2n3906 pulse current?

[Mosaic]

Hi, i am using the 2n3906/4 pair as a totem pole FET driver. Spice sims the peak pulse current @ 365mA to drive the NFET gate via a 10 Ohm gate resistor. This peaks past the 200mA continuous rating of the transistors,
(incl rise/fall) for about 400nS.
The switching frequency is 4Khz making for a 0.16% DCycle pulse.


Is this a sustainable design? I can't find pulse specs. for the transistors.

 

[crutschow]

The transistor will likely carry that peak current without a problem, but if you are concerned about reliability, I would use a transistor with a higher peak current rating.

 

[Mr RB]

Use a higher value gate resistor, like 15, 18 or 22 ohms. The fractionally slower switch times of the FET will not matter when you are running at a real low frequency like 4kHz. That will also reduce stresses on the parts, and reduce radiated noise etc.

 

[crutschow]

Mr RB has a good idea. I would go with that.

 

[ronv]

If it is a totem pole it is probably "shoot thru" at the point where one transistor turns on before the other is fully off. Most totem poles have some resistance to limit the current. Assuming 5 volts you could add 27 ohms between the 2 transistors to limit the current or you could beta limit it by making the base resistors bigger, say 4.7k.

 

[audioguru]

Most FET driver circuits are complementary emitter-followers that do not produce shoot-through currents.
A 2N3904 and 2N3906 have poor current gain above only 100mA.