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problem with Bjt switching speed

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When using BJT as a switch, is there anyway to improve the switching speed(rising/falling time) and delay, or is it of the bjt characteristics?

When switching at 20Khz, do bjt and mosfet give the same rising/falling time?
 
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1.) Is there a way to change rise and fall tiems..definitely at the device level: i) change doping values ii) junction length iii) oxide width........
at the circuit level: you can think of reducing the capacitive effect of any load youhave attached to the BJT

2.) switching at 20KHz typically BJT and MOSFET will not have the same characteristics since normally the device characteristics are completely different except for being transistors.
 

at the circuit level: you can think of reducing the capacitive effect of any load youhave attached to the BJT.
I tried googling but couldn't find any info about it. Any tips? :roll:
 

For a given transistor type, you would want to optimize the base current waveforms for fastest switching.
When switching at 20Khz, do bjt and mosfet give the same rising/falling time?
Click to expand...
For a reasonable answer, you should refer to specific voltage and current ranges. By present state of technology, MOSFET power switches can be expect to perform better.

P.S.:
at the circuit level: you can think of reducing the capacitive effect of any load you have attached to the BJT.
Click to expand...
What dod you exactly mean with "reducing the capacitive effect"? Reducing the load capacitance won't be particularly related to BJT versus MOSFET. The capacitive "effect" is charge C*U, you can't reduce it at will.

I think it would be better to present a circuit example to clarify your intentions.
 
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Yes, showing a circuit will help. If your circuit is in anyway involved in switching currents as opposed to mere voltage switching (on/off kind of a thing where mosfet are preferable) then try not to put the bjt in saturation because it will need more time to recover from that state.
 

I think, that capacitance doesn't play that big role. I've done my masters thesis no junction capacitance measurements and it turned out that small bjt's like bc547 have very small base-emitter capacitance (in the order of single pF). High power ones have like up to 200 pF. I think the saturation effect is the main problem. You may want to consider Schottky transistors.
 

If you saturate the BJT your storage time will go way up. The lower your
"forced beta" (Ic/Ib, actual) the worse this gets. Applying a Schottky
diode clamp can help that. I haven't ever seen a datasheet for a
"Schottky transistor", not to say they don't exist but I've only ever
seen them as integrated circuit devices. A peaking base drive circuit
can improve turnon and turnoff while keeping the bottomed "on" base
drive to an acceptable minimum.

Faster collector risetime wants lower base impedance. Cjc*dV/dt opposes
your attempt to turn the device off. This is also true for FETs.
 

In order to switch off a saturated BJT, the charge (majority carriers) stored in the base must be removed. This is usually what takes time and puts a delay.
You must provide a strong negative current (i.e. exiting from base terminal in a NPN bjt). This can be achieved with the help of a negative supply or a capacitor in parallel with the R in the driving circuit.
For fast switch-on, you need a strong positive current (much higher than steady-state) for fast slope in Ic. The same capacitor gives that result.
Regards

Z
 

I will try putting speedup capacitor and schottky diode clamp see if it works.
thank you
 

Unfortunately, the kind of switching problem is still unclear. Schottky clamps are very effective to guarantee unsaturated switching - for those problems where it's appropriate. You'll mostly find them in signal level switching, but 20 kHz won't be a problem even with saturated operation.
 

Basically, it will be helpful to speed up BJT switching by avoiding deep satuation.
 

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