Calculating Vebo on a high side switch with NPN+PNP: 5v->12v

Other Hardware General Electronics
1

(See attached circuit diagram)

I am building a high-side switch using an NPN and PNP transistor. I will be driving a 12v load from an arduino 5v output pin. I am unsure of how to calculate the Vebo the NPN, Q1, and the PNP, Q2, will be exposed to in this circuit.

For Q2:
If I am understanding Vebo correctly I think the Vebo I need for Q2, the PNP, is 12v since the emitter is connected to +12v and the base is connected to GND at its base.

For Q1:
If I am understanding this correctly the Vebo at Q1 is 0, since GND is on the emitter and the potential on the base is 0v when the output pin of the arduino is LOW.

The problem:
Most BJT PNP transistors I find have a Vebo well below 12v, usually in the neighborhood of 5v, which makes me think they can't be used to drive more than a 10v load using a 5v base, but that just doesn't seem correct.

My questions:

  1. Do I understand correctly how to calculate Vebo in a circuit?
  2. If so, how is one to drive a load 6, 7, or even more volts above their base voltage using BJT's?

Screen Shot 2016-05-02 at 9.27.18 PM.png

2

You don't understand what this parameter means in the data sheet. It is the voltage on the actual base. When the transistor is on it will be 0.7V because of the transistor action. The resistors prevent any further voltage increase as they drop the excess voltage. You are going to have to worry about the base current much more than the base voltage.

3

I would do this:

  1. Find/measure the load current. Q2 drops ~0.5volt across EC when saturated, so the load gets ~11.5volt with a BJT.

  2. Calculate R2/base current for Q2 (Q2 drops ~0.7volt across EB, and Q1 drops ~0.5volt across CE, so calculate for 10.8volt).
    Base current must be ~1/10 for big loads and big/old power transistors.
    But can be ~1/20 for small loads and small transistors.
    Saturation is important here to keep Q2 temp down.

  3. Calculate R1 for 1/20 to 1/40 of Q2 base current (Q1 drops ~0.7volt across BE, so calculate for 4.3volt).

  4. Use a 10k 'bleed' resistor for R3.

Post a diagram with parts values, so we can check.
Leo..

4

Hi

Vebo is Base Emitter Open Collector Breakdown Voltage.

Emitter voltage wrt Base, open Collector

Not the forward conducting voltage
Vebo is not calculated but stated as a parameter.

Vceo is Collector voltage wrt Emitter, open Base
Vcbo is Collector voltage wrt Base, open Emitter

But I would think the OP means the forward bias voltage, ie 0.6 or 0.7, depending on what University of Life you went to.

Tom...

5

ie 0.6 or 0.7, depending on what University of Life you went to.

Or 0.3V if you were brought up on germanium.

6
  1. Do I understand correctly how to calculate Vebo in a circuit?

In the circuit you posted, the base-emitter junctions will never be reverse biased, so you don't have to worry about the Vebo parameter.

7

In circuits where the emitter-base junction can be reverse biased its very important, as its
v. easy to fry the whole device if you breakdown the emitter-base junction.

However most circuits you'll see (especially switching circuits), this isn't an issue (except for
very high current switching where emitter stray inductance can lead to reverse Vbe).