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2022-02-25
In recent years, with the automobile, communication, energy, consumption, green industry and a large number of applications of FETS products in recent years to the rapid development of the industry. In the future, FET will still occupy the dominant position. The field effect tube will still be many new engineers will be exposed to the device, microbi semiconductor will start from the basics, discuss some basic knowledge of the field effect tube, including selection, the introduction of key constants, the consideration of the system and heat dissipation for you to do some introduction.
First, the basic selection of field effect tube
There are two main types of FET: N-channel and P-channel. The first step is to decide whether to use N-channel or P-channel MOS tube. In typical power applications, when a MOS tube is grounded and the load is connected to the mains voltage, the MOS tube is structured as a low voltage side switch. In low-voltage side switches, n-channel MOS tubes should be used for consideration of the voltage required to turn off or turn on the device. When the MOS tube is connected to the bus and the load is grounded, the high voltage side switch is used. P-channel MOS tubes are usually used in this topology, again for the purpose of voltage drive.
Determine the desired rated voltage, or the maximum voltage the device can withstand. The higher the rated voltage, the higher the cost of the device. As a rule of thumb, the rated voltage should be greater than the mains voltage or bus voltage. This provides enough protection to keep the MOS tube from failing. For MOS tube selection, it is necessary to determine the maximum possible voltage between drain and source, i.e., maximum VDS. It is important to know that the maximum voltage a MOS tube can withstand varies with temperature. We must test the range of voltage variation over the whole operating temperature range. The rated voltage must have a sufficient margin to cover this variation to ensure that the circuit does not fail. Other safety factors to consider include voltage transients induced by switching electronics such as motors or transformers. The rated voltage varies from application to application; Typically, 20 v for portable devices, 20 to 30V for FPGA power supplies, and 450 to 600V for 85 to 220VAC applications. The MOS tube designed by KIA semiconductor has strong pressure resistance and wide application fields, which are favored by customers.
Two. Determine the rated current of MOS tube
The rated current shall be the maximum current the load can withstand under all circumstances. As in the case of voltage, ensure that the MOS tube selected can withstand this rated current, even when the system generates spikes in current. The two current cases considered are the continuous mode and the pulse spike. In the continuous conduction mode, the MOS tube is in a steady state, and the current continuously passes through the device. A pulse spike is when there is a large surge (or spike) flowing through the device. Once the maximum current under these conditions is determined, simply select the device that can withstand this maximum current.
After selecting the rated current, the conduction loss must also be calculated. In practice, MOS tubes are not ideal devices, because there will be electrical energy loss in the conduction process, which is called conduction loss. The MOS tube acts as a variable resistor when "ON", determined by the RDS (ON) of the device and varies significantly with temperature. The power loss of the device can be calculated from Iload2×RDS (ON), and since the on-resistance varies with temperature, the power loss also varies proportionally. The higher the voltage VGS applied to MOS tube, the smaller RDS (ON) will be. Otherwise, the RDS (ON) is going to be higher. Note that the RDS (ON) resistance rises slightly with the current. The various electrical parameters for RDS (ON) resistors can be found in the technical data sheet provided by the manufacturer.
Three. The next step in selecting MOS tubes is the heat dissipation requirements of the system
There are two different scenarios to consider, the worst-case scenario and the true scenario. Worst-case calculations are recommended because they provide a greater margin of safety and ensure that the system does not fail. There are also some measurements that need to be noted on the MOS tube data sheet; The junction temperature of the device is equal to the maximum ambient temperature plus the product of thermal resistance and power dissipation (junction temperature = maximum ambient temperature + [thermal resistance × power dissipation]). According to this formula, the maximum power dissipation of the system can be solved, which is equal to I2×RDS (ON) by definition. We have calculated RDS (ON) at different temperatures from the maximum current of the device. In addition, the circuit board and its MOS tube heat dissipation.
Avalanche breakdown refers to the reverse voltage on the semiconductor device exceeds the maximum value, and the formation of a strong electric field to increase the current in the device. Increasing chip size improves avalanche resistance and ultimately device robustness. Therefore, choosing a larger package can effectively prevent avalanches.
Four. The final step in selecting a MOS tube is to determine the switching performance of the MOS tube
There are many parameters that affect switch performance, but the most important are gate/drain, gate/source, and drain/source capacitance. These capacitors cause switching losses in the device because they are charged each time they are switched on or off. Therefore, the switching speed of MOS tube is reduced, and the device efficiency is also reduced. In order to calculate the total loss of devices in the switching process, the loss in the switching process (Eon) and the loss in the switching process (Eoff) should be calculated. The total power of the MOSFET switch can be expressed in the following equation: Psw= (Eon+Eoff) x switching frequency. Gate charge (Qgd) has the greatest influence on switch performance.
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