Reverse Conducting Thyristor (RCT)

Reverse-Conducting Thyristir RCT (Reverse-Conducting Thyristir) is also called reverse-conducting thyristor. Its characteristic is that a diode is connected in reverse parallel between the anode and cathode of the thyristor, so that the emitter junctions of the anode and cathode are short-circuited. Due to this special circuit structure, it has excellent properties such as high voltage resistance, high temperature resistance, short off-time, and low on-state voltage. For example, the turn-off time of a reverse-conducting thyristor is only a few microseconds and the operating frequency reaches tens of kilohertz, which is better than that of a fast thyristor (FSCR). This device is suitable for switching power supplies and UPS uninterruptible power supplies. One RCT can replace one thyristor and one freewheeling diode. It is not only easy to use, but also simplifies circuit design. The symbol and equivalent circuit of the reverse conduction thyristor are shown in Figure 1(a) and (b). Its volt-ampere characteristics are shown in Figure 2. It can be seen from the figure that the volt-ampere characteristics of the reverse conduction thyristor are asymmetric. The forward characteristics are the same as those of ordinary thyristors SCR, while the reverse characteristics are the same as those of silicon rectifiers (only the coordinate positions are different). A typical product of reverse conduction thyristor is S3900MF produced by Radio Corporation of America (RCA), whose appearance is shown in Figure 1(c). It is packaged in TO-220, and the three terminals are gate G, anode A, and cathode K. The main parameters of S3900MF are as follows: Off-state repetitive peak voltage VDRM: >750V On-state average current IT (AV): 5A Maximum on-state voltage VT: 3V (IT=30A) Maximum reverse conduction voltage VTR: <0.8V Maximum gate Trigger voltage VGT: 4V Maximum gate trigger current IGT: 40mA Off time tooff: 2.4μs On-state voltage critical rise rate du/dt: 120V/μs On-state surge current ITSM: 80A Use a multimeter and megger to check the inverse The quality of the conductor thyristor. The test content is mainly divided into three parts: 1. To check the reverse conductivity, select R×1 level of the multimeter, connect the black test lead to the K pole, and the red test lead to the A pole (see Figure 3(a)). The resistance value should be 5~10Ω. If the resistance is zero, it proves that the internal diode is short-circuited; if the resistance is infinite, it indicates that the diode is open-circuit. 2. Measure the forward DC transition voltage V (BO). Connect the circuit as shown in (b), and then shake the megohmmeter at the rated speed to make the RCT break down in the forward direction. Read the V (BO) value from the DC voltmeter. 3. Example of checking triggering capability: Use a 500-type multimeter and a ZC25-3 megger to measure an S3900MF reverse-conducting thyristor. Select the R×1k, R×100, R×10 and R×1 gears in order to measure the reverse resistance between AK poles, and use the reading voltage method to find the reverse conduction voltage VTR of the internal diode (actually the forward voltage of the diode VF). Then use a megohmmeter and a multimeter at 500VDC to measure the V (BO) value. All data are organized into Table 1. This proves that the quality of the tested RCTs is good. Notes: (1) The VTR of S3900MF is <0.8V, so it is advisable to choose R×1 level for measurement. (2) If the ITR value is calculated using the reading current method, the reverse volt-ampere characteristics can also be drawn.