Must read! Summary of parameters of discrete semiconductor devices

Overview

In the field of electronic circuits, diodes, triodes and MOS tubes are key basic components for controlling current and voltage, and their selection is crucial. There are many parameters involved in the selection process, and the reasonable selection of these parameters is directly related to the performance, stability and cost-effectiveness of the circuit. Next, the selection parameters of these three components will be introduced in detail.

PART.1

Diode selection

When selecting a diode, the following key parameters should be considered:

Type: Choose the right type of diode based on actual application requirements. Common types include rectifier diodes, which are used to convert AC to DC; switching diodes, which have fast switching characteristics; voltage regulator diodes, which are used to stabilize circuit voltage; and light-emitting diodes (LEDs), which can convert electrical energy into light energy.

Maximum rectifier current (IF): For rectifier diodes, it is necessary to ensure that its maximum rectifier current is greater than or equal to the maximum forward current in the circuit to ensure that the diode is not burned during normal operation.

Reverse breakdown voltage (VRRM): This parameter represents the maximum reverse voltage that the diode can withstand. When selecting, ensure that this parameter is greater than or equal to the maximum reverse voltage in the circuit to prevent the diode from being damaged by breakdown due to excessive reverse voltage.

Forward voltage drop (VF): refers to the voltage drop across the diode under the action of forward current. For light-emitting diodes such as LEDs, the forward voltage drop is an important parameter and needs to be matched with the drive circuit to ensure that the LED emits light normally.

Reverse current (IR): At a specified temperature, when the diode is subjected to a specified reverse voltage, a certain reverse current will pass through. When selecting, it is necessary to ensure that the reverse current is within the specified range to avoid affecting the circuit performance due to excessive reverse current.

Junction temperature (Tj): the maximum allowable temperature of the diode junction. During the operation of the diode, it must be ensured that its temperature does not exceed the junction temperature limit, otherwise the diode performance may be degraded or even damaged.

Thermal resistance (Rth): This parameter describes the heat dissipation capacity of the diode. For high-power application scenarios, thermal resistance is a key indicator. Lower thermal resistance helps the diode to dissipate heat better and ensure its stable operation.

Frequency characteristics: In high-frequency applications, parameters such as the frequency response and reverse recovery time of the diode need to be considered. The frequency response determines the diode's ability to handle high-frequency signals, and the reverse recovery time affects the performance of the diode in high-frequency switching circuits.

Packaging type: According to the application environment and space constraints, select the appropriate packaging type, such as the plug-in type is suitable for manual welding and circuit boards with larger spaces, and the surface mount type is more suitable for automated production and compact circuit design.

Cost and availability: Under the premise of meeting performance requirements, the cost and market availability of the diode need to be comprehensively considered to maximize cost-effectiveness.

PART.2

Transistor selection

When selecting a transistor, you need to pay attention to the following important parameters:

Type (NPN or PNP): Select the appropriate transistor type according to the circuit design and required functions. NPN and PNP transistors have different connection methods and current flow directions in the circuit.

Maximum voltage and current: VCEO (collector-emitter open circuit voltage) indicates the maximum voltage that the transistor can withstand in the open circuit state; VCBO (collector-base open circuit voltage) is the maximum voltage between the collector and the emitter when the base is open; VEBO (emitter-base open circuit voltage) is the maximum voltage between the collector and the base when the emitter is open; ICM (maximum collector current) is the maximum continuous collector current that the transistor can withstand. These parameters need to be reasonably selected according to the voltage and current conditions in the circuit.

Gain hFE (DC current transfer ratio): This parameter indicates the ratio of the base current change to the collector current change, which directly affects the amplification factor of the transistor. In the amplification circuit, the appropriate gain needs to be selected according to the amplification requirements.

Frequency response fT (characteristic frequency): refers to the frequency point at which the transistor's amplification ability begins to decline. For high-frequency applications, it is necessary to select a transistor with a higher characteristic frequency to ensure that it can still maintain good amplification performance under high-frequency signals.

Power consumption PD (dissipated power): indicates the maximum power that the transistor can safely dissipate under specific conditions. When selecting, it is necessary to ensure that the power consumption of the transistor meets the circuit requirements to avoid damage due to overheating.

Package and size: According to the circuit board space and heat dissipation requirements, select the appropriate package type, such as TO-92 package is small in size and suitable for small circuits; TO-220 package has good heat dissipation performance and is often used in high-power circuits.

Temperature characteristics: Tj (junction temperature) is the highest junction temperature at which the transistor can safely work, and Ta (ambient temperature) is the ambient temperature range when the transistor works normally. When selecting, it is necessary to ensure that the transistor can work stably within the expected operating temperature range.

Stability and reliability: Especially in harsh environments such as high humidity and high temperature, it is necessary to consider the long-term stability and reliability of the transistor and select products with reliable quality.

Cost: According to the budget and performance requirements, choose transistors with high cost performance to reduce costs while meeting the circuit requirements.

PART.3

MOS tube selection

When selecting a MOS tube, the following parameters should be considered:

Type (NMOS or PMOS): Select the appropriate MOS tube type according to the circuit design and required functions. NMOS and PMOS have different conduction characteristics and connection methods in the circuit.

Voltage parameters: The rated voltage (Vdss) is the maximum drain-to-source voltage that the MOSFET can withstand. When selecting, ensure that it is higher than the maximum expected voltage in the circuit; the gate threshold voltage (Vgs(th)) is the minimum gate-to-source voltage required to turn on the MOSFET. The appropriate threshold voltage is critical to ensure that the MOSFET is turned on under the required conditions.

Current parameters: The continuous drain current (Id) is the maximum current that the MOSFET can continuously pass under normal operating conditions. It is necessary to ensure that the Id of the selected MOSFET is greater than the maximum expected current in the circuit; the pulse drain current (Ipulse) is for applications that need to handle short-term high currents. Consider the pulse current tolerance of the MOSFET.

Resistance parameters: On-resistance (Rds(on)) is the resistance of the MOSFET in the on state, which directly affects its heat dissipation and efficiency. Lower Rds(on) usually means higher efficiency and lower temperature rise, but it may also be accompanied by higher costs.

Thermal parameters: Maximum junction temperature (Tj) is the highest junction temperature at which the MOSFET can safely operate. It is necessary to ensure that the Tj of the selected MOSFET is higher than the expected operating temperature; thermal resistance (Rth) is the thermal resistance of the MOSFET from the junction to the environment, which is crucial for evaluating the heat dissipation capability of the MOSFET.

Switching characteristics: Switching time includes rise time, fall time, etc. These parameters are very important for applications that require fast switching; input capacitance (Ciss, Coss, Crss) affects the switching speed of the MOSFET and the required drive power.

Packaging: Selecting the appropriate package can ensure that the MOSFET is suitable for specific application scenarios and environmental conditions, and it will also affect its heat dissipation performance and mechanical strength.

Reliability: Consider the failure mode and expected life of the MOSFET to ensure that the selected product is suitable for long-term applications.

Cost: The most cost-effective MOSFET should be selected based on budget while meeting performance requirements.

PART.4

Summarize

In the process of transistor selection, it is necessary to closely combine the specific application scenarios and circuit requirements and comprehensively consider the above parameters. Only in this way can we ensure that the selected transistor meets the design requirements and has good performance and reliability. At the same time, be sure to refer to the transistor's data sheet and technical specifications to ensure the accuracy of the selection and achieve correct use. Our company represents well-known domestic brands, and friends from all walks of life are welcome to consult!