Why is it always difficult to test high power MOSFET use and replacement with a multimeter?

About high-power MOSFET has been one of the engineers keen to discuss the topic, so we have organized the common and uncommon knowledge of MOSFET, I hope to help engineers. Let's talk about MOSFET, a very important component!

Anti-static protection

High-power MOSFET is an insulated gate field effect tube, the gate is no direct current circuit, the input impedance is extremely high, it is very easy to cause static charge aggregation, resulting in a high voltage will be the gate and the source of the insulating layer between the breakdown.

Most of the early production of MOSFETs do not have anti-static measures, so be very careful in the custody and application, especially the smaller power MOSFETs, due to the smaller power MOSFET input capacitance is relatively small, when exposed to static electricity generates a higher voltage, easily caused by electrostatic breakdown.

The recent enhancement of high-power MOSFET is a relatively big difference, first of all, due to the function of a larger input capacitance is also larger, so that contact with static electricity has a charging process, resulting in a smaller voltage, causing breakdown of the possibility of smaller, and then again, now the high-power MOSFET in the internal gate and the source of the gate and source of a protected regulator DZ, the static embedded in the protection of the regulator diode voltage regulator value Below, effectively protect the gate and source of the insulating layer, different power, different models of MOSFET protection regulator diode voltage regulator value is different.

Although high-power MOSFET internal protection measures, we should operate in accordance with the anti-static operating procedures, which is a qualified maintenance staff should have.

Detection and replacement

In the repair of televisions and electrical equipment, will encounter a variety of component damage, MOSFET is also among them, which is how our maintenance staff to use the commonly used multimeter to determine the good and bad, good and bad MOSFET. In the replacement of MOSFET if there is no same manufacturer and the same model, how to replace the problem.

1, high-power MOSFET test:

As a general electrical TV repair personnel in the measurement of crystal transistors or diodes, generally using an ordinary multimeter to determine the good and bad transistors or diodes, although the judgment of the transistor or diode electrical parameters can not be confirmed, but as long as the method is correct for the confirmation of crystal transistors "good" and "bad" or "bad" for the confirmation of crystal transistors. "Bad" or no problem. Similarly, MOSFET can also be

To apply the multimeter to determine its "good" and "bad", from the general maintenance, can also meet the needs.

Detection must use a pointer type multimeter (digital meter is not suitable for measuring semiconductor devices). For power-type MOSFET switching tube are N-channel enhancement, the products of the manufacturers are almost all using the same TO-220F package form (refers to the switching power supply for the power of 50-200W of the field effect switching tube), the three electrode arrangement is also consistent, that is, the three

Pins down, print model facing the self, the left pin for the gate, the right test pin for the source, the middle pin for the drain.

(1) multimeter and related preparations:

First of all, before the measurement should be able to use the multimeter, especially the application of ohm gear, to understand the ohm block will be the correct application of ohm block to measure the crystal transistor and MOSFET.

With the multimeter ohm block ohm center scale can not be too large, preferably less than 12 Ω (500-type table for 12 Ω), so that in the R × 1 block can have a larger current, for the PN junction of the forward characteristics of the judgment is more accurate. Multimeter R × 10K block internal battery is best greater than 9V, so that in measuring the PN junction inverse leakage current is more accurate, otherwise the leakage can not be measured.

Now due to the progress of the production process, the factory screening, testing is very strict, we generally judge as long as the judgment of the MOSFET does not leakage, does not break through the short circuit, the internal non-circuiting, can be amplified on the way, the method is extremely simple:

Using a multimeter R × 10K block; R × 10K block internal battery is generally 9V plus 1.5V to 10.5V this voltage is generally judged to be enough PN junction inversion leakage, the red pen of the multimeter is negative potential (connected to the negative terminal of the internal battery), the black pen of the multimeter is positive potential (connected to the positive terminal of the internal battery).

(2) Test procedure:

Connect the red pen to the source of the MOSFET S; connect the black pen to the drain of the MOSFET D. At this time, the needle indication should be infinity. If there is an ohmic index, indicating that the tube under test has a leakage phenomenon, this tube can not be used.

Maintain the above state; at this time with a 100K ~ 200K resistor connected to the gate and drain; at this time the needle should indicate the number of ohms the smaller the better, generally can be indicated to 0 ohms, this time it is a positive charge through the 100K resistor on the MOSFET gate charging, resulting in a gate electric field, due to the electric field generated by the conductive channel resulting in the drain and source conduction, so the multimeter needle deflection, deflection angle is large (Ohm's index is small) to prove that the discharge performance is good.

And then connected to the resistor removed, then the multimeter pointer should still be the MOSFET on the index remains unchanged. Although the resistor to take away, but because the resistor to the gate charged by the charge does not disappear, the gate electric field continues to maintain the internal conductive channel is still maintained, which is the characteristics of the insulated gate type MOSFET.

If the resistor to take away the needle will slowly and gradually return to high resistance or even return to infinity, to consider that the measured tube gate leakage.

At this time with a wire, connected to the gate and source of the tube under test, the multimeter's pointer immediately returned to infinity. The connection of the wire so that the measured MOSFET, gate charge release, the internal electric field disappears; conductive channel also disappears, so the drain and source between the resistance and become infinite.

2, high-power MOSFET replacement

In the repair of televisions and all kinds of electrical equipment, encountering component damage should be replaced with the same type of components. However, sometimes the same components are not on hand, it is necessary to use other types of replacement, so that we must take into account all aspects of performance, parameters, dimensions, etc., such as television inside the line output tube, as long as the consideration of the voltage, current, power can generally be replaced (line output tube almost the same dimensions of the appearance), and the power tends to be bigger and better.

For MOSFET replacement, although also this principle, it is best to prototype the best, in particular, do not pursue the power to be larger, because the power is large; input capacitance is large, changed and excitation circuits do not match the excitation of the charge current limiting resistor of the irrigation circuit of the size of the resistance value and the input capacitance of the MOSFET is related to the selection of the power of large despite the capacity of large, but the input capacitance is also large, and the input capacitance is also large, and the power is not large.

Input capacitance is also large, the excitation circuit is not good, which in turn will make the MOSFET on and off performance worse. Shows the replacement of different models of MOSFETs, taking into account the input capacitance of this parameter.

For example, there is a 42-inch LCD TV backlight high-voltage board damage, after checking the internal high-power MOSFET damage, because there is no prototype number of replacement, the choice of a voltage, current, power are not less than the original MOSFET replacement, the result is the backlight tube appears to be a continuous flicker (startup difficulties), and finally replaced with the same type of the original to solve the problem.

Detected damage to the high-power MOSFET, replacement of its peripheral components of the perfusion circuit must also be replaced, because the damage to the MOSFET may also be poor perfusion circuit components caused by the damage to the MOSFET. Even if the MOSFET itself is damaged, the moment the MOSFET breaks down, the perfusion circuit components are also harmed and should be replaced.

Just as we have a lot of clever repair master in the repair of the A3 switching power supply; as long as the switching tube is found to break down, it is also the front of the 2SC3807 excitation tube together with the replacement of the same reason (although the 2SC3807 tube, measured with a multimeter is good).