Condition to eliminate thermal run away in BJT

Electronic Devices & Circuits
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The condition to eliminate the thermal runaway in BJT is to bias the transistor in such a way that the Vce < Vcc/2, where Vcc is the DC power supply, Vce is collector to emitter voltage. This is derived on the fact that the rate at which heat is removed from the junction should be more compared to the rate at which heat is dissipated at the junction.

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Bipolar Junction Transistors (BJTs) can suffer from a phenomenon known as thermal runaway, which is a self-reinforcing condition where the BJT begins to conduct too much current, leading to increased heat, which in turn increases current flow, and so on. This can quickly lead to the destruction of the transistor.

BJT Thermal Tunaway

The key conditions required to prevent or eliminate thermal runaway include:

  1. Negative Temperature Coefficient: When the transistor is designed so that the collector current decreases as the temperature increases, this is known as a negative temperature coefficient. This can help prevent thermal runaway, as an increase in temperature will result in a decrease in current.
  2. Thermal Stability: The transistor should have proper heat dissipation mechanisms in place, such as heat sinks, to carry away excess heat and prevent the device from becoming too hot. Adequate ventilation and cooling should also be provided.
  3. Biasing: Proper biasing of the transistor can help to stabilize the operating point and reduce the risk of thermal runaway. A common technique is to use emitter degeneration, where a small resistor is inserted in the emitter lead of the BJT, providing negative feedback and stabilizing the transistor.
  4. Safe Operating Area (SOA): It is essential to operate the transistor within its defined safe operating area, which is specified by the manufacturer. The SOA takes into account parameters such as collector-emitter voltage, collector current, and power dissipation. Operating within the SOA will prevent the device from reaching a state of thermal runaway.
  5. Temperature Compensation: Circuits can be designed with temperature compensation mechanisms to counteract the temperature effect on the BJT. This might include diodes or resistors that are also sensitive to temperature and can provide a counteracting bias as temperature changes.

It’s important to remember that thermal management is a critical part of electronic device design and that careful consideration should be given to how devices like BJTs are used in a circuit to avoid situations that could lead to thermal runaway.