Electronics > Beginners
Speed up BJT switching
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xavier60:
I found some BCX52-16 transistors here. With the 10 ohm load, the final saturation is 0.8v, 0.2v with a 20 ohm load.
The turn on has the same sluggish look but faster than the 2SB1010, taking some time to reach final saturation voltage.
The turn off speed is slightly slower than the 2S1010. Also at turn off there is a 200ns delay where the saturation voltage slowly increases then quickly turns off over the next 200ns. This must be the Storage Time.

I'd call the BCX52-16 and possibly the BCX53-16 suitable for the job so long as the current doesn't go much over 300ma without the frequency being made much over 100Khz.
T3sl4co1l:

--- Quote from: xavier60 on October 02, 2018, 11:54:25 pm ---The turn off speed is slightly slower than the 2S1010. Also at turn off there is a 200ns delay where the saturation voltage slowly increases then quickly turns off over the next 200ns. This must be the Storage Time.

--- End quote ---

Bingo.  If you watch base voltage, you'll see it kind of floats along, dropping slightly, before dropping out at the same time collector voltage rises.  Indeed, that's seen in the simulations above. :)

The B-E junction is actually very much a tiny battery.  It has an exponential charge dependency, just as an electrochemical cell does; it just so happens to have extremely rapid self-discharge (~10us for average silicon), and SFA charge storage (a paltry, er... ~nC's worth?).  It would take a very wide silicon junction to store any practical amount of charge at all, but alas, it would still leak away in microseconds. :P

Indeed, you could call BJTs a charge-controlled device as well; although at this level of subtlety, it's not any difficulty to express it any other way (Ic ~ e^Vbe, or as a current-controlled device with gain being some function β(Ic), or as a leaky charge device, or..).

Tim
xavier60:
Adding the Schottky diode eliminates the Storage delay but adds another 0.3v to the saturation voltage.
Adding 2 Schottky diodes in series gives a good compromise but still totally unnecessary.
T3sl4co1l:
2 in series isn't going to have any effect... you're better off without them, saving the Miller effect speed hit.

Have you proven that 0.3V of saturation is more dissipation than that due to switching loss?

Increasing voltage drop of components is a very real win when it is done to save losses elsewhere.

Tim
xavier60:

--- Quote from: T3sl4co1l on October 03, 2018, 05:14:00 am ---2 in series isn't going to have any effect... you're better off without them, saving the Miller effect speed hit.

Have you proven that 0.3V of saturation is more dissipation than that due to switching loss?

Increasing voltage drop of components is a very real win when it is done to save losses elsewhere.

Tim

--- End quote ---
Without any diodes, the C-E voltage goes from 0.2v to 0.4v during the 200ns of Storage delay. So this would be responsible for adding a very small loss compared to adding 0.3v for most of the On time. 
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