Really
Newbie level 1
Hello,
And thank you to anyone who wants to take a stab at this one.
I would like to use an npn bjt to conduct in forward mode and then block or approximately block current in reverse active mode when the base will be switched to the emitter. My circuit will be switching the base terminal to the emitter terminal so that they are almost shorted(except for the base resistor), then applying the negative voltage across the collector emitter. The voltages present will exceed 25Volts, well above the emitter base reverse breakdown voltage, and forward currents will be less than 2 amps for very short pulses with a low duty cycle.
My questions are:
A.) First, how to protect the emitter-to-base junction voltage from exceeding maximum voltage when that diode is reverse biased and the bjt is in the reverse active region. My concern is that since the base collector diode will be forward biased, that almost the full emitter to collector voltage will be seen across the emitter base junction, and there is nothing I can do about it since these connections are all internal. I.e., I can't add resistors to limit current/voltages. Do the datasheet max Vebo values apply here, or is there something I can do about it?
B.) Second, how to obtain sufficiently low current and gain from emitter to collector when in the reverse active mode. Yes, I said LOW. Ideally it would be completely off, but my hope is that the bjt asymmetry would be great enough that I might be able to get away with the resulting "low" beta hence low reverse current.
To clarify a bit, here are the basic steps that the circuit will be performing and to which the above questions pertain. In order:
1.) NPN is conducting in forward active or saturation.
2.) Base is "shorted" to emitter terminal. The external base resistor exists between the base and emitter, so not a
true short.
3.) A negative voltage is applied across the collector and emitter. It is in this phase that I am worried about
exceeding the Vebo voltage, and it is also the phase in which I would like to have no current or small
currents(positive) flowing from emitter to collector. Placing a diode in series with the bjt is not an option for me.
Thank you in advance,
Mark
And thank you to anyone who wants to take a stab at this one.
I would like to use an npn bjt to conduct in forward mode and then block or approximately block current in reverse active mode when the base will be switched to the emitter. My circuit will be switching the base terminal to the emitter terminal so that they are almost shorted(except for the base resistor), then applying the negative voltage across the collector emitter. The voltages present will exceed 25Volts, well above the emitter base reverse breakdown voltage, and forward currents will be less than 2 amps for very short pulses with a low duty cycle.
My questions are:
A.) First, how to protect the emitter-to-base junction voltage from exceeding maximum voltage when that diode is reverse biased and the bjt is in the reverse active region. My concern is that since the base collector diode will be forward biased, that almost the full emitter to collector voltage will be seen across the emitter base junction, and there is nothing I can do about it since these connections are all internal. I.e., I can't add resistors to limit current/voltages. Do the datasheet max Vebo values apply here, or is there something I can do about it?
B.) Second, how to obtain sufficiently low current and gain from emitter to collector when in the reverse active mode. Yes, I said LOW. Ideally it would be completely off, but my hope is that the bjt asymmetry would be great enough that I might be able to get away with the resulting "low" beta hence low reverse current.
To clarify a bit, here are the basic steps that the circuit will be performing and to which the above questions pertain. In order:
1.) NPN is conducting in forward active or saturation.
2.) Base is "shorted" to emitter terminal. The external base resistor exists between the base and emitter, so not a
true short.
3.) A negative voltage is applied across the collector and emitter. It is in this phase that I am worried about
exceeding the Vebo voltage, and it is also the phase in which I would like to have no current or small
currents(positive) flowing from emitter to collector. Placing a diode in series with the bjt is not an option for me.
Thank you in advance,
Mark