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Low-voltage rail for transistor bases?

Started by Lauri Alanko September 21, 2013
Hello.

When power transistors are used as switches, the base current can be
significant, as the saturation beta may be as low as 10. On the other
hand, the base _voltage_ need only be at Vbe(sat), and any additional
voltage needs to be dropped by e.g. a resistor. Given the high
currents, this voltage drop may be a significant loss.

This suggests to me that ideally the base of a low-side NPN switch
would be powered by a low-voltage rail, only slightly higher than
Vbe(sat). This way the conduction loss would be minimized.

Has such a rail been used with bipolar switches? If not, what's wrong
with my thinking?

(Please don't tell me to use MOSFETs because they are better. I know
that already. I'm looking for understanding, not solutions.)


Lauri
On Sat, 21 Sep 2013 19:30:44 +0000 (UTC), Lauri Alanko <la@iki.fi>
wrote:

>Hello. > >When power transistors are used as switches, the base current can be >significant, as the saturation beta may be as low as 10. On the other >hand, the base _voltage_ need only be at Vbe(sat), and any additional >voltage needs to be dropped by e.g. a resistor. Given the high >currents, this voltage drop may be a significant loss. > >This suggests to me that ideally the base of a low-side NPN switch >would be powered by a low-voltage rail, only slightly higher than >Vbe(sat). This way the conduction loss would be minimized. > >Has such a rail been used with bipolar switches? If not, what's wrong >with my thinking? > >(Please don't tell me to use MOSFETs because they are better. I know >that already. I'm looking for understanding, not solutions.) > > >Lauri
--- If you had a low voltage rail just to use as a base-to-emitter supply, you'd still have to switch it into the base, which would require another transistor: (View using a fixed-pitch font.) Vcc>-----------------------+ | Vbb>-------------+ | | [RL] E | | O--[R]--B PNP C -->| C--[R]--B NPN | O-+ E | | GND>----+-----------------+ Which would work, and the dissipation would be lower than driving the NPN's base from Vcc. -- JF
On Sat, 21 Sep 2013 19:30:44 +0000 (UTC), Lauri Alanko <la@iki.fi> wrote:

>Hello. > >When power transistors are used as switches, the base current can be >significant, as the saturation beta may be as low as 10. On the other >hand, the base _voltage_ need only be at Vbe(sat), and any additional >voltage needs to be dropped by e.g. a resistor. Given the high >currents, this voltage drop may be a significant loss. > >This suggests to me that ideally the base of a low-side NPN switch >would be powered by a low-voltage rail, only slightly higher than >Vbe(sat). This way the conduction loss would be minimized. > >Has such a rail been used with bipolar switches? If not, what's wrong >with my thinking? > >(Please don't tell me to use MOSFETs because they are better. I know >that already. I'm looking for understanding, not solutions.) > > >Lauri
Couple of issues: Vbe_sat isn't really well defined. If you jam a fixed voltage onto the base, the base current could vary a lot with temperature, or between different transistors. Transistors have equivalent emitter resistance on-chip, and more in the leads and circuit board traces. At high currents, the drop in the total emitter resistance reduces the voltage across the actual, internal b-e junction, so reduces the base current, which is what you don't want to do. And if you tune it for the loaded condition and then there were no load current for some reason, the base current might get huge. I practice one rarely sees (ie, I've never seen) a circuit that tries to drive a switching transistor base at a fixed voltage. There's usually enough resistance in the driver to force the current to be pretty constant against changes in Vbe. Mosfets are better! -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
"Lauri Alanko" <

> When power transistors are used as switches, the base current can be > significant, as the saturation beta may be as low as 10. On the other > hand, the base _voltage_ need only be at Vbe(sat), and any additional > voltage needs to be dropped by e.g. a resistor. Given the high > currents, this voltage drop may be a significant loss. > > This suggests to me that ideally the base of a low-side NPN switch > would be powered by a low-voltage rail, only slightly higher than > Vbe(sat). This way the conduction loss would be minimized. > > Has such a rail been used with bipolar switches?
** No doubt - but it would still have been a 5 or 12 V rail. But the DC voltage being switched might be several hundred. ... Phil
On Saturday, September 21, 2013 12:30:44 PM UTC-7, Lauri Alanko wrote:

> When power transistors are used as switches, the base current can be > significant, as the saturation beta may be as low as 10. On the other > hand, the base _voltage_ need only be at Vbe(sat)...
> This suggests to me that ideally the base of a low-side NPN switch > would be powered by a low-voltage rail... > Has such a rail been used with bipolar switches? If not, what's wrong > with my thinking?
Yes, of course. Old NIM (nuclear instrumentation module) power for switching transistors included +24, +12, +6, -12, -24V power supplies, and it was common to make use of whatever was most convenient and efficient from the plethora. Vacuum tube radios typically had A battery for filaments (1 to 6 V), B battery for tube bias (20 to 30V) and C battery for plate supplies (as high as you can get). Sometimes, you see Vbb for base drive voltage supplies, Vcc for collector.
On Sun, 22 Sep 2013 19:52:10 -0700 (PDT), whit3rd <whit3rd@gmail.com>
wrote:

>On Saturday, September 21, 2013 12:30:44 PM UTC-7, Lauri Alanko wrote: > >> When power transistors are used as switches, the base current can be >> significant, as the saturation beta may be as low as 10. On the other >> hand, the base _voltage_ need only be at Vbe(sat)... > >> This suggests to me that ideally the base of a low-side NPN switch >> would be powered by a low-voltage rail... >> Has such a rail been used with bipolar switches? If not, what's wrong >> with my thinking? > >Yes, of course. Old NIM (nuclear instrumentation module) power for switching >transistors included +24, +12, +6, -12, -24V power supplies, and >it was common to make use of whatever was most convenient and >efficient from the plethora. > >Vacuum tube radios typically had A battery for filaments (1 to 6 V), B battery for >tube bias (20 to 30V) and C battery for plate supplies (as high as you can get).
--- The "B" battery was for the plate supply and the "C" for bias. ---
>Sometimes, you see Vbb for base drive voltage supplies, Vcc for collector.
-- JF