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Various questions regarding optocouplers

M

Michael

Jan 1, 1970
0
Hi there - I'm working on finalizing component selectrion for a high
side switching circuit (as discussed here a month or so back). I was
recommended the FOD617 optocoupler, but I'd like to find a similar if
not better surface mount part.

I'm looking at using a darlington optocoupler. I think the Sharp
PC452J00000F looks nice, with a 1000% current transfer ratio at 1ma.
The datasheet for the part mentions a VCE saturation voltage (http://
document.sharpsma.com/files/pc452_eJ.pdf, figure 8) of about 1V. What
does a saturation voltage even mean with a phototransistor? I mean
with a normal transistor that means a large percentage of the base
current is not being amplified. Is it sort of the same with an
optocoupler? Or is this just the lowest that VCE for the part to
conduct current?

Similarly - with just a normal transistor optocoupler, what would a
VCE saturation voltage mean?

Also, are there any major advantages between transistor optocouplers
and darlington optocouplers? From what I can see, you have a much
higher (~5-10x higher) current transfer ratio with a darlington, but
you also have a higher VCE saturation voltage (about 1V as compared to
about 0.1-0.2V). Is one kind faster than the other, or are there any
other major differences?

Thanks!

-Michael
 
T

Tom Bruhns

Jan 1, 1970
0
Hi there - I'm working on finalizing component selectrion for a high
side switching circuit (as discussed here a month or so back). I was
recommended the FOD617 optocoupler, but I'd like to find a similar if
not better surface mount part.

I'm looking at using a darlington optocoupler. I think the Sharp
PC452J00000F looks nice, with a 1000% current transfer ratio at 1ma.
The datasheet for the part mentions a VCE saturation voltage (http://
document.sharpsma.com/files/pc452_eJ.pdf, figure 8) of about 1V. What
does a saturation voltage even mean with a phototransistor? I mean
with a normal transistor that means a large percentage of the base
current is not being amplified. Is it sort of the same with an
optocoupler? Or is this just the lowest that VCE for the part to
conduct current?

Similarly - with just a normal transistor optocoupler, what would a
VCE saturation voltage mean?

Also, are there any major advantages between transistor optocouplers
and darlington optocouplers? From what I can see, you have a much
higher (~5-10x higher) current transfer ratio with a darlington, but
you also have a higher VCE saturation voltage (about 1V as compared to
about 0.1-0.2V). Is one kind faster than the other, or are there any
other major differences?

Thanks!

-Michael

The saturation voltage is the voltage you'll measure at the collector,
emitter grounded, at the specified coupler input current and collector
current. "Saturation" means the same thing as in a bipolar transistor
excited by injected base current. It doesn't matter a whole lot if
it's an electrical base connection that injects charge, or photons
that create it in the same region. It's the region of the Vce vs Ic
curves, loosely, where the collector voltage changes very little with
changes in collector current. Clearly, it's the input transistor of
the darlington that's saturated, and the base-emitter drop of the
output transistor adds to the saturation voltage of the input
transistor.

Optos with photodiode outputs are generally much faster (especially in
turn-off) than transistor output optos, and the darlingtons are even
slower. A key problem is that there's a lot of stored charge to get
rid of. It's like if you connect up a bipolar amplifier transistor,
drive the base hard enough to saturate it, and then disconnect the
base and let it float. You'll have a long turn-off time to deal
with. It helps to not allow the transistors to saturate: operate
with a collector-emitter voltage greater than the saturation voltage,
and at an appropriate current. See fig. 10 of the data sheet you
referred to.

Cheers,
Tom
 
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