IGBTs have three electrodes, called the gate (G), collector (C), and emitter (E).
I. Using an analog multimeter to diagnose field-effect transistors
(1) Identifying the electrodes of a junction field-effect transistor using the resistance measurement method
Based on the difference in forward and reverse resistance values of the PN junction in a junction field-effect transistor (JFET), the three electrodes of the JFET can be identified. The specific method is as follows: Set the multimeter to the R×1k range, randomly select two electrodes, and measure their forward and reverse resistance values respectively. When the forward and reverse resistance values of two electrodes are equal and both are several thousand ohms, then these two electrodes are the drain (D) and source (S), respectively. Because the drain and source are interchangeable for a JFET, the remaining electrode must be the gate (G). Alternatively, you can touch any electrode with the black probe of the multimeter (the red probe will also work), and then touch the other two electrodes in turn, measuring their resistance values. When two approximately equal resistance values are obtained, the electrode touched by the black probe is the gate, and the other two electrodes are the drain and source, respectively. If both measured resistance values are very high, it indicates a reverse PN junction, meaning both are reverse resistances, indicating an N-channel MOSFET, and the black probe is connected to the gate. If both measured resistance values are very low, it indicates a forward PN junction, meaning both are forward resistances, indicating a P-channel MOSFET, and the black probe is also connected to the gate. If neither of these conditions is met, the black and red probes can be swapped, and the test can be repeated until the gate is identified.
(2) Using the resistance measurement method to determine the quality of a field-effect transistor
The resistance measurement method involves using a multimeter to measure the resistance between the source and drain, gate and source, gate and drain, and gate G1 and gate G2 of a field-effect transistor (FET). The method compares the measured resistance with the values specified in the FET's datasheet to determine its condition. Specifically: First, set the multimeter to the R×10 or R×100 range and measure the resistance between the source (S) and drain (D). This resistance is typically in the range of tens to thousands of ohms (the resistance values vary depending on the FET model, as can be found in the datasheet). If the measured resistance is greater than normal, it may be due to poor internal contact; if the measured resistance is infinite, it may indicate an internal open circuit. Next, set the multimeter to the R×10k range and measure the resistance between gates G1 and G2, between the gate and source, and between the gate and drain. If all these resistances are infinite, the FET is normal; if the measured resistances are too low or show a short circuit, the FET is faulty. Note that if both gates are internally open, a component substitution method can be used for testing.
(3) Estimating the amplification capability of the field-effect transistor using the inductive signal input method
Specific method: Using a multimeter set to the R×100 range, connect the red probe to the source (S) and the black probe to the drain (D). Apply a 1.5V power supply to the MOSFET. The meter needle will then indicate the resistance between the drain and source. Next, hold the gate (G) of the MOSFET by hand and apply a voltage signal induced by your body to the gate. Due to the transistor's amplification effect, both the drain-source voltage (VDS) and the drain current (Ib) will change, resulting in a change in the drain-source resistance. This will be observed as a significant swing in the meter needle. If the needle swings only slightly when the gate is held, the transistor has poor amplification capability; a larger swing indicates a larger amplification capability; and no movement indicates a faulty transistor.
Following the above method, we used the R×100 range of a multimeter to test the 3DJ2F junction field-effect transistor. First, we opened the gate (G) of the transistor and measured the drain-source resistance (RDS) to be 600Ω. Then, we held the G terminal by hand, and the meter needle swung to the left, indicating a resistance of 12kΩ. The large swing of the meter needle indicates that the transistor is good and has a large amplification capability.
Several points should be noted when using this method: First, when testing a field-effect transistor (FET), the multimeter needle may swing to the right (resistance decreases) or to the left (resistance increases) when the gate is held by hand. This is because the AC voltage induced by the human body is relatively high, and different FETs may have different operating points when measured with resistance mode (either operating in the saturation region or the unsaturation region). Experiments show that for most transistors, the RDS increases, causing the needle to swing to the left; for a few transistors, the RDS decreases, causing the needle to swing to the right. However, regardless of the direction of the needle swing, a large swing amplitude indicates that the transistor has a large amplification capability. Second, this method is also applicable to MOS FETs. However, it should be noted that MOS FETs have high input resistance, and the gate G should not be allowed to be too high. Therefore, do not hold the gate directly with your hand; instead, use the insulated handle of a screwdriver and touch the gate with the metal rod to prevent the induced charge from the human body from being directly applied to the gate, causing gate breakdown. Third, after each measurement, the gate and source terminals should be short-circuited briefly. This is because a small amount of charge will be charged on the GS junction capacitor, establishing a VGS voltage, which may cause the meter needle to not move when measuring. Only by short-circuiting and discharging the charge between the GS electrodes can the problem be solved.
(4) Identify unmarked field-effect transistors using the resistance measurement method.
