Thermal runaway is a phenomenon that occurs in electronic components, including transistors, when their temperature rises uncontrollably due to excessive power dissipation. This increase in temperature can lead to further increase in power dissipation, creating a positive feedback loop that can ultimately result in the destruction of the component.
To understand the relationship between thermal runaway and Ohm's Law in transistors, let's first briefly explain Ohm's Law:
Ohm's Law states that the current (I) flowing through a conductor between two points is directly proportional to the voltage (V) across the two points, and inversely proportional to the resistance (R) of the conductor. Mathematically, Ohm's Law is expressed as:
V = I * R
Now, let's consider transistors:
Transistors are semiconductor devices used in electronic circuits to amplify or switch electronic signals. They have three terminals: the emitter, the base, and the collector. In a typical NPN transistor, the current flowing from the emitter to the collector (Ic) is controlled by the current flowing from the base to the emitter (Ib).
When a transistor operates in its normal range, the relationship between Ic and Ib is well-defined and controlled by the transistor's characteristics and the external circuitry. However, if the transistor experiences a sudden increase in temperature, its characteristics can change.
Here's where Ohm's Law comes into play:
Collector Current (Ic) and Base Current (Ib):
As the temperature of a transistor increases, its internal resistance can change, altering the relationship between Ic and Ib. The transistor's datasheet provides information on how the current levels should be controlled under different temperatures to avoid thermal issues. If the relationship between Ic and Ib changes significantly due to increased temperature, it can lead to unexpected and uncontrolled behavior, including excessive collector current.
Power Dissipation (P):
When a transistor operates, it dissipates power in the form of heat. This power dissipation is given by:
P = Vce * Ic
Where Vce is the voltage between the collector and the emitter. If the transistor is not properly biased or controlled, excessive power dissipation can occur, leading to a temperature increase.
Thermal Runaway:
The increase in temperature caused by power dissipation can, in turn, affect the transistor's resistance and current characteristics. If the change in temperature further increases the collector current (Ic), it will lead to even higher power dissipation. This positive feedback loop can result in a continuous rise in temperature, potentially reaching a point where the transistor fails or gets permanently damaged.
To prevent thermal runaway in transistor circuits, proper design, biasing, and heat management are crucial. By ensuring that the transistor operates within its specified safe operating limits, you can avoid the adverse effects of thermal runaway and ensure reliable and stable performance.