Thyristor AC switch module with zero-crossing trigger circuit

1 Introduction

Since the invention of thyristors in 1957, they have been widely used in various fields of the national economy due to their simple structure, convenient use and stable and reliable performance, playing a significant role in industrial development, technological progress and energy conservation. At present, the manufacturing process and application technology of thyristors have become quite mature, and are developing in the direction of modularization with smaller size, lighter weight, more compact structure, higher reliability, different internal wiring circuits and different functions. There are also various so-called "thyristor intelligent modules" that integrate phase-shift trigger systems, protection systems and thyristor chips in the same housing. This article will briefly introduce the structure, process, application scenarios, overcurrent and overvoltage protection, heat dissipation selection and other technical issues of the thyristor AC switch module with zero-crossing trigger circuit (hereinafter referred to as AC switch module), which was successfully developed by Changzhou Ruihua Power Electronics Co., Ltd. and has obtained a national patent and is a national key new product.

2Structural technology, application fields and technical parameters

The AC switch module is a four-terminal contactless electronic switch, which consists of two anti-parallel thyristor chips and a zero-crossing trigger circuit, and is mixed and integrated in the same insulating resin shell, as shown in Figure 1. The insulating module forms an insulation between the input and output terminals and the copper bottom plate, and its insulation withstand voltage is ≥2500VRMS. When the trigger signal is applied to the input terminal, its main circuit is in the on state, and when there is no trigger signal, it is in the blocking state. However, because the trigger circuit is a zero-crossing trigger, the turn-on time of the output device will be delayed to the vicinity of the zero-point crossing of the AC sine wave voltage (generally about ±10V), and its working waveform is shown in Figure 2. As can be seen from Figure 2, when ωt=ωt1, after the input control current is added, the thyristor does not turn on immediately, until the power supply voltage crosses the zero point (that is, about ±10V), the thyristor is turned on, and the load flows current. When ωt=ωt2, the input control current is stopped, and the thyristor is not turned off immediately until the power supply voltage crosses near the zero point (i.e., about ±10V), and then the load current stops. In this way, the load current is a sine wave, reducing interference to the power grid.

Figure 3 is a simplified structural diagram of the AC switch module. The copper base plate in the figure adopts pre-bending technology so that it can still contact the heat sink well after heat welding, thereby greatly reducing the contact thermal resistance of the module. The use of DBC ceramic plates that are both insulating and thermally conductive can reduce the thermal resistance of the module and improve its current carrying capacity on the one hand, and on the other hand, the insulation withstand voltage between the main electrode and control electrode of the module and the copper base plate is ≥2500VRMS. Therefore, several AC switch modules can be installed on the same grounded heat sink at the same time, which is convenient and safe to use and greatly reduces the size of the device. The thyristor chips are all imported glass passivated square chips, which makes the module electrical parameters consistent and highly reliable. The module uses multiple sealing protections such as RTV silicone rubber, elastic silicone gel and epoxy resin to improve the air tightness and moisture resistance of the module, so that the module can operate stably and reliably for a long time.

The zero-crossing trigger circuit is made on a multi-layer PCB board and placed on top of the power device. The circuit board is coated with RTV insulating silicone rubber to improve the insulation performance between the high and low voltages on the board. The zero-crossing trigger circuit adopts advanced optical coupling devices, built-in integrated zero-crossing detection circuit, and optoelectronic isolation technology to make the module have strong static dv/dt capability. The insulation withstand voltage between the input control terminal and the output power terminal is as high as 7.5kV, which overcomes the previous trigger mode that is easily affected by grid voltage fluctuations and power waveform distortion, making the output waveform sinusoidal, with no waveform distortion, small electromagnetic interference, and no noise. The trigger circuit is simple and reliable. By adjusting the input resistance value of the input control circuit, it can adapt to the requirements of different input voltages (5V, 12V, 24V), but the input control current must be controlled at about 12mA, which can double the life of the trigger device.

Compared with mechanical switches, AC switch modules have the advantages of small size, light weight, fast switching speed, high sensitivity, no noise, explosion-proof, no spark, long life, maintenance-free, vibration-resistant, shock-resistant, and high reliability. Therefore, it has been widely used in computer peripheral interfaces and devices, and is used for constant temperature control, AC motor control, power regulation, solenoid valve control, single-phase and three-phase AC contactless power switches, CNC machinery, remote control and various industrial automation devices, and thyristor switched capacitor (TSC) static reactive power compensation, etc., which provides a reliable device material basis for solving mechatronics, miniaturization, automation, anti-interference, explosion-proof, etc.

The main technical parameters of the AC switch module are shown in Table 1.

