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Old 16th Apr 2019, 4:05 am   #7
Argus25
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Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
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Default Re: Replacement power transistors hfe.

Quote:
Originally Posted by Radio Wrangler View Post
The answer is that it depends on how the supply was designed.

A lower Hfe device will demand more base current from the device driving it. This device may overheat and fail as a result or it may be fine.

David
Its hard to know that without knowing the design of the supply. In a typical pass transistor situation where the base current is part of the output current (like an emitter follower) then the transistor's base current will increase for any output load if the hfe is lower, putting more demand on the driver, but that demand normally comes via a resistor which turns the pass transistor on, not the driver transistor that turns it off, unless of course there is an extra transistor that turns the pass transistor into a Darlington, then that transistor would have increased demands/dissipation.

In addition, if the output from the supply comes from the pass transistor's collector, a lower gain pass transistor has little effect on the demands on the driver circuit, all that happens in that case (the hfe too low) is that the feedback loop runs out of dynamic range and the driver saturates (protecting itself too) in an attempt to get the output voltage up to the normal regulated voltage.

Most manufacturers appear to set the design parameters/values so that even the poorest hfe specimen of a transistor will work in the circuit (or at least they should). Most Germanium power transistor I have tested appear to have an hfe in the range of 18 to 50, so if the design allows for one with an hfe of 10, they will all work.

Also, of note, for transistor switching in Automotive applications (with early transistors), the base currents Lucas chose for their power transistors were mostly very close to 1/10 the collector currents, which is interesting.
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