Re: IGBT in a MOSFET application ?

From: Winfield Hill (Winfield_member_at_newsguy.com)
Date: 09/19/04 

Date: 19 Sep 2004 16:37:14 -0700

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. 

-- 
 Thanks,
    - Win
 (email: use hill_at_rowland-dotties-org for now)

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