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Bama Slamma 6th March 2007 03:21 AM

IGBT amplifiers
 
I've been reading a little about IGBT's. from What I read, they seem like a cross between a bipolar transistor and a mosfet. I know very little about these devices. Anybody built an amp with them? Either class D or class AB.

djk 6th March 2007 05:22 AM

Most IGBTs have poor safe area for a linear (class AB) amplifier, and are too slow for anything but a subwoofer amp in class D.

MOER 6th March 2007 06:15 AM

IGBTs
 
IGBTs are not slow. We are using them at a switching frequency of 100 and 150KHz and the newer generation of IGBTs from IXYS can switch even faster.

Stephen
Zed Audio Corp

stahlight 6th March 2007 06:34 AM

IGBT characteristics
 
The IGBT or Insulated Gate Bipolar Transistor is indeed a hybrid MOSFET/BJT device. MOSFET's traditionally have low transconductance (gain). The old name for an IGBT is a GEMFET or Gain Enhanced MOSFET. As its name implies they have higher gain, but are a voltage controlled device (BJT are current controlled).

They function similarly to MOSFET's but with one limitation: secondary breakdown, just like a BJT, and like a BJT they also dont share loads without balancing resistors (i.e. operating in parallel). Think of them as a MOSFET input with a BJT output.

Amplifier considerations: I have never seen a P-channel IGBT, just N-Channel, so you will need to design a dual N-Channel output.

Hope this burned away some of the fog.

~G6~

Eva 6th March 2007 08:10 AM

Some facts about IGBTs:

- Fast IGBTs intended for high frequency switching are actually much faster than standard audio output bipolar transistors. There is a tradeoff between turn-off speed and Vce-sat. Slower devices tend to saturate at Vce=1.0-1.5V while the faster devices saturate at Vce=2.0-3.0V. IGBT model selection is very important.

- There are some IGBT models with positive temperature coefficient intended for direct paralleling, while most models have negative temperature coefficient and can't be paralleled without emitter resistors or some other current sharing aid. See SKP10N60 from Infineon for example.

- There are P-channel IGBTs too. The GT20D201/GT20D101 complemantary N/P audio IGBT pair was once available from Toshiba, and a circuit using them was published in Elektor some years ago. Check the following link: http://www.arky.ru/audio/shem/igbt/igbt.htm

- All modern switching IGBTs feature square SOA, like MOSFETs.

- IGBTs are easier to drive than MOSFETs because the capacitances and gate charges involved are almost 5 times lower.

- The main advantage of IGBT over MOSFET in high voltage switching applications is much lower conduction losses given the same package or die size. Switching losses are higher, though. Check SGH80N60UF for example, Vce-sat for 40A is only 2.1V in the 25șC to 125șC temperature range. Any similar MOSFET would exhibit a two or three-fold increase in Rds-on at 125șC with respect to 25șC.

djk 7th March 2007 03:53 AM

"IGBTs are not slow. We are using them at a switching frequency of 100 and 150KHz and the newer generation of IGBTs from IXYS can switch even faster."

Slow for a full range amp, OK (as I said) for a sub amp.

"All modern switching IGBTs feature square SOA, like MOSFETs."

Most data sheets do not show forward bias SOA at DC (needed for linear use) because they are intended for switching (they show turn-off SOA instead).

"a rugged device with a square switching SOA" (quote from IR sheet)

For a class D amp with a B+ below 200V I would choose a Hexfet.

As Nelson Pass once observed, we can build an IGBT amp (and he did with the afore-mentioned Toshiba parts), but should we?


Upupa Epops 7th March 2007 07:00 AM

Don't use in construction devices, which exist only in two types ( complementary ), made only by one firm. You have not guarantee, that tomorow they will be out of production...

Piersma 7th March 2007 05:40 PM

Elektor IGBT amp
 
2 Attachment(s)
Hi,

This IGBT amplfier was published some years ago by Elektor.
The specs of this amplifier were quite good according to measurements done by Elektor.

best regards,
Piersma

jacco vermeulen 7th March 2007 06:01 PM

Elektor did 2 IGBT amps.
The first was an IGBT upgrade in 1995 of the Giesberts designed HEXFET amplifier.
The second IGBT design came in 1997.
An early 1995 Elektor issue had an page on IGBTs, Februari or March number.

Eva 8th March 2007 12:26 PM

2 Attachment(s)
See the attached SGH80N60UF SOA figures, which demonstrate that SOA is only restricted by dissipation and not by secondary breakdown as stated. IGBTs are secondary-breadown free.


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