Must-read: Field Effect Transistor Testing Methods and Practical Operation Experience Sharing

1. Use a pointer multimeter to identify the field effect tube

(1) Using the resistance measurement method to identify the electrodes of junction field effect transistors

According to the phenomenon that the forward and reverse resistance values ​​of the PN junction of the field effect tube are different, the three electrodes of the junction field effect tube can be identified. Specific method: Set the multimeter to the R×1k position, select any two electrodes, and measure their forward and reverse resistance values ​​respectively. When the forward and reverse resistance values ​​of two electrodes are equal and are several thousand ohms, the two electrodes are the drain D and the source S respectively. Because for junction field effect tubes, the drain and source are interchangeable, and the remaining electrode must be the gate G. You can also touch the black probe (red probe) of the multimeter to any electrode, and touch the other probe to the remaining two electrodes in turn to measure their resistance values. When the resistance values ​​measured twice are approximately equal, the electrode touched by the black probe is the gate, and the other two electrodes are the drain and source respectively. If the resistance values ​​measured twice are both very large, it means that it is the reverse direction of the PN junction, that is, both are reverse resistance, it can be determined that it is an N-channel field effect transistor, and the black test pen is connected to the gate; if the resistance values ​​measured twice are both very small, it means that it is a forward PN junction, that is, forward resistance, it is determined to be a P-channel field effect transistor, and the black test pen is also connected to the gate. If the above situation does not occur, you can swap the black and red test pens and test according to the above method until the gate is identified.

(2) Using the resistance measurement method to determine the quality of field effect transistors

The resistance measurement method is to use a multimeter to measure the resistance between the source and drain, gate and source, gate and drain, gate G1 and gate G2 of the field effect tube to determine whether it is consistent with the resistance value indicated in the field effect tube manual to determine whether the tube is good or bad. Specific method: First, set the multimeter to R×10 or R×100, and measure the resistance between the source S and the drain D, which is usually in the range of tens of ohms to thousands of ohms (it can be seen in the manual that the resistance values ​​of various types of tubes are different). If the measured resistance value is greater than the normal value, it may be due to poor internal contact; if the measured resistance value is infinite, it may be internal disconnection. Then set the multimeter to R×10k, and then measure the resistance between the gate G1 and G2, the gate and the source, and the gate and the drain. When the measured resistance values ​​are all infinite, it means that the tube is normal; if the above resistance values ​​are too small or are connected, it means that the tube is bad. It should be noted that if the two gates are disconnected in the tube, the component replacement method can be used for detection.

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(3) Estimating the amplification capability of field effect tubes using the induction signal input method

Specific method: Use the multimeter resistance R×100, connect the red test lead to the source S, the black test lead to the drain D, and add a 1.5V power supply voltage to the field effect tube. At this time, the needle indicates the resistance value between the drain and the source. Then pinch the gate G of the junction field effect tube with your hand and add the induced voltage signal of the human body to the gate. In this way, due to the amplification of the tube, the drain-source voltage VDS and the drain current Ib will change, that is, the resistance between the drain and the source has changed, so it can be observed that the needle has a large swing. If the needle swings less when the gate is pinched, it means that the tube has a poor amplification ability; if the needle swings more, it means that the tube has a large amplification ability; if the needle does not move, it means that the tube is broken.

According to the above method, we use the R×100 range of the multimeter to measure the junction field effect tube 3DJ2F. First, open the G pole of the tube, and measure the drain-source resistance RDS to be 600Ω. After pinching the G pole with your hand, the needle swings to the left, and the indicated resistance RDS is 12kΩ. The swing amplitude of the needle is large, indicating that the tube is good and has a large amplification capacity.

There are a few points to note when using this method: First, when you hold the gate of the field effect tube with your hand during the test, the multimeter needle may swing to the right (the resistance value decreases) or to the left (the resistance value increases). This is because the AC voltage induced by the human body is high, and different field effect tubes may have different working points when measured with the resistance range (either working in the saturation zone or in the unsaturated zone). The test shows that the RDS of most tubes increases, that is, the needle swings to the left; the RDS of a few tubes decreases, causing the needle to swing to the right. But no matter what the direction of the needle swing, as long as the swing amplitude of the needle is large, it means that the tube has a large amplification capacity. Second, this method is also applicable to MOS field effect tubes. However, it should be noted that the input resistance of the MOS field effect tube is high, and the induced voltage allowed by the gate G should not be too high, so do not pinch the gate directly with your hands. You must use the insulated handle of the screwdriver to touch the gate with a metal rod to prevent the human body induced charge from being directly added to the gate, causing the gate to break down. Third, after each measurement, the GS poles should be short-circuited. This is because a small amount of charge will be stored on the GS junction capacitor, which will build up the VGS voltage. This may cause the needle to not move when measuring again. This can only be done by short-circuiting the charge between the GS electrodes and discharging it.

