What is a field-effect transistor? Several Selection Techniques for Field Effect Transistors

The Field Effect Transistor (FET) semiconductor is a type of transistor that uses an electric field to regulate the flow current. Transistors have three terminals: source, gate, and drain. FET regulates the flow of current by applying voltage to the gate, thereby changing the conductivity between the drain and source electrodes. Due to the involvement of single carrier class operations, field-effect transistors are also known as monopole transistors. In other words, electrons or holes are applied as charge carriers in the operation of field-effect transistors, but cannot be used together. There are many different types of field-effect transistors. At low frequencies, field-effect transistors typically exhibit very high input impedance. MOSFET (metal oxide semiconductor field-effect transistor) is the most widely used field-effect transistor. The selection of field-effect transistors may directly affect the efficiency and cost of the entire power supply. The following is a quick and accurate selection of field-effect transistors by electronic engineers, and share the following six selection methods.

Method for selecting field-effect transistors:

1. Channel type

The first step in selecting a field-effect transistor device is to measure the selection of N-channel or P-channel field-effect transistors. In typical power applications, when the field-effect transistor is grounded and the load is connected to the main voltage, the field-effect transistor forms a low-voltage side switch. N-channel field-effect transistors should be used in low-voltage side switches based on the voltage required to shut down or cut off the equipment. When the field-effect transistor is connected to the bus and load grounding, a high-voltage side switch should be used. P-channel field-effect transistors are commonly used in this topology due to voltage driven considerations.

2. Rated current

Determine the required rated current or maximum voltage that the device can accept. The higher the rated current, the higher the cost of the equipment. Based on practical experience, the rated current should exceed the main line voltage or total phase voltage. Only in this way can sufficient maintenance be provided so that the field-effect transistor does not become ineffective.

In terms of the selection of field-effect transistors, it is important to determine the maximum voltage that can be withstood between the drain electrodes, which is the larger VDS. It is important to understand that the maximum voltage that a transistor can accept varies with temperature. Everyone needs to detect the range of voltage changes throughout the entire working temperature range. The rated current must have sufficient capacity to cover the range of changes to ensure that the circuit will not fail. Other safety factors to consider include voltage transients caused by switching electronic products such as motors or transformers. The rated current used is also different; Generally speaking, portable devices are 20V, FPGA power supplies are 20-30V, and 85-220VAC450 applications are 600V.

3. Rated voltage

The rated voltage should be the maximum current that the load can withstand under all conditions. Similar to voltage, even if the device generates a peak current, the selected field-effect transistor can withstand the rated voltage. The two currents considered are continuous mode and pulse peak. The field-effect transistor is in a stable state in continuous on/off mode, and the current continues to flow according to the device. Pulse peak refers to the amount of surge (or peak current) flowing through the device. In these cases, once the maximum current is determined, simply select the device that can withstand the maximum current.

4. On Off Consumption

In fact, field-effect transistors are not ideal devices, as there may be electrical energy loss during the conduction process, which is known as on off consumption. When the field-effect transistor is "cut off", it is determined like a variable resistor (ON) made of device RDS and changes significantly with temperature. Iload2 can consume the power of the device × Due to the variation of the conduction resistance with temperature, the power consumption of RDS (ON) will gradually change proportionally. The higher the voltage VGS applied to the field-effect transistor, the opposite is RDS (ON), and the smaller is RDS; (ON) will be higher. Note that with a slight increase in current, the resistance of RDS (ON) will increase. Various electrical parameter changes of the resistance can be found in the technical data sheet provided by the manufacturer regarding RDS (ON).

5. System heat removal

Two different scenarios must be considered, namely the worst-case scenario and the real situation. It is recommended to use the worst-case value as it provides more safety margin to ensure that the system does not fail. There are still some measurement data to be noted on the data sheet of the field-effect transistor; The junction temperature of the equipment is equal to the high operating temperature, thermal resistance, and power loss (junction temperature=high operating temperature [thermal resistance × Power loss]. The maximum power loss of the system can be solved according to this algorithm, which is defined as equivalent to I2 × RDS (ON). Based on the maximum current of the device, we are about to calculate the RDS (ON) at different temperatures. In addition, heat removal work for circuit boards and field-effect transistors should also be done well.

Avalanche breakdown refers to the reverse voltage on a semiconductor device exceeding its maximum value, forming a strong electric field and increasing the current inside the device. The increase in chip size will enhance the ability to resist landslides and ultimately improve the stability of the device. Therefore, choosing more packaging components can effectively prevent landslides.

6. Switch characteristics

There are many parameter values that affect switch characteristics, but the most important ones are gate/drain, gate/source, and drain/source capacitance. Due to the need to charge them every time they switch, these capacitors will experience switching losses in the device. As a result, the switching speed of field-effect transistors decreases, and the equipment efficiency also decreases. In order to calculate the total loss of equipment during the switching process, it is necessary to calculate the consumption during the switching process (Eon) and the consumption during the closing process (Eoff). The total power of a transistor switch can be represented by the following equation: Psw=(Eon Eoff) × Switching frequency. The gate charge (Qgd) has a significant impact on the switch performance.

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