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Num：50

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2020-11

Is it always difficult to use and replace MOS tubes for multimeter tests?

MOS tube has always been one of the topics that engineers are keen to discuss, so we have sorted out the common and uncommon knowledge of MOS tube, hoping to be of some help to engineers. Now let's talk about the MOS tube, a very important component.

Antistatic protectionAlthough the MOS tube has internal protection measures, we should also operate in accordance with the anti-static operation rules, which is a qualified maintenance personnel should have.

While recent enhanced power MOS tube has a larger difference, the first input capacitance is big because of its high function, this one comes into contact with the electrostatic charging process, the resulting voltage is small, cause breakdown may be small, moreover the power MOS tube inside the gate and the source has a protection of voltage regulator tube DZ (as shown in the figure below), the static embedded below the protection zener diode voltage value, effective protection of the grid and the source of the insulating layer, different power, different models of MOS tube, its protection zener diode voltage values are different.

Most MOS tubes produced in the early stage have no anti-static measures, so care should be taken in storage and application, especially MOS tubes with small power. As the input capacitance of MOS tubes with small power is relatively small, the voltage generated when they come into contact with static electricity is relatively high, which is likely to cause static electricity breakdown.

MOS tube is an insulated gate field effect tube, the gate is a dc free path, the input impedance is very high, easy to cause electrostatic charge accumulation, produce a high voltage to break down the insulation layer between the gate and the source.

Detection and substitution
In the repair of television and electrical equipment, there will be a variety of components of the damage, MOS tube is one of them, this is how our maintenance personnel use the commonly used multimeter to judge the quality of MOS tube, the pros and cons. How to replace the MOS tube if there is no same manufacturer or the same model when replacing it?

1. MOS tube test:Pointer multimeters (digital meters are not suitable for measuring semiconductor devices) must be used for testing. Type for power MOSFET switch tube are N channel enhanced, every manufacturer of products is also almost the same the TO - 220 - f package form (used in switching power supply for 50-200 w power field effect of the switch tube), the three electrodes arrangement is consistent, the three pins down, print model for the third, on the left side of the pin gate, right pin measurement for the source, the middle pins TO drain as shown in the figure below.

As a general electric TV maintenance personnel when measuring transistor or the diode is generally adopt ordinary multimeter to judge the stand or fall of transistor or the diode, although the electrical parameters of the transistor or the diode judged can not confirm, but as long as the method to confirm the correct transistor "good" and "bad" or there is no problem. Similarly, THE MOS tube can also be used to judge its "good" and "bad" by a multimeter, which can also meet the requirements for general maintenance.

1) Multimeter and related preparation:With the multimeter ohm ohm center scale cannot too big, less than 12 best Ω (500 type table for 12 Ω), so that in R x 1 block can have larger current, more accurate for positive characteristics of PN junction judgment. The battery inside the R×10K block of the multimeter should be greater than 9V, so that it is more accurate when measuring the reverse leakage current of the PN junction, otherwise the leakage cannot be measured.

First of all, before measurement, you should use a multimeter, especially the application of ohmic gear. To understand the ohmic gear, you can correctly use the ohmic gear to measure the transistor and MOS tube.

Now due to the progress of production technology, factory screening and detection are very strict, we generally judge as long as the MOS tube is not leakage, not breakdown short circuit, internal continuous circuit, can be enlarged, the method is very simple:

Adopt R×10K gear of multimeter; The battery inside R×10K gear is usually 9V plus 1.5V to reach 10.5V, and the voltage is generally judged to be sufficient for reverse phase leakage at PN node. The red marker pen of the multimeter is negative potential (connected to the negative pole of the internal battery), and the black marker pen of the multimeter is positive potential (connected to the positive pole of the internal battery), as shown in the figure above.

2) Test steps:Connect the red watch pen to the source S of the MOS tube; Connect the black watch pen to the drain D of the MOS tube, and the indicator of the watch hand should be infinite, as shown in the figure below. If there is an ohm index, the tube under test has leakage phenomenon, the tube cannot be used.

Maintain the above state; At this point, a 100K ~ 200K resistor is connected to the gate and drain, as shown in the figure below; Whose hands indicates ohms should be as small as possible, then can general instructions to 0 ohm, at this moment is positively charged by 100 k resistor grid charging of MOS tube, produce grid electric field, because the electric field to produce conductive channel drain and source conduction, so the multimeter pointer deflection, deflection Angle is large (small) ohm index prove good discharge performance.

A wire is used to connect the gate and source of the tube under test, and the pointer of the multimeter immediately returns to infinity, as shown in the figure above. The connection of the conductor makes the measured MOS tube, the gate charge release and the internal electric field disappear. The conduction channel also disappears, so the resistance between the drain and the source becomes infinite again.

2. Replacement of MOS tube
In the repair of television and various electrical equipment, encountered components damage should be replaced with the same type of components. Sometimes don't have the same component, however, with other types of substitution, so consider to all aspects of the performance, parameters and overall dimensions, such as the inside of the television line output tube, as long as thinking of high voltage, current, power is generally can be substitution (line output tube appearance is almost the same size), and more power tend to be larger.

Although it is the same principle for MOS tube substitution, it is better to choose the original model. In particular, the power should not be higher because of the high power. Changed after the input capacitance is big, and the driver circuit mismatch, incentive perfusion charging current limiting resistor of resistance of the circuit and the size of the input capacitance is connected, MOS tube with large power in spite of the large capacity, but the input capacitance is large, the driver circuit with not good, it can make the MOS tube, the bad performance. As shown in the substitution of different types of MOS tube, its input capacitance should be taken into account.

For example have a 42 inch LCD TV backlight high pressure plate is damaged, after check is the internal power MOS tube damage, because without the original type of substitution, is to choose a, voltage, current and power are not less than the original MOS tube replacement, the result is a backlight tube continuous flashing difficult (start), finally put on the original model to solve the problem.

When the MOS tube is detected to be damaged, the components of the surrounding perfusion circuit must be replaced completely, because the damage of the MOS tube may also be caused by the poor perfusion circuit components. Even if the MOS tube itself is damaged, at the moment of breakdown of the MOS tube, the perfusion circuit elements are also damaged and should be replaced.

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