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How do MOS tubes be selected?
Publish：SHENZHEN ENXIC ELECTRONICS CO., LTD,Schottky diodes, rectifier diodes, fast recovery diodes, switching transistors, thermistors, MOS tube, TVS tube, Youze Electronics Time：2018-06-22 Views:1528
The most common application of the MOS tube may be the switch element in the power supply. Many small partners do not know how to choose the MOS tube, and the four main rules about the selection of MOS tubes are summarized.
1: using N channel or P channel
The first step to choose the MOS tube device 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 trunk voltage, the MOS tube forms a low-voltage side switch. In the low voltage side, the N channel MOS pipe should be adopted, which is due to the consideration of the voltage required for closing or conduction devices. When the MOS tube is connected to the bus and the load grounding, the high voltage side switch is used. The P channel MOS tube is usually used in this topology, which is also due to the consideration of voltage driving.
Two: determine the rated current and voltage of the MOS tube
The rated current should be the maximum current that the load can bear in all cases. Similar to the voltage, ensure that the selected MOS tube can withstand this rated current even when the system produces spike current. The two consideration is the continuous mode and pulse spike. In the continuous conduction mode, the MOS tube is static, and the current is continuously passed through the device. The pulse peak means that there is a lot of surge flow through the device. Once the maximum current under these conditions is determined, it is only necessary to directly select the device to bear the maximum current.
Determine the required rated voltage, or the maximum voltage that the device can withstand. The greater the rated voltage, the higher the cost of the device. According to practical experience, rated voltage should be greater than trunk voltage or bus voltage. Only in this way can adequate protection be provided so that the MOS tube will not fail. As far as the MOS tube is concerned, it is necessary to determine the maximum voltage that may be taken between the drain pole and the source, that is, the maximum VDS. is important to know that the maximum voltage that the MOS tube can withstand will change with the temperature. We must test the range of voltage in the whole operating temperature range. The rated voltage must have enough allowance to cover the change range to ensure that the circuit will not fail.
Three: select the heat dissipation requirements of the MOS tube system
Two different situations must be considered, namely, the worst case and the real situation. It is recommended to use the worst case calculation results, because this result provides a greater margin of safety to ensure that the system will not fail. In the data sheet of the MOS tube, there are some measurements to be paid attention to; the junction temperature of the device is equal to the maximum ambient temperature plus the product of the thermal resistance and power dissipation (the junction temperature = maximum ambient temperature + [thermal resistance * power dissipation]). According to this formula, the maximum power dissipation of the system can be solved, that is to say, it is equal to I2 * RDS (ON) by definition. We have to calculate the RDS (ON) at different temperatures through the maximum current of the device. In addition, the heat dissipation of the circuit board and its MOS tube should be well done.
Four: select the switching performance of the MOS tube
There are many parameters that affect the switching performance, but the most important are gate / drain, gate / source and source capacitance. These capacitors will cause switching losses in the device because they are charged at each switch. The switching speed of the MOS tube is thus reduced, and the device efficiency is also reduced. In order to calculate the total loss of devices in the switching process, the losses during the opening process and the losses in the closing process are calculated.
Above is the four rule of choosing MOS tube, and we hope to help you.
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