Is it always difficult to use and replace the multimeter test MOS tube? Check this out!

About MOS tube has always been one of the topics that engineers are keen to discuss, so we have sorted out the relevant knowledge of common and uncommon MOS tubes, hoping to help engineers. Let's talk about this very important component of MOS tube!
Anti-static protection
MOS transistors are insulated gate field effect transistors. The gate has no DC path, and the input impedance is extremely high. It is easy to cause static charges to accumulate and generate a higher voltage to break down the insulating layer between the gate and the source.
Most of the MOS tubes produced in the early days did not have anti-static measures, so be very careful in storage and application, especially MOS tubes with lower power, because the input capacitance of lower power MOS tubes is relatively small, and the voltage generated when exposed to static electricity Higher, it is easy to cause electrostatic breakdown.
However, the recent enhanced high-power MOS tube has a relatively big difference. First of all, due to the larger function and the larger input capacitance, there will be a charging process when exposed to static electricity, and the generated voltage is relatively small, which may cause breakdown. In addition, the current high-power MOS tube has a protective voltage regulator DZ (as shown in the figure below) on the internal gate and source of the MOS tube, and the static electricity is embedded below the voltage regulation value of the protection Zener diode, which effectively protects In addition to the insulating layer of the gate and the source, the voltage regulation value of the protection Zener diode is different for different power and different types of MOS transistors.
Although there are protective measures inside the MOS tube, we should also follow the anti-static operating procedures when operating. This is what a qualified maintenance person should have.

Is it always difficult to use and replace the multimeter test MOS tube? Check this out!
detection and substitution
When repairing TV sets and electrical equipment, various components will be damaged, and MOS tubes are among them. This is how our maintenance personnel use commonly used multimeters to judge the quality of MOS tubes. When replacing the MOS tube, if there is no same manufacturer and the same model, how to replace it.
1, MOS tube test:
As a general electrical television repairer, when measuring transistors or diodes, they usually use ordinary multimeters to judge whether the transistors or diodes are good or bad. Although the electrical parameters of the judged transistors or diodes cannot be confirmed, but as long as the method is correct There is no problem in confirming the "good" and "bad" of the transistor. Similarly, the MOS tube can also be judged "good" and "bad" by using a multimeter. From the general maintenance, it can also meet the demand.
A pointer multimeter must be used for testing (digital meters are not suitable for measuring semiconductor devices). The power MOSFET switch tubes are all N-channel enhancement type, and the products of each manufacturer almost all adopt the same TO-220F package form (referring to the field effect switch tube with the power of 50-200W in the switching power supply). The arrangement of the three electrodes is also the same, that is, the three pins are facing downwards, the printed model faces the self, the left pin is the gate, the right measurement pin is the source, and the middle pin is the drain, as shown in the figure below.

Is it always difficult to use and replace the multimeter test MOS tube? Check this out!
1) Multimeter and related preparations:
First of all, you should use a multimeter before measuring, especially for the application of ohms. It is necessary to understand the ohms to correctly use the ohms to measure transistors and MOS tubes.
The ohm center scale of the ohm block of the multimeter should not be too large, less than 12Ω (12Ω for the 500 type meter), so that there can be a larger current in the R×1 block, and the judgment of the forward characteristic of the PN junction is more accurate. The battery inside the R×10K block of the multimeter is greater than 9V, so it is more accurate when measuring the PN junction reverse leakage current, otherwise the leakage cannot be measured.

Is it always difficult to use and replace the multimeter test MOS tube? Check this out!
Now due to the advancement of production technology, the screening and testing of the factory are very strict. We generally judge that as long as we judge that the MOS tube has no leakage, no breakdown short circuit, internal continuous circuit, and amplification, the method is very simple:
Use the R×10K block of the multimeter; the battery inside the R×10K block is generally 9V plus 1.5V to reach 10.5V. This voltage is generally judged to be enough for the PN junction inverted leakage current. The red test pen of the multimeter is at a negative potential (connected to the internal battery) Negative), the black test lead of the multimeter has a positive potential (connected to the positive pole of the internal battery), as shown in the figure above.
2) Test steps:
Connect the red test lead to the source S of the MOS tube; connect the black test lead to the drain D of the MOS tube, the needle indicator should be infinite, as shown in the figure below. If there is an ohmic index, it means that the tube under test is leaking and the tube cannot be used.

Is it always difficult to use and replace the multimeter test MOS tube? Check this out!
Keep the above state; at this time, use a 100K～200K resistor to connect to the gate and drain, as shown in the figure below; at this time, the ohms indicated by the needle should be as small as possible. Generally, it can indicate 0 ohms. At this time, the positive charge passes through. The 100K resistor charges the gate of the MOS tube to generate a gate electric field. Due to the electric field, the conductive channel causes the drain and source to be turned on. Therefore, the pointer of the multimeter deflects, and the large deflection angle (small ohmic index) proves that the discharge performance is good.

Is it always difficult to use and replace the multimeter test MOS tube? Check this out!
At this time, it is in the state of the above figure; then remove the connected resistance, at this time the pointer of the multimeter should still be the index of the MOS tube conduction, as shown in the figure below. Although the resistance is taken away, because the electric charge charged by the resistance to the gate does not disappear, the electric field of the gate continues to be maintained, and the internal conductive channel is still maintained. This is the characteristic of the insulated gate MOS transistor.
If the resistance is removed from the meter needle, it will gradually return to high resistance or even to infinity. The grid leakage of the tube under test must be considered.

Is it always difficult to use and replace the multimeter test MOS tube? Check this out!
At this time, a wire is used to connect the grid 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 wires causes the gate charge to be released and the internal electric field disappears; the conductive channel also disappears, so the resistance between the drain and the source becomes infinite again.

Is it always difficult to use and replace the multimeter test MOS tube? Check this out!
2. Replacement of MOS tube
When repairing TV sets and various electrical equipment, if the components are damaged, the components of the same model should be used for replacement. However, sometimes the same component is not available at hand, so it is necessary to replace it with another model. In this way, various aspects of performance, parameters, and dimensions must be considered. For example, the line output tube inside the TV, as long as the withstand voltage and current are considered , The power can generally be replaced (the line output tube appearance size is almost the same), and the power is often better.
Although it is the same principle for MOS tube replacement, it is the original model, especially do not pursue a higher power, because the power is large; the input capacitance is large, and it will not match the excitation circuit after the replacement, and the charging of the excitation perfusion circuit The resistance of the current-limiting resistor is related to the input capacitance of the MOS tube. Although the large power is selected, the input capacitance is also large, and the coordination of the excitation circuit is not good, which will cause the MOS The opening and closing performance of the tube deteriorates. When replacing different types of MOS tubes as shown, the input capacitance must be taken into consideration.
For example, the backlight high-voltage board of a 42-inch LCD TV is damaged. After inspection, the internal high-power MOS tube is damaged. Because there is no replacement of the original model, one is selected, and the voltage, current, and power are not less than the original MOS tube. , The result is that the backlight tube flashes continuously (difficult to start), or replace the original model to solve the problem.
After detecting that the MOS tube is damaged, all the components of the perfusion circuit around it must be replaced during replacement, because the damage of the MOS tube may also be the damage of the MOS tube caused by the poor perfusion circuit components. Even if the MOS tube itself is damaged, at the moment the MOS tube breaks down, the perfusion circuit components are also damaged and should be replaced.
Just like when we have many skilled repairers repairing the A3 switching power supply; as long as the switching tube is found to be broken, it is the same as replacing the previous 2SC3807 excitation tube (although the 2SC3807 tube is measured with a multimeter).