Transistor limit parameters

The following introduces the main limiting parameters of transistors. The voltage, power dissipation and current that a transistor can withstand are all limited. When they exceed the rated value, the working performance of the transistor will be affected at best, and it will be damaged at worst.

1. Maximum allowable collector power dissipation Pcm
Pcm refers to the maximum power consumed by the collector when the change in the parameters of the transistor due to temperature rise does not exceed the specified value. When the transistor is working normally, the collector junction is applied with a reverse bias voltage, and the reverse resistance of the collector junction is very high. In this way, a large amount of heat will be generated when the collector current flows through the collector junction, and the junction temperature will rise. If the temperature is too high, it will cause irreversible damage to the transistor. People determine the maximum allowable power dissipation based on the maximum allowable junction temperature of the transistor. In order to reduce the junction temperature, for high-power transistors, people often have to set up a heat sink. The larger the surface area of ​​the heat sink, the better the heat dissipation effect, and the Pcm of the transistor can be appropriately increased.

2. The maximum allowable collector current Icm
The increase of collector current will cause the current magnification factor β of the transistor to decrease. When β drops to the rated multiple of the low-frequency current magnification factor βo (usually specified as one-half or one-third), the collector current at this time is called the maximum allowable collector current Icm. Therefore, when the collector current of the transistor reaches Icm, the transistor will not be damaged, but the current magnification factor has been greatly reduced.

3. Collector-emitter breakdown voltage BVCEO
BVCEO refers to the maximum allowable voltage between the collector and emitter of a transistor when the base of the transistor is open. For an NPN transistor, the collector is connected to the positive pole of the power supply, and the emitter is connected to the negative pole of the power supply; for a PNP transistor, the collector is connected to the negative pole of the power supply, and the emitter is connected to the positive pole of the power supply.

When the voltage applied between the collector and emitter of the transistor is greater than the value of BVCEO, the current flowing through the transistor will suddenly increase, causing permanent damage to the transistor. This phenomenon is called breakdown.