- BJT as a Switch Definition: A BJT (bipolar junction transistor) is defined as a device that acts as a switch by controlling the base-emitter current to change the emitter-collector resistance.
- Regions of Operation: BJTs operate in three regions: cutoff (switch is OFF), saturation (switch is ON), and active (not used for switching due to high power loss).
- Cutoff and Saturation Regions: For switching, BJTs are used in cutoff and saturation regions to ensure minimal power loss.
- Transition Period Power Loss: Maximum power loss occurs during transitions between ON and OFF states, requiring heat sinks to manage heat dissipation.
- Load and Voltage Considerations: The transistor must handle the load current in the ON state and withstand the open circuit voltage in the OFF state to prevent damage.
A BJT is used as a switch by setting the base-emitter current so the collector-emitter path looks open or closed. OFF is an open circuit (very high resistance). ON is a short (very low resistance).
A transistor characteristic plot has three regions:
- Cutoff Region
- Active Region
- Saturation Region
In the active region IC stays nearly constant over a wide VCE. That current times voltage is wasted heat, so the active region is not used for switching. An ideal OFF switch carries no current, so it dissipates nothing.
An ideal ON switch has zero voltage across it, so it also dissipates nothing. A BJT switch is therefore biased so ON and OFF dissipation stay very low.
Those low-loss points are the edges of the characteristic: cutoff and saturation. The same two regions apply to npn transistors and to pnp transistors.
In the figure, when base current is zero, the collector current (IC) has very small constant value for a wide range of collector-emitter voltage (VCE). So when the transistor is operated with base current ≤ 0, the collector current (IC ≈ 0) is very tiny, hence the transistor is said to be in OFF condition but at the same time, power loss across the transistor switch i.e. IC × VCE is negligible because of very tiny IC.
The transistor is connected in series with an output resistance RC. Hence, current through the output resistance is
If the transistor is operated with a base current IB3 for which collector current is IC1. IC is less than IC1, then the transistor is operated in the saturation region. Here, for any collector current less than IC1, there will be very tiny collector-emitter voltage (VCE < VCE1). Hence in this situation, the current through the transistor is as high as load current, but the voltage across the transistor (VCE < VCE1) quite low, hence power loss in the transistor is negligible.
The transistor behaves as an ON switch. So for using the transistor as a switch we should make sure that the applied base current must be sufficiently high to keep the transistor in the saturation region, for a collector current.
So, from the above explanation, we can conclude that bipolar junction transistor behaves as a switch only when it is operated in the cutoff and saturation region of its characteristic. In switching application, the active region or active region of characteristics is avoided. As we already told, the power loss in transistor switch is very low but not zero. So, it is not an ideal switch but accepted as a switch for specific applications.
When you choose a transistor as a switch, stay inside its ratings. ON, it must carry the full load current. More than the rated collector current overheats the die. OFF, it must stand the open-circuit load voltage without breakdown. A heat sink is needed for the remaining heat. The device also takes a finite time to change state.
Though the switching time is very brief, often less than a few microseconds, it is not zero. During the ON switch period, the current (IC) increases while the collector-emitter voltage (VCE) decreases towards zero. There is a moment when both current and voltage are at their maximum, causing peak power loss. This also occurs when switching from ON to OFF. Maximum power loss happens during these transitions, but the energy dissipated is moderate due to the short transition period. At low frequencies, heat generation is manageable, but at high frequencies, significant power loss and heat occur.
This is to be noted that, heat generation does not occur only during transient condition also during steady ON or OFF condition of the transistor but the amount of heat during steady condition is quite small and negligible.
