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Speed up BJT switching
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xavier60:
I used to see 2SB1010 transistors used in Buck regulators in VCR power supplies. I was never able to find a BC or BD series transistor to substitute it that had anywhere near the switching speed.
 I can't find a proper data sheet for the 2SB1010 to see what is so special about it.
I suspect that it is a Switching class transistor and that's what's needed.
I have also generally noticed that BJTs with low voltage ratings have better specs in other respects such as current gain.
Something like this, https://www.rohm.co.kr/datasheet/2SAR512R/2sar512rtl-e
npelov:
@T3sl4co1l It doesn't matter what diode I put. But as you say I tried BAT54 and there is no difference. I'm not sure exactly why. As I said it may have something to do with the inductive load.

Yes, I could have PNP pull up transistor, but how do I switch it on from 0V and switch it off from 5V ? Can you draw me a schematic how to drive it? I also must make sure that both transistors don't switch on at the same time.


--- Quote from: T3sl4co1l on October 02, 2018, 04:02:18 pm ---Also, consider MOSFETs.  These have lower gain (= needs more drive voltage), but that's not such a big deal (e.g., use a bootstrap gate driver IC), and you don't have stored charge or Vce(sat) to worry about.

--- End quote ---

But I have gate charge to worry about. And gate charge is a lot more than a base-emiter capacitance. I'll need two drivers for at least the top two transistors. The bottom one could work directly from the MCU if I use logic level one. But the bottom one is fine as it is. The best MOSFET that can work with >12V on the gate is twice the price of the BJT and it has 4-5 times more the capacitance on the input. Technically I could use Vgs<=12V mosfets but I have to put some kind of divider on the gate which will increase the effect of gate charge.Well IRLML5203 has similar price to BCX53-16, but has 10nC total gate charge at 12V. That's about 1nF - 7-8 times more than the bjt.

@Hero999
Why does Ib go negative and how is R2 helping with it.
npelov:

--- Quote from: xavier60 on October 02, 2018, 08:47:20 pm ---I used to see 2SB1010 transistors used in Buck regulators in VCR power supplies. I was never able to find a BC or BD series transistor to substitute it that had anywhere near the switching speed.
 I can't find a proper data sheet for the 2SB1010 to see what is so special about it.
I suspect that it is a Switching class transistor and that's what's needed.
I have also generally noticed that BJTs with low voltage ratings have better specs in other respects such as current gain.
Something like this, https://www.rohm.co.kr/datasheet/2SAR512R/2sar512rtl-e

--- End quote ---

I wonder if it'll make things better if I choose a transistor with higher transition frequency.
2SAR513P is available at Farnel hfe=180 min. I don't know how that affects the switching.
xavier60:

--- Quote from: npelov on October 02, 2018, 09:07:54 pm ---
--- Quote from: xavier60 on October 02, 2018, 08:47:20 pm ---I used to see 2SB1010 transistors used in Buck regulators in VCR power supplies. I was never able to find a BC or BD series transistor to substitute it that had anywhere near the switching speed.
 I can't find a proper data sheet for the 2SB1010 to see what is so special about it.
I suspect that it is a Switching class transistor and that's what's needed.
I have also generally noticed that BJTs with low voltage ratings have better specs in other respects such as current gain.
Something like this, https://www.rohm.co.kr/datasheet/2SAR512R/2sar512rtl-e

--- End quote ---

I wonder if it'll make things better if I choose a transistor with higher transition frequency.
2SAR513P is available at Farnel hfe=180 min. I don't know how that affects the switching.

--- End quote ---
I would have said yes a while ago. I think there is more to it.
The photo shows a 2SB1010 driving a 10 ohm resistor from a 4v rail. It is being driven in the same way as you have. I have used a 680 and 330 ohm drive resistors.
Although the turn off speed is good with no tail current, the turn on speed is rather sluggish. I have only seen these transistors driving inductors. When driving an inductive load, the voltage transitions look faster because the voltage doesn't move until the Collector current matches the inductor current.
 When driving inductors  in discontinuous mode, transistor turn on speed has little effect on efficiency.
In my test, the saturation voltage is about 0.4v. When I add a signal Schottky diode between Collector and Base, the saturation voltage increases but there no significant improvement in turn off speed.
T3sl4co1l:
A nice circuit for testing effective best-case BJT switching speed is something like this,
https://www.seventransistorlabs.com/Images/LED_Light2.png
Notice the positive current feedback in the coupling transformer.  For testing purposes, rather than the oscillator circuit (and the mains-derived supply(!)), a function generator can control the driver transistor, turning on the circuit.  Turn-off is controlled by magnetic saturation in this case, so the output transistor latches on until the coupling transformer saturates, effectively short-circuiting the base, turning it off as quickly as possible.

Transistors with high internal base resistance (poor base contact / interdigitation, thin base layer; usually general-purpose parts like TIP31C, or high hFE like 2SD1273) have very sloppy switching: a long transition period, and an even longer tail (the tail is easily seen with a resistive load, but has to be measured with a shunt resistor when an inductive load is used, like the buck configuration used here).

A lot of power transistors have surprisingly good switching performance, MJE15024 for example.
The difference between classic (hometaxial) 2N3055s and modern (multi-diffused or epitaxial) production is dramatic, with most 80s+ production having similar performance as an MJ15011 or the like (indeed, I wouldn't be surprised if similar dies were tested into both product lines).

Modern HV switching BJTs perform reasonably well, of course, but do suffer from very long storage time, even with as strong base drive as this circuit affords.  This is partly due to geometry alone: high voltage transistors store a lot of charge in a thick junction.  Line output transistors (typically rated 1500Vceo) are particularly dramatic, with t_stg in the 2-3us range, and a recommended hFE(sat) of only 2.5!

(As for general purpose transistors, I'm not sure exactly how fast BC847 and the like can go, but I have operated 2N4401/3 in the range of 20ns transition time.  You do have to beat the crap out of the base though, nothing you're going to do with a mere resistor.  Well, not around battery powered circuits anyway, one would hope -- too inefficient.)

Tim
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