Precautions for transistor replacement

Precautions for transistor replacement

Types and materials of transistors:

Firstly, we need to understand the types and materials of transistors. There are two types of commonly used transistors: NPN and PNP. Due to the different polarity requirements for voltage between these two types of transistors, they cannot be replaced with each other.

The transistor materials include germanium material and silicon material. The biggest difference between them is the starting voltage. The conductivity voltage of germanium diode PN junction is about 0.2V, while the conductivity voltage of silicon diode PN junction is 0.6~0.7V. In amplification circuits, the same type of germanium diode can be used instead of the same type of silicon diode, and the same type of silicon diode can also be used instead of the same type of germanium diode. However, due to different starting voltages, the base bias voltage must be adjusted. But it is necessary to analyze in detail whether the transistors made of different materials in pulse circuits and switch circuits can be replaced blindly.

The main parameters of a transistor are:

Choosing a transistor requires understanding the main parameters of the transistor. It's best to have a transistor characteristic manual. There are many parameters for a transistor. Based on practical experience, I believe that ICM, BVCEO, PCMICM, BVCEO, PCM, and FT can meet over 95% of usage needs.

ICM is the maximum allowable current of the collector. When the transistor is working, the current amplification factor β will decrease when the collector current exceeds a certain value. Therefore, when the current amplification factor β of the transistor does not exceed the allowable value, the maximum current at the collector is called ICM. Therefore, when the collector current IC exceeds ICM, it will not damage the transistor, but will reduce the beta value, affecting the performance of the circuit.

BVCEO is the reverse breakdown voltage between the collector and emitter when the transistor base is open circuited. If the voltage between the collector and emitter exceeds this value, the transistor may generate a large collector current, which is called breakdown. The breakdown of a transistor can cause permanent damage or performance degradation.

PCM is the maximum allowable dissipated power in the collector. When a transistor is working, the collector current generates heat at the collector junction, heating the transistor. If the dissipated power is too high, the transistor will burn out. If the transistor operates above PCM for a long time, the transistor will be damaged. It should be noted that the maximum allowable dissipated power given by the high-power transistor is a parameter when it comes to the heat sink of the high-power transistor grid. Be sure to pay attention to this when using it.

4. Characteristic frequency ft. As the operating frequency increases, the amplification capability of the transistor decreases, and the frequency ft corresponding to β=1 is called the characteristic frequency of the transistor.

Generally, low-power transistors are used:

Low power transistors are widely used in electronic circuits. Mainly used for small signal amplification, control, or oscillator. When choosing a transistor, the first thing to understand is the operating frequency of the electronic circuit. For example, the highest frequency of the oscillator in a medium wave radio is about 2mHz; The maximum oscillation frequency of FM radio is about 120mHz; The highest oscillation frequency in the VHF band is about 250mHz; The highest oscillation frequency in the UHF band is approximately 1000MHz. Engineering design generally requires that the actual operating frequency of the transistor FT be three times or more. Therefore, the characteristic frequency ft of the transistor can be selected according to this requirement. FT parameters cannot be considered in audio electronic circuits, as high-frequency transistors made of silicon materials typically have a FT of no less than 50mHz.

Select the low-power transistor BVCEO based on the power supply voltage of the circuit. Generally speaking, as long as the BVCEO of the transistor is greater than the highest voltage of the power supply in the circuit. When the transistor load is an inductive load, such as a transformer, coil, etc., The selection of BVCEO value should be cautious, as the induced voltage on inductive loads can reach 2-8 times the power supply voltage (such as the boost transistor in energy-saving lamps). Generally speaking, the BVCEO of low-power transistors is not less than 15V, so this parameter does not need to be considered in low-voltage circuits without inductive components.

Generally speaking, the ICM of low-power transistors is between 30-50ma, and small signal circuits are not considered. But it is necessary to carefully calculate the pipelines that drive the relays and high-power speakers. Of course, the first thing to know is how many milliamps the relay's pull in current is in order to determine the ICM of the transistor.

When estimating the operating current (i.e. collector current) of a transistor in a circuit and the voltage between the collector and emitter of the transistor, we can calculate the maximum allowable dissipated power PCM of the transistor collector based on P=U × I.

There are many models of low-power transistors produced domestically and internationally, some with the same parameters and some with different ones. According to the above analysis of usage conditions, BVCEO high transistor can replace BVCEO low transistor; ICM large transistor can replace ICM small transistor, The transistor can be freely applied.

Choose high-power transistors:

For high-power transistors, as long as they are not high-frequency transmission circuits, there is no need to consider the characteristic frequency ft of the transistor. The limit parameters of BVCEO are the same as those of low-power transistors. The selection of the maximum allowable current ICM for the collector is mainly calculated based on the load of the transistor. The maximum allowable dissipated power PCM of the collector of a transistor is a key issue for high-power transistors. It should be noted that high-power transistors must have good heat sinks. Even a high-power transistor of 40-50 watts can only withstand power dissipation of two to three watts without a heat sink. The selection of high-power transistors should also have sufficient margin. In addition, when selecting high-power transistors, their installation conditions should also be considered to determine whether to use plastic sealed diodes or metal sealed diodes.

If the parameters of a transistor cannot be found, their parameters can be inferred based on their shape. At present, the most common low-power transistor is TO-92 packaged plastic sealed diode, and some are packaged in metal shell. PCM is generally between 100~500mW, with the maximum not exceeding 1W. ICM is generally between 50~500mA, with a maximum not exceeding 1.5A. Other parameters are difficult to determine