4
\$\begingroup\$

A simple current source using a bipolar transistor uses the voltage over an emitter resistor as feedback of the current. The current is given by the base voltage, the \$U_{eb}\$ of the transistor, which is about 0.7V and the voltage over the emitter resistor. The interesting important constant value is \$U_{eb}\$.

Valid XHTML http://files.hottis.de/Stromquelle.JPG.

I'm wondering whether it would be possible to use an IGBT instead of an regular bipolar transistor. Has an IGBT a constant \$U_{eg}\$ as the regular bipolar transistor has a constant \$U_{eb}\$?

\$\endgroup\$

1 Answer 1

Reset to default
4
\$\begingroup\$

No. Not as shown without "select on test"ing.

For input purposes an IGBT looks like a MOSFET. It will have a minimum gate voltage at which conduction begins and this will vary with the model of component and across a significant range for a given model.

For example here's the data sheet dor a FGD2N40L IGBT - a baby one by most standards. TO252 package, 400V, 7A continuous. On page 2 it lists Vge(th) - the turn on voltage as 0.70V minimum, 0.85 V typical, 1.2V maximum. If you used this part in your circuit the "constant current" for a range of devices would be significantly variable unless you adjusted them for each circuit used. Use of a large voltage across Re would minimise error by "swamping" the variation in Vge(th).

However, if you monitor \$V_{RE}\$ with an opamp or similar and use it to hold \$V_{RE}\$ constant by driving the IGBT as requird, you could have an acceptably accurate current source.

\$\endgroup\$
2
  • \$\begingroup\$ I've seen this done in purpose-built lab equipment, and briefly considered using IGBTs to replace some old power Darlington transistors in a product. IF you can control the gate voltage of the IGBT to get the C-E voltage you want, you can drive an IGBT in the analog region just fine. Analog region isn't bad if you're sinked properly to handle the added losses. The problem is that the IGBT isn't designed to be controlled in the analog region, which means you have no guarantee batch-to-batch or minute-to-minute that its gain is going to remain consistent. \$\endgroup\$
    – Stephen Collings
    Commented Mar 19, 2013 at 18:45
  • \$\begingroup\$ @Remiel - AN IGBT is capable of being controlled with losed loop control using proper external feedback (such as the op amp I mentioned.) The problem is that when you use open-loop voltage control, as is suggested by the questioner, the required control voltage for a given current varies widely between devices and you have about zero certainty of the result. \$\endgroup\$
    – Russell McMahon ♦
    Commented Mar 19, 2013 at 23:53

Not the answer you're looking for? Browse other questions tagged or ask your own question.