In this data sheet for the TIP31A:
The spec is given: VCE(sat) * Collector-Emitter Saturation Voltage IC = 3A, IB = 375mA:
1.2 VDoes this mean then that the resistance between the collector and emitter is (1.2V) / (3A) = 0.4 ohms, when the transistor is carrying
3A through the collector to the emitter?If so, that's a bit higher than the Rds_on for the IRF530, at 0.1 ohm.
Thanks,
Michael
Pretty much. But the BJT needs a whole lot of base current to keep it there and it needs a long time to come back out of saturation. IOW if you want to switch something at a reasonable efficiency you are usually better off with a FET.
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** You have just discovered why power MOSFETS have been preferred over BJTs for high current switching applications for the last 20 years or so.
...... Phil
-- Yes
You can pull the base negative when you turn off; this sucks charge out of the base, thereby speeding things up considerably. Zetex has app notes, and bipolar transistors that could kinda sorta compete with MOSFETs for overall circuit efficiency.
Dunno if they managed to keep it up, though -- switching FETs have gotten better since the last time I really scrutinized the Zetex line.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" gives you just what it says. See details at http://www.wescottdesign.com/actfes/actfes.html
-- Indeed, but there are always those applications where Vbe(sat)for a BJT at
Yes, but don't get too close to the abs max negative Vbe or the transistor will literally degrade. Most FETs can safely be swung to -15V with gusto. Do that with a BJT ... phssst ... *BANG*.
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Looking at the _typical_ (rather than max spec) curve for Ic/Ib=10, Figure 2, it shows under 0.5V for Vce at Ic=3A and, by definition, Ib=0.3A. So the typical is more like 0.5V/3A or 1/6th (.17) Ohm. The IRF530 data sheet I looked at lists typical .14 Ohm and max .16 Ohm. But the max spec on the TIP31A appears worse, as you both agree about, and there is no question about the BJT's base drive power for Ic=3A, which even using typical figures from the curve is about 1.05V*0.3A or more than 300mW, by itself.
Jon
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