(4) Using the resistance measurement method to identify unmarked field effect transistors

First, use the resistance measurement method to find two pins with resistance values, that is, the source S and the drain D. The remaining two pins are the first gate G1 and the second gate G2. Write down the resistance value between the source S and the drain D measured by the two test pens, swap the test pens and measure again, and write down the measured resistance value. The larger resistance value is the one connected to the black test pen as the drain D; the red test pen is connected to the source S. The S and D poles identified by this method can also be verified by estimating the amplification capacity of the tube, that is, the black test pen with large amplification capacity is connected to the D pole; the red test pen is connected to the ground pole, and the detection results of the two methods should be the same. After determining the position of the drain D and the source S, install the circuit according to the corresponding position of D and S. Generally, G1 and G2 will also align in turn, which determines the position of the two gates G1 and G2, and thus determines the order of the D, S, G1, and G2 pins.

(5) Determine the transconductance by measuring the change in reverse resistance

When measuring the transconductance performance of a VMOS N-channel enhancement field effect transistor, the red test lead can be connected to the source S and the black test lead to the drain D, which is equivalent to adding a reverse voltage between the source and the drain. At this time, the gate is open and the reverse resistance value of the tube is very unstable. Set the ohm range of the multimeter to the high resistance range of R×10kΩ, and the voltage in the meter is relatively high. When the hand touches the gate G, it will be found that the reverse resistance value of the tube has a significant change. The greater the change, the higher the transconductance value of the tube; if the transconductance of the tube being measured is very small, the reverse resistance value will not change much when measured by this method.

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2. Precautions for using field effect tubes

(1) In order to use field effect transistors safely, the circuit design must not exceed the limit values ​​of the tube's dissipated power, maximum drain-source voltage, maximum gate-source voltage, and maximum current.

(2) When using various types of field effect transistors, they must be connected to the circuit in strict accordance with the required bias, and the polarity of the field effect transistor bias must be observed. For example, the gate, source and drain of a junction field effect transistor are PN junctions, and the gate of an N-channel transistor cannot be positively biased; the gate of a P-channel transistor cannot be negatively biased, and so on.

(3) Due to the extremely high input impedance of MOS field effect tubes, the lead pins must be short-circuited during transportation and storage, and metal shielding packaging must be used to prevent external induced potential from breaking down the gate. In particular, it is important to note that MOS field effect tubes cannot be placed in plastic boxes. It is best to store them in metal boxes, and also pay attention to moisture-proofing the tubes.

(4) In order to prevent the gate of the field effect tube from being induced and broken down, all test instruments, workbenches, electric soldering irons, and the circuit itself must be well grounded. When soldering the tube pins, solder the source first. Before connecting to the circuit, all the lead ends of the tube should be kept short-circuited to each other, and the short-circuit material should be removed after soldering. When removing the tube from the component rack, the human body should be grounded in an appropriate manner, such as using a grounding ring. Of course, if an advanced gas-heating electric soldering iron can be used, soldering the field effect tube is more convenient and safe. When the power is not turned off, the tube must never be inserted into or removed from the circuit. The above safety measures must be taken into account when using field effect tubes.

(5) When installing the field effect tube, pay attention to the installation position to avoid being close to the heating element as much as possible; in order to prevent the tube from vibrating, it is necessary to tighten the tube body; when bending the pin lead, it should be 5 mm larger than the root size to prevent breaking the pin and causing air leakage.

For power FETs, good heat dissipation conditions are required. Because power FETs are used under high load conditions, sufficient heat sinks must be designed to ensure that the shell temperature does not exceed the rated value, so that the device can work stably and reliably for a long time.

In short, to ensure the safe use of field effect tubes, there are many things to pay attention to and various safety measures to be taken. The vast number of professional and technical personnel, especially the vast number of electronics enthusiasts, must proceed from their actual situation and take practical measures to use field effect tubes safely and effectively.

3. VMOS Field Effect Transistor

VMOS field effect transistor (VMOSFET) is abbreviated as VMOS tube or power field effect tube, and its full name is V-groove MOS field effect tube. It is a new high-efficiency, power switching device developed after MOSFET. It not only inherits the high input impedance (≥108W) and small drive current (about 0.1μA) of MOS field effect tube, but also has excellent characteristics such as high withstand voltage (up to 1200V), large working current (1.5A~100A), high output power (1~250W), good linearity of transconductance, and fast switching speed. It is precisely because it combines the advantages of electron tubes and power transistors that it is widely used in voltage amplifiers (voltage amplification can reach thousands of times), power amplifiers, switching power supplies and inverters.

