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Parameter and test of FET


In fact, there are many parameters related to FETs, including DC parameters, AC parameters and limit parameters, but generally pay attention to the following main parameters:


1. I DSS - saturated drain-source current. It refers to the drain-source current when the gate voltage U GS = 0 in the junction or depletion IGBT.


2, UP - pinch off voltage. It refers to the gate voltage when the drain and source are just turned off in the junction or depletion IGBT.


3. UT - turn-on voltage. It refers to the gate voltage when the drain and source are just turned on in the enhanced insulated gate field effect transistor.


4, gM - transconductance. is the control ability of the gate-source voltage U GS to the drain current ID, that is, the ratio of the variation of the drain current ID to the variation of the gate-source voltage UGS. gM is an important parameter to measure the amplification ability of FET.


5. BUDS - drain-source breakdown voltage. It refers to the maximum drain-source voltage that the FET can withstand when the gate-source voltage UGS is constant. This is a limit parameter, and the operating voltage applied to the FET must be less than BUDS.


6. PDSM - maximum power dissipation. It is also a limit parameter, which refers to the maximum allowable drain-source dissipation power without the performance of the FET being deteriorated. When in use, the actual power consumption of the FET should be less than that of the PDSM with a certain margin.


7. IDSM - maximum drain-source current. is a limit parameter, which refers to the maximum current allowed to pass between the drain and the source when the FET is working normally. The operating current of the FET should not exceed IDSM.


The role of field effect transistors


1. Field effect tube can be used for amplification. Since the input impedance of the FET amplifier is high, the coupling capacitor can be small, and the electrolytic capacitor is not necessary.


2. The high input impedance of the FET is very suitable for impedance transformation. It is often used in the input stage of multi-stage amplifier for impedance transformation.


3. The FET can be used as a variable resistor.


4. The FET can be easily used as a constant current source.


5. FETs can be used as electronic switches.


FET testing


1. Pin identification of junction field effect transistors


The gate of the FET corresponds to the base of the transistor, and the source and drain correspond to the emitter and collector of the transistor, respectively. Put the multimeter in the R×1k gear, and use two test pens to measure the forward and reverse resistances between each two pins respectively. When the forward and reverse resistances between two pins are equal, and both are several KΩ, the two pins are the drain D and the source S (interchangeable), and the remaining pin is the gate G . For junction field effect transistors with 4 pins, the other pole is the shielding pole (grounded in use).


2. Judgment grid


Touch one electrode of the tube with the black test lead of the multimeter, and touch the other two electrodes with the red test lead respectively. If the resistance values measured twice are very small, it means that it is a forward resistance, the tube is an N-channel field effect transistor, and the black test lead is also connected to the grid.


The manufacturing process determines that the source and drain of the FET are symmetrical and can be used interchangeably without affecting the normal operation of the circuit, so there is no need to distinguish them. The resistance between source and drain is about several thousand ohms. Note that this method cannot be used to determine the gate of the IGBT. Because the input resistance of this kind of tube is extremely high, and the inter-electrode capacitance between the gate and source is very small, as long as there is a small amount of charge during measurement, a high voltage can be formed on the inter-electrode capacitance, which is easy to damage the tube.


3. Estimate the amplification capability of the FET


Turn the multimeter to R×100, the red test lead is connected to the source S, and the black test lead is connected to the drain D, which is equivalent to adding a 1.5V power supply voltage to the FET. At this time, the pointer indicates the resistance value between the D-S electrodes. Then pinch the grid G with your fingers, and add the induced voltage of the human body to the grid as an input signal. Due to the magnification of the tube, both UDS and ID will change, which is also equivalent to the change in the resistance between the D-S electrodes. It can be observed that the needle has a relatively large swing. If the needle swings very little when the grid is pinched by hand, it means that the amplifying ability of the tube is weak; if the needle does not move, it means that the tube has been damaged.


Since the 50Hz AC voltage induced by the human body is relatively high, and different FETs may have different working points when measuring with the resistance gear, the needle may swing to the right or left when the grid is pinched by hand. The RDS of a few tubes is reduced, causing the hands to swing to the right, and the RDS of most tubes is increased, causing the hands to swing to the left. Regardless of the swing direction of the hands, as long as there is a significant swing, the tube has the ability to magnify. This method is also suitable for measuring MOS tubes. In order to protect the MOS field effect tube, it is necessary to hold the insulating handle of the screwdriver by hand, and touch the grid with a metal rod, so as to prevent the human body induced charge from being directly added to the grid and damaging the tube.


After each measurement of the MOS tube, the G-S junction capacitor will be charged with a small amount of charge, and the voltage UGS will be established, and then the time measurement needle may not move. At this time, the G-S pole is short-circuited.


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