Test methods and precautions of FET

Date：2022-03-28 14:26:27Views：1236

Field effect transistor is abbreviated as FET, which can be divided into junction field effect transistor (JFET) and insulated gate field effect transistor (MOSFET). We usually call it MOS transistor for short. The MOS tube can be divided into enhanced and depleted, and the enhanced MOS tube is commonly used in our common motherboard. FET is a voltage controlled semiconductor device in which most carriers participate in conduction. It has the advantages of high input resistance (107 ~ 1015 Ω), low noise, low power consumption, large dynamic range, easy integration, no secondary breakdown phenomenon, wide safe working area and so on. It has become a strong competitor of bipolar transistor and power transistor.

Pointer multimeter detects FET

The source and drain of Junction FET are generally interchangeable, so it is generally only necessary to distinguish its grid G. When judging, put the multimeter in R × 1K gear, select any two electrodes, and measure the forward and reverse resistance between them respectively. If the forward and reverse resistance values are equal (about several thousand ohms), the two poles are drain D and source s, and the rest are gate G.

The multimeter can also be placed in R according to the guiding principle of PN statement × 1K block, contact the black probe with one electrode of the tube and the red probe with the other two electrodes of the tube respectively. If the measured resistance is very small, the black probe is connected to the grid and is an n-channel FET. For p-channel FET, the red probe contacts one electrode of the tube and the black probe contacts the other two electrodes of the tube respectively. When the measured resistance is very small, the red probe is connected to the grid.

During detection, if there is a fixed resistance between grid g, drain D and source s, it indicates that the FET is in good condition; If the resistance between them tends to zero or infinity, it indicates that the FET has been damaged.

Digital multimeter detects FET

The digital multimeter can not only distinguish the electrode of the FET, but also measure the amplification factor of the FET. Set the digital multimeter to HFE gear, and insert the G, D and S poles of the FET into HFE respectively. The B, C and e holes of the measuring socket (the n-channel tube is inserted into the NPN socket, and the p-channel tube is inserted into the PNP socket). At this time, a value will be displayed on the display screen, which is the amplification factor of the FET; If the electrode is inserted incorrectly or the polarity is inserted incorrectly, the display screen will display "000" or "1".

In order to use FET safely and effectively, pay attention to the following matters:

(1) Before using the field effect tube, the type of field effect tube and its electrode must be found out first, and the test shall be carried out through the instrument if necessary. The s and D poles of Junction FET are interchangeable, and the s and D poles of insulated gate FET are generally interchangeable. However, the S pole of some products is connected with the substrate, and the S pole and D pole cannot be interchanged at this time.

(2) In the circuit design, the type and parameters of FET shall be selected according to the needs of the circuit. It is not allowed to exceed the limit values of dissipated power, maximum drain source current and voltage of FET.

(3) All types of FET should be connected to the circuit in strict accordance with the required bias, and pay attention to the bias polarity of FET.

(4) When installing the FET, pay attention to the installation position and try to avoid being close to the heating element; In order to prevent pipe vibration, fasten the pipe during installation; When the lead of the pin is bent, it shall be carried out at a place more than 5mm larger than the root size of the pipe to prevent air leakage caused by bending the pin.

(5) For insulated gate field effect transistor (MOS tube), because the gate is in the insulated state, the induced charge is not easy to release, and the insulating layer is very thin, which is very easy to break down. Therefore, the following problems should be paid attention to when using this type of field effect transistor.

① During transportation and storage, the lead out pin must be short circuited or packaged with metal shielding to prevent the grid from being broken down by external induced electromotive force. When taking the FET, you should take its shell instead of its pin, because the human body has a small amount of charge. If you take the pin of the FET and a small amount of charge runs to the grid, the grid and drain junction will be inductively charged and it is easy to breakdown the FET.

② The shell of electric soldering iron for welding shall be grounded, or the waste heat after power failure of soldering iron shall be used for welding. The sequence of welding insulated grid FET is: drain → source → grid. The order of disassembly is reverse. In order to prevent the breakdown of FET, when connecting to the circuit, short circuit each lead of the tube, and cut off the short wiring after welding.

③ Before welding, the power line and ground wire of the circuit board shall be short circuited and separated after the welding of MOS devices.

④ Before installing the circuit board, use the grounding clamp to touch each terminal of the machine, and then connect the circuit board.

⑤ The testing instrument and workbench shall be well grounded and anti-static measures shall be taken.

⑥ If the gate of MOS FET is allowed, it is better to connect the protective crystal diode to prevent the gate of MOS FET from being broken down.

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