Low on-resistance bipolar transistor - does it exist?
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Posted by tempus fugit on April 6, 2007, 2:03 pm
Please log in for more thread options Hey all; A transistor thread in this group got me wondering about transistor characteristics. I have seen low (and very low) on resistance MOSFETS (2 ohms and less) but haven't seen a similar spec in a standard BJT. In fact, sifting through the Fairchild site, one can sort the MOSFETS based on Ron, but the same isn't true for the BJTs. Are some BJTs lower Ron than others, or is low Ron something that BJTs simply are not capable of? It's my understanding that a MOSFET is more susceptible to damage from static discharge than a BJT, and so it would seem to limit it's applications as a switch if you weren't supposed to have any voltage at the drain before things were powered up. Thanks | |||||||||||||
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Posted by John Popelish on April 6, 2007, 12:27 pm
Please log in for more thread options tempus fugit wrote: Saturated BJTs are not as simply ohmic as mosfets are, but they can have pretty low forward drops. I don't know what range of voltage and current you are talking about switching, but Zetex makes some small, very low drop transistors. E.g. ZTX1049A http://www.zetex.com/3.0/pdf/ZTX1049A.pdf has a collector to emitter drop of no more than 220 mV while carrying 4 amps of collector current and driven with only 50 mA of base current. The resistance with a similar drop at 4 amps would be 0.055 ohms. And this is a little TO-92 sized device. Unfortunate, the resistance is not linear with variable current. In other words, at zero collector current, the collector to emitter voltage does not go to zero. | |||||||||||||
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Posted by John Popelish on April 6, 2007, 12:35 pm
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John Popelish wrote: Here is another impressive bit of silicon from Zetex: http://www.zetex.com/3.0/pdf/ZXTN23015CFH.pdf | |||||||||||||
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Posted by tempus fugit on April 6, 2007, 3:03 pm
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>
> > tempus fugit wrote:
> > > Hey all;
(2
> > > > > > A transistor thread in this group got me wondering about transistor > > > characteristics. I have seen low (and very low) on resistance MOSFETS > > > ohms and less) but haven't seen a similar spec in a standard BJT. In
> fact,
> > > sifting through the Fairchild site, one can sort the MOSFETS based on
> Ron,
> > > but the same isn't true for the BJTs. Are some BJTs lower Ron than
> others,
> > > or is low Ron something that BJTs simply are not capable of? It's my
as
> > > understanding that a MOSFET is more susceptible to damage from static > > > discharge than a BJT, and so it would seem to limit it's applications > a
> > > switch if you weren't supposed to have any voltage at the drain before
> > > things were powered up. > >
> > Saturated BJTs are not as simply ohmic as mosfets are, but > > they can have pretty low forward drops. I don't know what > > range of voltage and current you are talking about > > switching, but Zetex makes some small, very low drop > > transistors. E.g. ZTX1049A > > http://www.zetex.com/3.0/pdf/ZTX1049A.pdf > > has a collector to emitter drop of no more than 220 mV while > > carrying 4 amps of collector current and driven with only 50 > > mA of base current. The resistance with a similar drop at 4 > > amps would be 0.055 ohms. And this is a little TO-92 sized > > device. Unfortunate, the resistance is not linear with > > variable current. In other words, at zero collector > > current, the collector to emitter voltage does not go to zero. >
the
> Thanks for the reply John. I'm switching 12v at the collector with 5v at > base. The 12v is coming through a 100K resistor from the previous device,
so
> the current is pretty small. I've found that using a standard (2N2222
type)
> transistor doesn't quite switch things all the way off ( there is an LED
Oh, one other thing - is this how the Ron is calculated, i.e., Collector to
> indicator that still glows faintly). > > emitter drop divided by collector current? If this were the case, my low current would increase Ron. | |||||||||||||
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Posted by John Popelish on April 6, 2007, 1:00 pm
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tempus fugit wrote: > Oh, one other thing - is this how the Ron is calculated, i.e., Collector to
> emitter drop divided by collector current? I can't say whether that is the method, or an incremental method is used (small change in collector current causes a small change in collector voltage drop, Ron= delta V / delta I ). > If this were the case, my low current would increase Ron.
Yes. The base current leakage into the collector produces a minimum collector voltage drop for a given base current, even if the external collector current goes to zero. I think that for any given collector current, there is an optimum base drive, for lowest collector to emitter drop. More is not necessarily better. | |||||||||||||
>
> A transistor thread in this group got me wondering about transistor
> characteristics. I have seen low (and very low) on resistance MOSFETS (2
> ohms and less) but haven't seen a similar spec in a standard BJT. In fact,
> sifting through the Fairchild site, one can sort the MOSFETS based on Ron,
> but the same isn't true for the BJTs. Are some BJTs lower Ron than others,
> or is low Ron something that BJTs simply are not capable of? It's my
> understanding that a MOSFET is more susceptible to damage from static
> discharge than a BJT, and so it would seem to limit it's applications as a
> switch if you weren't supposed to have any voltage at the drain before
> things were powered up.