Re: Solid State Voltage and Current Regulator (fwd)

From: 	Jim Lux[SMTP:jimlux-at-earthlink-dot-net]
Sent: 	Friday, January 09, 1998 10:07 AM
To: 	Tesla List
Subject: 	Re: Solid State Voltage and Current Regulator (fwd)

> From: 	Bill Lemieux[SMTP:gomez-at-netherworld-dot-com]
> Sent: 	Wednesday, January 07, 1998 5:22 PM
> To: 	Tesla List
> Subject: 	Re: Solid State Voltage and Current Regulator (fwd)
> Richard Wayne wrote:
> > Harri's observation about these series pass transistors are analogous
> > to large variable resistors is partially correct.  Resistor are always
> > fully on and are incapable of functioning in a cut off mode.  These
> > transistors dissapate virtually no power in the full on mode or cutoff
> > mode. 
> Cut off mode I'll believe, but no power dissipation in full on mode?
> That's hard to believe.  What is their forward drop?  Since any BPT is 
> essentially two back-to-back diodes, the forward drop is usually at
> least a few tenths of a volt. 
A BPT can have a much lower drop when conducting than the diode drop of the
BE junction.  The base injects the charge carriers into the area between C
and E. The VceSat ( drop in saturation) is mostly due to ohmic resistance
of the device, which is in the milliohms range.

 That equates to a lot of dissipated power
> when they are fully conducting.  Are these by any chance Insulated Gate
> Bipolar Transistors?  I hear that IGBTs have _very_ low forward drop, 
Low drop for same reason as regular old BPT's have low drop. Think of the
IGBT as a clever combination of a FET (high input impedance) amplifier and
a BPT (high current capability with low drop) power stage. A gross
oversimplification, of course.

> and I see them being used in some of the higher-end lighting dimmers in
> place of SCRs these days- their ability to turn on at any desired speed
> eliminates the need for a filter inductor.
Of course, turning on slow increases the power dissipation. Probably what
they are doing is PWM, and then using a much smaller filter to get rid of
the 20 kHz switching transients.

IGBT's switch so fast (nanoseconds), that without filtering, they cause
real problems in PWM motor speed controllers from the dv/dt transients
ringing the parasitic L/C in the wires to the motor. The older BPT and
VMOSFET drives had such slow switching times, that transients weren't an
issue (although "inverter whine" is).