VMOS field effect power tube has the advantages of extremely high input impedance and large linear amplification area, especially it has negative current temperature coefficient, that is, when the gate-source voltage remains unchanged, the on-current will decrease with the increase of tube temperature, so there is no tube damage caused by "secondary breakdown". Therefore, the parallel connection of VMOS tubes is widely used.

As we all know, the gate, source and drain of traditional MOS field effect tubes are roughly on the same horizontal plane of the chip, and its working current basically flows in the horizontal direction. VMOS tubes are different. From Figure 1, we can see its two major structural features: first, the metal gate adopts a V-groove structure; second, it has vertical conductivity. Since the drain is led out from the back of the chip, the ID does not flow horizontally along the chip, but starts from the heavily doped N+ region (source S), flows through the P channel into the lightly doped N-drift region, and finally vertically downward to the drain D. The direction of the current is shown by the arrow in the figure. Because the flow cross-sectional area is increased, a large current can pass. Since there is a silicon dioxide insulating layer between the gate and the chip, it still belongs to an insulated gate type MOS field effect tube.

The main domestic manufacturers of VMOS field-effect transistors include Factory 877, Tianjin Semiconductor Device Factory No. 4, Hangzhou Electron Tube Factory, etc. Typical products include VN401, VN672, VMPT2, etc.

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The following is a method for detecting VMOS tubes:

1. Determine the gate G

Set the multimeter to the R×1k position and measure the resistance between the three pins. If the resistance between a pin and its two pins is infinite, and it is still infinite after swapping the test leads, it proves that this pin is the G pole, because it is insulated from the other two pins.

2. Determine the source S and drain D

As shown in Figure 1, there is a PN junction between the source and drain. Therefore, the S pole and the D pole can be identified based on the difference in the forward and reverse resistance of the PN junction. Use the swapping probe method to measure the resistance twice. The one with a lower resistance value (usually several thousand ohms to more than ten thousand ohms) is the forward resistance. At this time, the black probe is connected to the S pole and the red probe is connected to the D pole.

3. Measure the drain-source on-state resistance RDS (on)

Short-circuit the GS pole, select the R×1 position of the multimeter, connect the black test lead to the S pole and the red test lead to the D pole, the resistance should be a few ohms to more than ten ohms.

Due to different test conditions, the measured RDS(on) value is higher than the typical value given in the manual. For example, using a 500-type multimeter with R×1 gear to measure an IRFPC50 VMOS tube, RDS(on) = 3.2W, which is greater than 0.58W (typical value).

4. Check transconductance

Set the multimeter to the R×1k (or R×100) position, connect the red probe to the S pole and the black probe to the D pole, and touch the gate with a screwdriver. The needle should have obvious deflection. The greater the deflection, the higher the transconductance of the tube.

Precautions:

(1) VMOS tubes are also divided into N-channel tubes and P-channel tubes, but the vast majority of products are N-channel tubes. For P-channel tubes, the positions of the test leads should be swapped during measurement.

(2) There are a few VMOS tubes with protection diodes between GS, so items 1 and 2 in this detection method are no longer applicable.

(3) There is also a VMOS power module on the market that is specifically used for AC motor speed regulators and inverters. For example, the IRFT001 module produced by IR Corporation of the United States has three N-channel and three P-channel tubes inside, forming a three-phase bridge structure.

(4) The VNF series (N-channel) products currently on the market are ultra-high frequency power field effect transistors produced by Supertex Corporation of the United States. Their maximum operating frequency fp=120MHz, IDSM=1A, PDM=30W, and common source small signal low-frequency transconductance gm=2000μS are suitable for high-speed switching circuits and broadcasting and communication equipment.

(5) When using a VMOS tube, a suitable heat sink must be added. Taking VNF306 as an example, the maximum power of this tube can reach 30W only after a 140×140×4 (mm) heat sink is added.

(6) When multiple tubes are connected in parallel, the inter-electrode capacitance and distributed capacitance increase accordingly, which deteriorates the high-frequency characteristics of the amplifier and easily causes high-frequency parasitic oscillation of the amplifier through feedback. For this reason, the number of parallel composite tubes is generally no more than 4, and an anti-parasitic oscillation resistor is connected in series on the base or gate of each tube.