Re: IXYS High Voltage current regulator



Winfield wrote:
Fred Bloggs wrote:

Winfield Hill wrote:

MooseFET wrote:

Fred Bloggs wrote:

Winfield wrote:

.... We do still have to deal with a
high-voltage MOSFET's propensity to oscillate.

Maybe a ferrite bead in the drain and a largeish
capacitor between G-D...

Chances are, you need to put the loss on the source
or gate to have a much effect.

From experience this oscillates:

60V-300V
------+----------+------
! !
\ !
/ 100K !
\ !
! !!- TO-220
+----+--!!- N Channel
! ! !!-
! ! !
20V /-/ --- +----- Vout
^ --- !
! ! ---
! ! --- 0.1u 2 places
! ! !
GND GND GND

Removing the 0.1 in parallel with the zener made the oscillation
stop. Putting a bead on the drain didn't. Putting the bead on
the source or gate did. I think the combination of the highish
impedance of the drain and the capacitance from the tab to the
heatsink prevented the bead on the drain from working.

Zener diodes have a fair amount of capacitance, which is
sometimes enough to cause trouble. One FET manufacturer
recommends a small resistor in series with the gate. A
ferrite bead should do the same thing.

Walt Jung has written a detailed lab report on all of
these circuits: http://waltjung.org/PDFs/Sources_101_P2.pdf


Yes indeed, Walt's two audioXpress articles are masterpieces
of low-voltage and high-voltage current-source exploration,
as they might be used in audio amplifiers. I get that little
magazine and saw his article when it arrived, but didn't give
it the attention it deserved.

Walt also suggests reading through the work of John Broskie,
on the www.tubecad.com website. That tube crowd really goes
at it.

Yesterday I received a shipment of 20 IXYS depletion-mode
mosfets, the IXCP 10m45s in a TO-220 package.

When my tests went above 35 volts I experienced the same
oscillation problem as I had with a Supertex dn2450. Walt
features both of these parts in his article (apparently he
hasn't found any other TO-220 depletion-mode power MOSFETs
available right now either). I see he uses a 100-ohm gate
damping resistor (fig 13C), so I'll try that. I wonder if
he added that after experiencing RF oscillation, or simply
from long experience with linear power MOSFET circuits.

Walt also ignores IXYS' naming of the 10m45s pins: K and A,
instead calling them D and S, thereby treating this part as
just another MOSFET, rather than a power IC with a built-in
voltage reference and an error amplifier driving the gate,
as shown in the IXYS block diagrams. The 10m45s data***
plots stop at 1mA, but I plan to explore at low currents;
a quick look last night down to 100uA showed the Id-vs-Vgs
plot continuing as one would expect for an ordinary MOSFET.


I did a cursory Google Patents search on patents assigned to IXYS to try to get to the bottom of the 10M45 and found nothing obvious unless it's US5629536 and/or US5751025 both High Voltage Current Limiter and Method for Making, but they're both just cross-sectional doping profiles and from the 1997/1998 time frame. Nothing earlier seems to match, but then again I was brain dead at the time.

.

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