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Drive LED from mcp23008. BJT vs Mosfet

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aheid:

--- Quote from: David Hess on March 28, 2019, 06:57:55 pm ---Bipolar transistor collector-emitter saturation voltage is much lower than 0.6 volts.
--- End quote ---

I should have checked more thoroughly, for some reason the 0.6V figure had gotten stuck and the first datasheet I pulled up (BC337) had 0.6V as well. My bad  |O

But AFAIK higher current capability is associated with higher saturation voltage, and given the LED was 12V I assumed it was a fairly high current thing.

Audioguru:
Yes, the collector-emitter saturation voltage of a bC337 transistor is 0.7V max, but only if it is almost overloaded with a collector current of 500mA.

David Hess:
The BC337 is designed specifically for audio and not saturated switching unlike the similar 2N4401 which uses a different process.  On Semiconductor shows a 50 millivolt Vce(sat) at a forced beta of 10 up to 100 milliamps.  Even at a forced beta of 100, it is still below 200 millivolts.

A modern saturated switching transistor under the same conditions would be more like 15 millivolts maximum or 0.15 ohms.  The best similar MOSFET I could find is 0.3 ohms and costs more than twice as much.

aheid:

--- Quote from: David Hess on March 30, 2019, 01:13:19 am ---The BC337 is designed specifically for audio and not saturated switching unlike the similar 2N4401 which uses a different process. 

--- End quote ---

On Semi says linear and switching for the BC337, copy/pasta error? https://www.onsemi.com/PowerSolutions/product.do?id=BC337
Checking further I see other manufacturers specifies it as an audio output driver.


--- Quote from: David Hess on March 30, 2019, 01:13:19 am ---On Semiconductor shows a 50 millivolt Vce(sat) at a forced beta of 10 up to 100 milliamps.  Even at a forced beta of 100, it is still below 200 millivolts.

A modern saturated switching transistor under the same conditions would be more like 15 millivolts maximum or 0.15 ohms.  The best similar MOSFET I could find is 0.3 ohms and costs more than twice as much.

--- End quote ---

I stand corrected. I looked at the max Vce(sat), I see the curves show much lower typical values.

Though, as I said, given that the LED was 12V I assumed much more current than 100mA (more like a 10W LED).

David Hess:
The comparison is difficult to make outside of specific applications.  I used TO-92 packages but checking SOT-23 devices now, MOSFETs have a general advantage in saturation voltage and price which I suspect has more to do with the economics of production than technology.  At 10 watts and 12 volts, the MOSFET has a clear advantage though because the base current of a bipolar transistor becomes excessive for a logic output.

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