Whats the trick to getting a Darlington pair to turn off quicker?

Replace the Darlington with one that has the resistor built-in ?:-)

...Jim Thompson

Reducing base current probably won't help in a reliable way -- you wouldn't have much control over the base current into the second transistor; if you could get it right at temperature 'A' it wouldn't work at all at temperature 'B'.

I assume you're using a Darlington for better current gain. Zetex has some super-Hfe transistors that may do, and may turn off better. Otherwise start a search for faster Darlingtons!

How about a negative bias on the base? Insert a diode in the Emitter if you can afford it.

Cheers

I like the Zetex solution. Good stuff.

It would help if we knew why you moved from a single to a Darlington.

Tom, Back before MOSFETS the way to speed up the turnoff was like this: | | -------->|--------------- | | | | | ------ | / | | | | | | /

--------->|-----------| | | | | | |->--| Z | Z |->- Z | | | _______ ______ / / / / / / The idea is that when the collector drops to very low voltage the top diode conducts and takes away the excess base drive, keeping the transistors from getting too saturated. If need be you can put two diodes in series for the bottom diode (especially if this is a single transistor and not a darlington). Oops, I just noticed you said common collector. No matter, just adapt the above topology.

Paul Probert University of Wisconsin

Common collector can't saturate, *unless* the base can be driven above the collector.

The problem is that, with no B-E resistor on the output device, there's no way to clear the output base.

...Jim Thompson

See if it can be replaced with a "biased" part (that is to say, the resistor is integrated into the darlington). Better yet, get a high beta non-darlington from Zetek?

Teece escreveu:

Tom ,

You can place a Capacitor in parallel with the resistor that connects to the base of the first transistor and adjust experimentally.

Bill

Right, and unlike many power Darlington transistors, which include an aggressive discharge resistor, Teece's poor little sot-23 doesn't have one. So the solution is to replace it with two discrete parts, as you suggested earlier. Sometimes it's necessary to provide an active base pull-down, rather than struggle with a wimpy resistor, like this: | ,---|>|----+------+ | | | | |/ | --+--/\\/\\--+-|>|----| | | |\\V | | | |/ | +----| | inverter | |\\V | | \\ |/ | '---| >O--/\\/\\---| gnd | / |\\V | gnd

Myself, I prefer an appropriately-sized MOSFET, not too big, not too small, with an aggressive gate-driver IC.

Thank you to everyone who responded to this post.

I have been looking at the Zetex, Diodes Inc, Fairchild websites and have found many Darlington transistors. However, I have not been able to come across one with the embedded turn off resistor. Does anyone have a suggestion for a part number?

Thanks again Tom

I'd pull down the upper driver base as well... marked with "X"

Same here. Darlington's are just a wee bit passé.

...Jim Thompson

Perhaps one of those Zetex single bipolar transistors would do, they have very high beta and might replace a darlington in some cases, if having only a single VBE drop is not a problem. It sounds like the OP doesn't want to change the PCB layout at all.

Chris

Geez, why are you using such a component if concerned with speed? See if you can find a pin-compatible N-chan FET with suitable ratings. Then, you might have to be concerned with how to slow it DOWN, but that is so much easier to accomplish!

Yes, reduce base current to the minimum, if you saturate the transistor then it takes longer to turn off.

Jon

Except in low VCC common collector situations where Vgs would eat away too much headroom. Of course one could include a charge pump for more gate drive but that all becomes quite eclectic.

In low VCC situations, Darlingtons probably have a prohibitive VCEsat.