What is the working principle of FET?

1.The biggest function of MOS in the main board is to reduce the voltage. The principle is to change the channel size of the MOS tube by controlling the voltage on the G pole of the MOS tube, and then to change the output voltage on the S pole, and to adjust the input voltage to the required voltage output.

The conduction characteristics of N channel MOS transistor: the higher the G pole voltage, the stronger the conduction degree between Dins. On the other hand, the lower the G pole voltage, the weaker the conduction degree of the D、S pole. When the G pole voltage reaches 12 V, the D、S pole is completely switched on.

The conduction characteristics of P channel MOS transistor are as follows: the higher the G pole voltage, the weaker the conduction degree of the D、S pole, whereas, when the G pole has no voltage, the DU S pole is completely switched on. Maintenance referred to as N-tube high-level conduction, P-tube low-level conduction.

In figure 1, Q27 is an N-channel MOS transistor. Q27 is switched on when U22A's one pin output is high, and the voltage of VCC_DDR memory is lowered to get 1.2V_HT bus power supply, while U22A's one-pin output low-current Q27 cutoff, 1.2V_HT bus voltage is 0V.

             Fig.1 An example of MOS step-down circuit

   2. MOS pipe model commonly used on motherboard.

(1) TO-252 encapsulated N-channel MOS transistors are commonly used in supply circuits, such as memory power supply, bridge power supply, CPU power supply and so on. The common models are 09N03, 06N03, 60N03, 45N02, 3055,3057, K3916,K3918,K3919, 9916, 85T03,70T02, D412,FR3707Z, FR3709Z, etc.

(2) In SOT-23 packaging, FET with very small appearance is often used in low current supply circuit to lower voltage, or to be used as switch. The common models are K72,S72,702,7002,351,024, 025,12W and so on, as shown in figure 2.

(3) In some Asus, Gigabytes, Biostar motherboard standby power supply, USB interface circuit using SOT-23 encapsulated small channel MOS tube to achieve dual and power supply. Common models are the A 36, commonly used next to the USB interface.

3. Special MOS pipe models used on motherboards.

(1) Junction FET (see figure 3). Junction FET is commonly used in ASUS motherboard and part of Asus main board, and the model is LD1010D,LD1014D.. Its characteristic is that when the power is off, the measurement of the Dos pole is completely interlinked. In the maintenance, you must pay attention to avoid miscalculation in the maintenance.

         Fig.2 Small MOS tube                     Fig.3 JFET

(2) 8 pin MOS tube.

①There are compound type, such as two N-channel field effect transistors in APM7313,7D03, and 4500,4501,4502,4609 are composed of one N-channel and one P-channel, as shown in Fig. 4.

      （a）APM7313 Double N tube    （b）4500N+P Composite MOS tube

                         Fig.4 Composite MOS tube

②The 8 pin single N-channel transistor, shown in figure 5, is commonly used in the Asus motherboard. Its three pin are s pole, four pin are G pole, and five pin is D pole.

                  Fig.7 5 pin single MOS transistor

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