3Overcurrent and overvoltage protection

The overvoltage and overcurrent protection of thyristor AC switch modules are the same as the protection methods of discrete thyristors. Module overcurrent protection can be achieved by external fast fuses, sensors, and fast overcurrent relays, but the commonly used method is to connect an external fast fuse. Figure 4 shows its connection method. The principle of selecting a fast fuse is that its rated voltage should be slightly greater than the normal working voltage of the circuit, such as a 500V fast fuse for a 380V voltage; its rated current should be selected according to the actual current IR (RMS) passing through the module it protects, rather than selecting a fast fuse according to the nominal rated current value of the module.

The module overvoltage protection generally adopts the combination of RC absorption and varistor. For overvoltage with short duration and small energy, the method of parallel RC absorption circuit at both ends of the module is generally adopted. The absorption capacitor converts the electromagnetic energy of the overvoltage into electrostatic energy storage. In addition to preventing circuit oscillation, the absorption resistor can also limit the turn-on loss and di/dt value caused by the discharge of the capacitor when the thyristor is turned on. The wiring method is shown in Figure 5. The RC value connected in parallel to the AC switch module can refer to the values ​​listed in Table 2. For overvoltage with longer duration and larger energy, such as overvoltage caused by lightning strike, varistor protection will be adopted. Its wiring method is shown in Figure 6. When selecting a varistor, its nominal voltage (V1mA) value must be determined first, which refers to the voltage at both ends of the varistor when 1mA current flows through it. Taking into account the fluctuation of the power grid and the safety factor, a 1000V varistor is generally used for a 380V power supply and a 630V varistor is used for a 220V power supply. The current carrying capacity of the varistor should be greater than the actual surge current value of the circuit, generally 3 to 15kA.

4. Radiator selection

During the operation of the AC switch module, the junction temperature of the thyristor chip will rise. In order to maintain the junction temperature below the maximum rated value of 125°C, a heat sink must be used. Therefore, the quality of heat dissipation directly affects the safe, stable and reliable operation of the module. At present, the heat dissipation methods include water cooling, air cooling (forced and natural air cooling) and heat pipe cooling. This article will briefly introduce the selection method of air-cooled heat sinks. The main circuit of the AC switch module is composed of two anti-parallel thyristors, as shown in Figure 7.

The thermal resistance of the heat sink can be calculated based on the average on-state current IT (AV) of a single thyristor in the module, the on-state peak voltage VTM, the module contact thermal resistance RTHCH and the average power PT (AV) = 0.85VTMIT (AV).

Where: TC is the shell temperature (i.e. the temperature of the copper base plate);

Ta is the ambient temperature.

If the maximum temperature TCmax of the copper base plate is unknown, then

Where: Tjm is the maximum junction temperature of the thyristor, 125°C;

Rthjc is the thermal resistance from junction to copper baseplate.

After finding the heat sink thermal resistance RTHHA, select the corresponding heat sink model specification (commonly used are DXC-450, DXC-616, DXC-573, etc.), and its thermal resistance should be smaller than the calculated thermal resistance, that is,

RTHHA (calculated value) > RTHHA (actual value of the selected heat sink) (3)

If there are several modules installed on a radiator, substitute ∑PT(AV)=nPT(AV) into formula (3) to obtain ∑RTHHA and select the radiator.

sinωtdωt, that is, IRMS≈2.22IT (AV) is calculated. According to the thermal resistance value of the optional heat sink (or the thermal resistance curve of the heat sink), the appropriate heat sink length can be selected. If necessary, the temperature measurement method of punching a hole in the geometric center of the long side of the module copper bottom plate can be used to check the correctness of the selected heat sink length.

5 Conclusion

At present, the application scope of thyristor AC switch module is very wide, and the application technology is also very mature. The AC switch module with zero-crossing trigger circuit produced by Changzhou Ruihua Power Electronic Devices Co., Ltd. has been widely used in AC motor soft starters, various industrial heating, ovens, automatic power regulation systems for drying room heating, steel mill conveyor belts, cranes and other drive systems, making the mechanical structure of the whole machine compact, simplified, reduced in size, lighter in weight and more reliable. In particular, after using the AC switch module in the TSC static reactive compensator to replace the mechanical contactor, it can not only achieve maintenance-free, save a lot of maintenance and replacement work, but also can invest in compensation capacitance when the power supply voltage passes through zero, thereby avoiding the generation of impact current and instantaneous grid voltage fluctuations, and realizing rapid reactive power compensation. Now the thyristor AC switch module has become the basic component of mechatronics and industrial automation, and will surely achieve greater development.