First, use resistance measurement to identify the two pins with resistance values: the source (S) and drain (D). The remaining two pins are the first gate (G1) and the second gate (G2). Record the resistance value measured between the source (S) and drain (D) using the two probes. Reverse the probes and measure again, recording the result. The pin with the larger resistance value is the drain (D) connected to the black probe, and the source (S) connected to the red probe. This method of identifying the S and D pins can be verified by estimating the transistor's amplification capability. The pin with the larger amplification capability (black probe) is the drain (D), and the pin with the larger amplification capability (red probe) is the source (S). Both methods should yield the same result. Once the positions of the drain (D) and source (S) are determined, assemble the circuit according to their corresponding positions. Generally, G1 and G2 will also align sequentially, thus determining the positions of the two gates (G1 and G2), and consequently, the pin order of D, S, G1, and G2.
(5) Determine the transconductance by measuring the change in the reverse resistance value.
When measuring the transconductance of a VMOSN channel enhancement-mode MOSFET, the red probe can be connected to the source (S) and the black probe to the drain (D), which is equivalent to applying a reverse voltage between the source and drain. At this time, the gate is open, and the reverse resistance value of the transistor is very unstable. Set the multimeter to the high resistance range of R×10kΩ; at this point, the internal voltage of the meter is relatively high. When you touch the gate (G) with your hand, you will find a significant change in the reverse resistance value of the transistor. The greater the change, the higher the transconductance value. If the transconductance of the transistor under test is very small, the change in reverse resistance value will be small when measured using this method.
II. Precautions for using field-effect transistors
(1) In order to use the field-effect transistor safely, the circuit design must not exceed the limit values of parameters such as the transistor's power dissipation, 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 according to the required bias and the polarity of the bias must be observed. For example, the gate-source-drain of a junction field-effect transistor is a PN junction, the gate of an N-channel transistor cannot be positively biased, and the gate of an N-channel transistor cannot be negatively biased, etc.
(3) Due to the extremely high input impedance, the leads of MOS field-effect transistors must be short-circuited during transportation and storage, and they must be packaged with metal shielding to prevent external induced potential from breaking down the gate. In particular, MOS field-effect transistors should not be placed in plastic boxes; they should be stored in metal boxes, and care should be taken to protect them from moisture.
(4) To prevent induced breakdown of the MOSFET gate, all testing instruments, workbenches, soldering irons, and the circuit itself must be properly grounded; when soldering the pins, solder the source first; before connecting to the circuit, keep all leads of the MOSFET short-circuited, and remove the short-circuiting material only after soldering; when removing the MOSFET from the component rack, ensure proper grounding of the user, such as by using a grounding ring; of course, if an advanced gas-heated soldering iron can be used, soldering MOSFETs is more convenient and safer; never insert or remove the MOSFET from the circuit while the power is on. The above safety measures must be observed when using MOSFETs.
(5) When installing the field effect transistor, the installation position should be kept away from the heating element as much as possible; in order to prevent the pipe from vibrating, it is necessary to tighten the pipe shell; when bending the lead wire, it should be done at a distance greater than 5 mm from the root to prevent the lead wire from breaking and causing air leakage.
For power MOSFETs, good heat dissipation is essential. Because power MOSFETs are used under high load conditions, sufficient heat sinks must be designed to ensure that the case temperature does not exceed the rated value, so that the device can operate stably and reliably for a long time.
In short, there are many things to pay attention to and various safety measures to ensure the safe use of MOSFETs. Professional technicians, especially electronics enthusiasts, should take practical measures based on their own circumstances to use MOSFETs safely and effectively.
III. VMOS Field-Effect Transistor
A VMOS field-effect transistor (VMOSFET), also known as a V-groove MOSFET or power MOSFET, is a high-efficiency, power switching device developed after the MOSFET. It inherits the high input impedance (≥10⁸ W) and low drive current (around 0.1 μA) of MOSFETs, while also possessing excellent characteristics such as high voltage withstand (up to 1200 V), high operating current (1.5 A to 100 A), high output power (1 to 250 W), good transconductance linearity, and fast switching speed. Because it combines the advantages of vacuum tubes and power transistors, 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 transistors have advantages such as extremely high input impedance and a large linear amplification region. In particular, they have a negative current temperature coefficient, meaning that with a constant gate-source voltage, the conduction current decreases as the transistor temperature increases. Therefore, there is no transistor damage caused by the "secondary breakdown" phenomenon. As a result, parallel connection of VMOS transistors is widely used.
As is well known, in traditional MOS field-effect transistors, the gate, source, and drain are roughly on the same horizontal plane on the chip, and the operating current flows primarily horizontally. VMOS transistors are different. Figure 1 shows two main structural features: first, the metal gate uses a V-groove structure; second, it has vertical conductivity. Since the drain is led out from the back of the chip, the current ID does not flow horizontally along the chip. Instead, it starts from the heavily doped N+ region (source S), flows through the P-channel into the lightly doped N- drift region, and finally reaches the drain D vertically downwards. The current direction is shown by the arrows in the figure. Because the current-carrying cross-sectional area is increased, a large current can pass through. Because there is a silicon dioxide insulating layer between the gate and the chip, it still belongs to the insulated-gate MOS field-effect transistor category.
