VMOS field effect tube detection method and precautions

VMOS field effect tube detection method
(1). Determine the gate G.
　　Set the multimeter to R×1k and measure the resistance between the three pins. If the resistance of a pin and its two pins is infinite, and it is still infinite after exchanging 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 the drain. Therefore, according to the difference in the forward and reverse resistance of the PN junction, the S pole and the D pole can be identified. Use the method of exchanging test leads to measure the resistance twice. The one with a lower resistance value (generally several thousand ohms to more than ten thousand ohms) is the forward resistance. At this time, the black test lead is the S pole and the red test lead 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 value should be several 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 at R×1 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 R×1k (or R×100), connect the red test lead to the S pole and the black test lead to the D pole, and hold a screwdriver to touch the gate. The needle should have obvious deflection. The greater the deflection, the higher the transconductance of the tube.

Notes:
(1) VMOS tubes are also divided into N-channel tubes and P-channel tubes, but most products belong to N-channel tubes. For P-channel tubes, the position of the test leads should be swapped during measurement.
(2) A few VMOS tubes have protection diodes between GS, and items 1 and 2 in this test method are no longer applicable.
(3) There is also a VMOS tube power module on the market, which is specially 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 tubes 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. It is suitable for high-speed switching circuits and broadcasting and communication equipment.
(5) When using VMOS tubes, they must be equipped with a suitable heat sink. Taking VNF306 as an example, the maximum power can reach 30W after the tube is equipped with a 140×140×4 (mm) heat sink.

Comparison between field effect transistors and transistors
(1) Field effect transistors are voltage-controlled elements, while transistors are current-controlled elements. When only a small amount of current is allowed to be drawn from the signal source, field effect transistors should be used; when the signal voltage is low and more current is allowed to be drawn from the signal source, transistors should be used.
(2) Field effect transistors use majority carriers to conduct electricity, so they are called unipolar devices, while transistors use both majority carriers and minority carriers to conduct electricity. They are called bipolar devices.
(3) The source and drain of some field effect transistors can be used interchangeably, and the gate voltage can be positive or negative, which is more flexible than transistors.
(4) Field effect transistors can work under very small current and very low voltage conditions, and their manufacturing process can easily integrate many field effect transistors on a silicon wafer, so field effect transistors have been widely used in large-scale integrated circuits.