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# Max peak current of bipolar transistor?

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#### MacIntoshCZ

##### Active Member
Hello is there any limit of peak current when driving capacitive load?
Peak currents in datasheets are in ms. I am switching mosfet gate capacitence.
Its okay to peak currents (5A) with 2n3904? In range of 100nS max..

It will stress the part but the answer depends on many factors - operating frequency, rise time of the signal to bjt base, gate capacitance, internal resistance of the gate capacitance (usually 0.5 to 5 ohms as ESR) to get gate charge time and, with frequency (above) you can estimate power dissipation into the device assuming everything is well under the thermal time constant of the transistor case and bond-wire connections.

however, little harm is done adding the appropriate resistor to limit current to 100mA (or 200mA depending on manufacturer). It is a good idea to put a 10ohm to 50 ohm resistor in series with the gate to cut down potential EMI.

Current gain peaks at 10mA collector current. Then drops to 1/10 at 200mA. There is a current limit function in the 2n3904 where is you sent 3mA into the base the Collector will open up at 100mA.
I have used the part to drive medium MOSFETS's gates. I don't think you can get 5A or near 5A.
If you drive Base with 20mA the part starts to open up at 200mA (gain of 10) With gain set to 10 the transistor will be slow.

Current gain peaks at 10mA collector current. Then drops to 1/10 at 200mA. There is a current limit function in the 2n3904 where is you sent 3mA into the base the Collector will open up at 100mA.
I have used the part to drive medium MOSFETS's gates. I don't think you can get 5A or near 5A.
If you drive Base with 20mA the part starts to open up at 200mA (gain of 10) With gain set to 10 the transistor will be slow.
Base current should be 500mA at peaks( when ideal switchinng appears - instant)

I was asking only if its possible to have high current peaks.
I will try to make pcb for my new gdt. Still trying new ways =)
Thanks for help ronsimpson, gophert and others.
VCC 15V; PWM 3V, generator unable to sink current

Current gain peaks at 10mA collector current. Then drops to 1/10 at 200mA. There is a current limit function in the 2n3904 where is you sent 3mA into the base the Collector will open up at 100mA.
I have used the part to drive medium MOSFETS's gates. I don't think you can get 5A or near 5A.
If you drive Base with 20mA the part starts to open up at 200mA (gain of 10) With gain set to 10 the transistor will be slow.
What should be value of gain? I missed probably some graph in datasheet. I know that lower Ic/Ib means faster switching. I though that hfe 10 should be enough. So i was wrong hmm

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There are many kinds of Gate drive ICs. Here is an example where you can pic inverting or non-inverting.

Here is an example where, by using diodes you can delay the on but not delay the off signal. You need to turn off one MOSFET before you turn on the other.

There are many kinds of Gate drive ICs. Here is an example where you can pic inverting or non-inverting.
View attachment 128480
Here is an example where, by using diodes you can delay the on but not delay the off signal. You need to turn off one MOSFET before you turn on the other.
View attachment 128482
So i need to implement some zero cross detection for VGS. Can it turn on "immediately" after zero cross or its some extra time needed - like 10ns or so?

Search "shoot through protection circuit". You can easy have both transistors on for 50nS. The current will be very high. It is a good idea to turn off transistor A, 100nS, turn on transistor B.

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anyway to speed up turn off time of q1?

looks like uselles circuit.
when frequency changes its not working.

eh a i forgot baker clamp diode...

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