Mike Diack wrote...
I need a 1Kv 200ma regulated variable supply, and thought that the
reference design in Win Hill's "A of E" was a good place to start. Trouble
is that MOSFETs hit the wall at 1Kv, wheras IGBTs are readily and cheaply
available up to 1200v (more than enough, the raw supply will be around
1160v). I cannot see any reason why the IGBT should not make a suitable
series pass element in a linear regulator like this - unless I missed
something. Any Gottchas?
I have a nice stock of 1200V MOSFETs, so wouldn't consider IGBTs for
the task. The problem I've faced in making fast amplifiers has been
getting small enough transistors. In the old days 1n120 parts (small
1.2kV fets) were available, but these were supplanted by 1n100 (1kV)
parts, and by 3n120 parts (1.2kV, but larger die size). The problem
with big FETs is their higher capacitance, which forces one to run
at higher operating currents. Now when adding IGBTs into the mix, we
observe that for a given current rating they have smaller dies, and
hence lower capacitance than similar MOSFETs. But sadly, the IGBTs
generally aren't offered in truly low-current versions, so the "small"
IGBTs may end up less appealing the larger MOSFETs they might replace.
Another factor, important for higher-power linear amplifiers, is that
good heat removal capability => good power dissipation capability, is
in direct proportion to the die size => higher capacitance. Another
serious aspect is which parts can you get from a distributor somehow?
They may be "active" parts, but non-stocked. In addition, there are
the 1700 volt IGBTs to consider. Finally, tests on individual FETs
will reveal that many entire runs work well to several hundred volts
above their rated values. And that some in fact fail to reach their
rated values. (Motorola discontinued all FETs above 900V for a time
until they got their fab process back on track, and meantime shipped
non-compliant parts. Later they discontinued 1.2kV parts, but their
1kV parts in fact were the same and as good as the older 1.2kV parts.
Finally ON Semi discontinued the entire high-voltage line.)
Alternately one can make cascode high-voltage MOSFET amplifiers.
Yep, Mike, it's all a giant trade-off game.
So good luck, have lots of fun, and let us know how it works out.