverse
> transconductance/temperature effect similar to bipolar transistors,
and
> the current will hog to the hottest part of the transistor.) IR's
driver
> chips are fine for driving them, but the Collector-Gate capacitance
> can turn them partially on during the power up ramp, and if the
driver chip
> can't hold the gate at Emitter potential, then they blow on power-
up.
>
> > The IR540 contains a silicon die that is about as big as you can
get
> > in a TO-220 package. It is rated at 100 volts. A 200 volt rated
> > device would have to have silicon with 4 times the area to keep
the
> > same on-resistance. It would not fit in a TO-220 package and would
> > have to go in the much larger and more expensive TO-247 package.
>
> The advantage of the IGBT is it doesn't have simple resistance, it
has
> a characteristic Vce (sat) which usually runs 1.8 V for small ones
and
> 2.5 V or so for the big ones. In some inverter and power factor
correction
> circuits I was working on a few years ago, I was getting 15 V Vds at
> about 75% rated current of the HEXFET. That will run up a big power
> loss very quickly!
>
> > If a 200 volt TO-220 part (IR640 perhaps) was used, current would
> > have to be scaled back 50% to keep dissipation normalized. Twice
the
> > voltage at half the current still yeilds the same output power.
> > Nothing would be gained by it.
> >
> > If someone was so inclined, they could use IR640s instead of
IR540s
> > and run it at 160VDC. I just don't see the point unless someone
wants
> > to kill themselves because they don't want to use a transformer.
>
> It isn't the transformer issue, but the current needed to run 1 Hp+
spindle
> motors. The amps are sky high if you can even get such motors below
> 90 V. (The aircraft motors run abot 45 A / Hp at 24 V.)
>
> You ought to check into IGBTs, anywhere over 100 V rated devices,
they
> look much better than MOSFETs.
>
> Jon