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How can I make transistor switch faster?

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deniz88

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How can we make transistor switch faster

Hi ,

I have following circuit, my purpose is to control current level digitally. Current rise time is like 100ns. It should be between 10-20ns
Any ideas how can i do it? Thanks a lot for helps
(negative current here).
(C1 represents the load).

best regards
 

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Re: How can we make transistor switch faster

10uF is a lot of capacitance. To charge it faster then its charging current must be increased.
But your graph shows a 50ns delay before anything happens which is close to the typical on time for the ZXTP2012 transistor on its datasheet.
 

Re: How can we make transistor switch faster

hi, thank you.
i used 10uf just to see max current can flow. all of the graph looks like that discharge_current.PNG

i am trying to reduce rise time to increase current from 3A to 3,5A (values are just sample). it needs 100 ns to rise from 3 to 3,5A, i am trying to make it between 10-20 ns.
 

Re: How can we make transistor switch faster

I'm missing a serious analysis of circuit behavior. You have all options to find out how the rise time is limited by the circuit and which component(s) and structure(s) are responsible for it.

Secondly, how's the particular circuit structure motivated?

In case of doubt, I would suggest faster transistors.
 

Re: How can we make transistor switch faster

Normal way is to minimise storage effects by not saturating or over over driving the transistor by using catching diodes.
I have to admit I don't understand your circuit, Q4 is a constant current generator? and Q5 is a variable load? So you are trying to discharge the capacitor (from -5V to -6V, I think in positive charge), by sucking the charge out via Q5 collector. You could try using a speed up capacitor to to the base of Q4 to reduce its Ic, so Q5 can handle the current faster.
Frank
 

Re: How can we make transistor switch faster

Hi FvM,

thanks for your message. Actually i found fastest BJTs i could. this on the picture is negative part of the circuit to discharge, and there is also positive part of the circuit to charge the load.

- - - Updated - - -

Hi chuckey,

thanks for the suggestions, i will try. on the other side there is similar circuit to charge the load, main current flows through the Q4 , it is also digitally controlled. positive part also has to be fast and charge step by step

actually voltage change will be from 15V to -5V.
when it is discharging, Q4 current is 0 already, it is turned off.
best regards
 

Re: How can we make transistor switch faster

Yse a High Frequency RF Transistor.
 

Re: How can we make transistor switch faster

Simple 1st effects are:
from Ic=CΔv/Δt or
Δt = CΔv / Ic

thus for Δt = 100 ns, C= 10 uF , let ΔV = 1 V, Ic= 100 Amps.. Is that what you used?

Also Consider ESR of 10 uF will have ultra low ESR*C product of 0.1 us best case and more likely 1us. Which do you have?

So to reduce Δt, now you can see the 1st order effects.

To improve you need to decouple 10 uF with a small choke and use a smaller C with a 100 MHz SRF in parallel to charge up quicker while the 10 uF takes longer. or use a cap rated for higher ripple current like teflon if you can afford it with very low ESL.
 

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