Six choice tips for field effect transistors
Published time: 2019-08-20
With the speed of upgrading electronic devices, the standards for the performance of electronic devices are getting higher and higher. In the circuit design and development of some electronic devices, not only in the switching power supply circuit, but also in the circuit of the portable electronic device. All of them will use better performance electronic components - field effect transistors. Therefore, the correct selection of field effect transistors is one of the most difficult problems that hardware engineers often encounter. It is also an extremely important one. The selection of field effect transistors has It may directly affect the rate and manufacturing cost of a whole integrated op amp. The following six techniques can be used to facilitate electronic engineers to quickly and accurately select FETs.
1, channel type
The first step in selecting a good FET electronic component is to choose an N-channel or P-channel FET. In typical power usage, when a field effect transistor is grounded and the load is applied to the mains voltage, the field effect transistor forms a low side switch. In low-side switches, N-channel field-effect transistors should be used, which are derived from the voltage required to turn off or turn on the electronic components. When the FET is connected to the bus and the load is grounded, a high side switch is required. P-channel FETs are typically used in this topology, again for voltage drive considerations.
2, rated voltage
Determining the required voltage rating or the highest voltage that the electronic component can carry. The higher the rated voltage, the higher the cost of the electronic components. According to practice, the rated voltage should be higher than the mains voltage or bus voltage. This provides sufficient protection so that the FET does not malfunction.
For the selection of field effect transistors, it is important to know the highest voltage that will be carried between the drain and the source, ie the maximum VDS. It is critical to understand that the highest voltage that a field effect transistor can carry will vary with temperature. We must detect the range of voltage variation over the entire operating temperature range. The rated voltage must have sufficient margin to cover this range of variation, so that the circuit will not be ineffective. Other safety factors to consider include voltage transients caused by switching electronics such as motors or transformers. The rated voltages for different uses are also different; in general, portable devices are 20V, FPGA power supplies are 20 to 30V, and 85 to 220VAC applications are 450 to 600V.
3, rated current
This rated current should be the highest current that the load can carry under all conditions. Similar to the voltage case, the selected field effect transistor is guaranteed to withstand this current rating even when the system is causing spike currents. The two considered current conditions are the continuous mode and the pulse spike. In the continuous conduction mode, the field effect transistor is in a steady state, at which time the current continues through the electronic components. A pulse spike refers to a large amount of surge (or spike current) flowing through an electronic component. Once the highest current under these conditions is specified, simply select the electronic component that can carry the highest current.
4, conduction loss
In practical situations, field effect transistors are not necessarily ideal electronic components. This is called conduction loss due to the power consumption during the conduction process. A field effect transistor is like a variable resistor when it is "on", which is confirmed by the RDS(ON) of the electronic component and varies significantly with temperature. The power loss of an electronic component can be estimated by Iload2 × RDS(ON), because the on-resistance varies with temperature, and the power loss alsovaries with the ratio. The higher the voltage VGS applied to the field effect transistor, the smaller the RDS(ON); otherwise the higher the RDS(ON). Note that the RDS(ON) resistance will increase slightly with current. The various electrical parameter variations for RDS(ON) resistors are available in the manufacturer's technical data sheet.
5, system cooling
Two different situations must be considered, namely the worst case and the specific situation. It is proposed to use the worst-case calculations, which provide a greater margin of safety and ensure that the system is not susceptible to failure. There are some measurement data that must be paid attention to in the material table of the field effect transistor; the junction temperature of the electronic component is equivalent to the product of the maximum ambient temperature reheating resistance and power dissipation (junction temperature = maximum ambient temperature + [thermal resistance × power dissipation] ]). According to this formula, the maximum power loss of the system can be solved, that is, equivalent to I2 × RDS (ON) by definition. We are already estimating the RDS(ON) at different temperatures by the maximum current of the electronic components. In addition, the heat dissipation of the board and the FET should be done well.
Avalanche breakdown refers to the fact that the reverse voltage on the semiconductor device exceeds the highest value and generates a strong electric field to increase the current in the electronic component. An increase in the size of the wafer enhances the avalanche resistance and ultimately improves the robustness of the electronic components. Therefore, selecting a larger package can effectively avoid avalanche.
6, switching performance
There are many parameters that affect the performance of the switch, but the most critical are the gate/drain, gate/source and drain/source capacitances. These capacitors create switching losses in the electronics because they are charged every time they are switched. The switching speed of the field effect transistor is thus reduced, and the efficiency of the electronic component is also lowered. In order to calculate the total loss of the electronic components during the switching process, the loss (Eon) during the opening process and the loss (Eoff) during the closing process are calculated. The total power of the field effect transistor switch can be expressed by the following equation: Psw = (Eon + Eoff) x switching frequency. The gate charge (Qgd) has the greatest impact on switching performance.
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