Major domestic manufacturers of VMOS field-effect transistors include Factory 877, Tianjin Semiconductor Device Factory No. 4, and Hangzhou Electron Tube Factory, with typical products such as VN401, VN672, and VMPT2.
The following describes the method for testing VMOS transistors.
1. Determine the gate G
Set the multimeter to the R×1k range and measure the resistance between the three pins. If you find that the resistance between a certain pin and the other two pins is infinite, and it remains infinite even after swapping the probes, then this pin is the gate (G) terminal, 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 source (S) and drain (D) terminals can be identified based on the difference in forward and reverse resistance of the PN junction. Measure the resistance twice using the probe-swapping method. The lower resistance value (typically several thousand to tens of thousands of ohms) is the forward resistance. In this case, the black probe is connected to the source (S), and the red probe is connected to the drain (D).
3. Measure the drain-source on-state resistance RDS(on)
Short-circuit the GS terminals, select the R×1 range on the multimeter, connect the black probe to the S terminal and the red probe to the D terminal, the resistance should be a few ohms to a dozen ohms.
Due to different testing conditions, the measured RDS(on) value is higher than the typical value given in the manual. For example, when measuring an IRFPC50 VMOS transistor using a 500 multimeter in the R×1 range, the RDS(on) is 3.2W, which is greater than 0.58W (typical value).
4. Check transconductance
Set the multimeter to the R×1k (or R×100) range, connect the red probe to the source (S) terminal and the black probe to the drain (D) terminal. Hold a screwdriver and touch the gate. The meter needle should deflect noticeably. The greater the deflection, the higher the transconductance of the transistor.
Precautions:
(1) VMOS transistors are also divided into N-channel transistors and P-channel transistors, but the vast majority of products are N-channel transistors. For P-channel transistors, the positions of the test leads should be reversed during measurement.
(2) A few VMOS transistors are located between GS and have protection diodes, so items 1 and 2 in this detection method are no longer applicable.
(3) There is also a type of VMOS power module on the market, which is specifically designed for use in AC motor speed controllers and inverters. For example, the IRFT001 module produced by IR Corporation of the United States contains three N-channel and three P-channel transistors, forming a three-phase bridge structure.
(4) Currently available VNF series (N-channel) products are ultra-high frequency power MOSFETs manufactured by Supertex Corporation of the United States. Their maximum operating frequency is fp=120MHz, IDSM=1A, PDM=30W, and common-source small-signal low-frequency transconductance gm=2000μS. They are suitable for high-speed switching circuits and broadcasting and communication equipment.
(5) A suitable heat sink must be added when using a VMOS transistor. Taking VNF306 as an example, the maximum power can only reach 30W after a 140×140×4 (mm) heat sink is added.
(6) When multiple transistors are connected in parallel, the inter-electrode capacitance and distributed capacitance increase accordingly, which deteriorates the high-frequency characteristics of the amplifier and can easily cause high-frequency parasitic oscillations in the amplifier through feedback. Therefore, the number of parallel composite transistors is generally no more than four, and an anti-parasitic oscillation resistor is connected in series on the base or gate of each transistor.
A simple method for testing the quality of an insulated gate bipolar transistor (IGBT).
1. Determine polarity
First, set the multimeter to the R×1KΩ range. When measuring with the multimeter, if the resistance between one terminal and the other two terminals is [value missing], then [the measurement will be performed].
If the resistance is infinite, and after swapping the probes, the resistance between this terminal and the other two terminals remains infinite, then this terminal is determined to be the gate (G). The other two terminals are then measured with a multimeter. If the measured resistance is infinite, and after swapping the probes, the measured resistance is smaller, then the terminal connected to the red probe is determined to be the collector (C), and the terminal connected to the black probe is determined to be the emitter (E).
2. Judging good from bad
Set the multimeter to the R×10KΩ range. Connect the black probe to the collector (C) of the IGBT and the red probe to the emitter (E). The multimeter pointer should be at zero. Touch both the gate (G) and collector (C) simultaneously with your finger. The IGBT will be triggered and turn on, and the multimeter pointer will swing towards the lower resistance and remain stationary. Then touch both the gate (G) and emitter (E) simultaneously again. This time, the IGBT will be turned off, and the multimeter pointer will return to zero. This indicates that the IGBT is working properly.
3. Precautions
Any analog multimeter can be used to test IGBTs. Note that when determining if an IGBT is good or bad, the multimeter must be set to the R×10KΩ range. Ranges below R×1KΩ have too low an internal battery voltage, preventing the IGBT from conducting and thus making it impossible to determine its condition. This method can also be used to test power MOSFETs (P-MOSFETs), inverters, soft starters, PLCs, HMIs, low-voltage electrical appliances, electrical automation engineering, constant pressure water supply equipment, musical fountain control systems, and for inverter repair.
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