# switching speed of a BJT

Discussion in 'General Electronics Chat' started by bug13, Feb 27, 2013.

1. ### bug13 Thread Starter Senior Member

Feb 13, 2012
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Hi guys

Could someone explain to me how to work out (by calculation or from graph) the maximum switch speed of a BJT on a give condition.

say this one for exmaple:
http://www.fairchildsemi.com/ds/BC/BC547.pdf

and Ib=1mA @ 25 degree C

Thanks

2. ### #12 Expert

Nov 30, 2010
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I've never pushed a transistor for speed, but the datasheet says the GainBandwidth product is 300 MHz and there are 1 to 6 pf's between the collector and base for your Miller effect calculations.

That's all I have.

Last edited: Feb 27, 2013
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3. ### crutschow Expert

Mar 14, 2008
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The switching time is not stated in the data sheet thus you can't really calculate an accurate switching time since the storage time of bipolar transistors is dependent upon the fabrication parameters. So you would have to measure the switching speed in a circuit or simulate it (LTspice has a BC547 model, for example) to determine its value.

If you want a fast switch, then a MOSFET is generally better since they do not have storage time that delays the time to come out of saturation.

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4. ### bug13 Thread Starter Senior Member

Feb 13, 2012
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The reason I want to know the switching speed of a BJT is I want it to speed up a big MOSFET (as below), if I replace those BJT, does it still be able to switch on/off the big MOSFET properly?

And also, can I swap Q3 and Q4?

EDIT: the big MOSFET is dealing with large current, say 10A or more

5. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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What parts are Q4 and Q5, and what is your desired switching frequency?

6. ### #12 Expert

Nov 30, 2010
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and now we see a circuit that has to drive current into the gate capacitance. The MOSFET doesn't take time to come out of saturation, but the drivers might.

7. ### bug13 Thread Starter Senior Member

Feb 13, 2012
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Hi

For your second question, I don't know what's my desired switching frequency, but let's say 500kHz to 1MHz?

I came cross this when I was researching how to reduce switching loss in high switching frequency.

8. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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Which specific MOSFETs have you chosen for the outputs?

Their specifications play a large role in maximum switching frequency.

9. ### crutschow Expert

Mar 14, 2008
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The driver circuit you show with the NPN and PNP should switch fairly fast since they are used as emitter followers and never go into saturation.

But switching a large MOSFET at 1MHz requires a lot of drive current to charge/discharge the MOSFET gate capacitance. Why do you need such a high switching frequency?

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10. ### bug13 Thread Starter Senior Member

Feb 13, 2012
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Hi thatoneguy and crutschow,

I came cross this question when I was reading this app note.

But I think I will put an hold on this question for now, as I don't know exactly what MOSFET I will be using and what frequency it will be switching at.

You guys are way ahead of me, so it might be a good idea for me to do a bit more research/reading on this, and when I know better, then come back to this question.

You helps are really appreciated, thanks a lot