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Abstract: MOS tube is a sensitive device, its own input resistance is very high, it is very easy to be charged by the external electromagnetic field or static electricity, and because it is difficult to discharge the charge in the place with strong static electricity, it is easy to cause an electrostatic breakdown.

First, the input resistance of the MOS tube itself is very high, and the capacitance between the gate and the source is very small, so it is very easy to be charged by the external electromagnetic field or electrostatic induction, and a small amount of charge can form a very high voltage on the capacitance between the electrodes. (U=Q/C), damage the tube.

Although the input end of the MOS tube in PCB components has anti-static protection measures, it still needs to be treated with care. It is best to use metal containers or conductive materials for storage and transportation, and do not place it in chemical materials or chemical fiber fabrics that are prone to high static electricity…

When assembling and debugging, tools, instruments, workbenches, etc., should be well-grounded. It is necessary to prevent the damage caused by the electrostatic interference of the operator. For example, it is not suitable to wear a nylon or chemical fiber clothes. It is best to connect the hands or tools to the ground before touching the integrated block. When the device leads are straightened and bent or manually soldered, the equipment used must be well-grounded.

Second, the protection diode at the input end of the MOS tube circuit generally has a current tolerance of 1mA. When there may be excessive transient input current (over 10mA), an input protection resistor should be connected in series. Therefore, a MOS tube with an internal protection resistor can be selected for the application.

In addition, because the instantaneous energy absorbed by the protection circuit is limited, too large instantaneous signals and too high electrostatic voltage will make the protection circuit useless.

Therefore, the electric soldering iron must be reliably grounded during soldering to prevent leakage of electricity from penetrating the input end of the device. In general use, the residual heat of the electric soldering iron can be used for soldering after the power is turned off, and the grounding pin should be soldered first.

The MOS tub in the PCB components is a voltage-driven component, which is very sensitive to voltage. The floating G can easily accept external interference to turn the MOS on. The external interference signal charges the G-S junction capacitance. This tiny charge can be stored for a long time.

In the test, it is very dangerous for G to hang in the air. Many pipes burst because of this, and G is connected to a pull-down resistor to the ground, and the bypass interference signal will not pass through. Generally, it can be 10~20K.

This resistance is called gate resistance. Function 1: Provides a bias voltage for the field-effect tube; Function 2: Plays the role of a discharge resistor (protecting the gate G~source S).

 

 

The first role is easy to understand. Here is an explanation of the principle of the second function: protecting the grid G~source S: the resistance between the GS electrodes of the field-effect tube is very large, so as long as there is a small amount of static electricity, it can make the GS electrodes wait. A very high voltage is generated across the effective capacitor.

If these small amounts of static electricity are not discharged in time, the high voltage at both ends of it may cause the FET to malfunction and may even break down its GS pole; at this time, the resistance added between the gate and the source will be able to The above-mentioned electrostatic discharge is discharged, thus playing the role of protecting the field-effect tube.

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