Abstract: Based on the testing and analysis of the main performance of domestically produced electrostatic induction thyristors (SITH), several driving circuits were developed, and three power supply circuits were constructed using them as main switching devices. Tests on these three circuits show that the electrostatic induction thyristor is a power electronic device with excellent characteristics. Domestically produced devices can be modified for specific applications to achieve optimal compatibility. Keywords: Electrostatic induction thyristor; Switching power supply; Power factor; Efficiency 1 Introduction In our previously published article, "Application Research of Electrostatic Induction Thyristors (SITH)" (see issue 12 of this journal, 2001), we studied the basic characteristics of domestically produced SITH devices and developed four driving circuits. Under the drive of these four circuits, the SITH devices achieved switching speeds below 0.2 ms. Now, we further expand the driving circuits and SITH devices together into practical switching power supply application circuits. Testing revealed relatively advanced performance indicators. This makes the application research of SITH devices more comprehensive, laying a solid foundation for its widespread application. 2 Circuit Research 2.1 Application Circuit (I) This application circuit is a switching power supply, designed based on the following considerations: (1) For high-power applications such as motor speed regulation and temperature control, it is determined to be an AC-DC converter, where AC specifically refers to a 50Hz single-phase 220V power frequency grid; (2) The power input must have a high power factor of 0.95 or higher and low noise, meeting or exceeding national standards; (3) The power efficiency is required to be above 90%; (4) It should reflect the advantages of the SITH transistor and avoid its weaknesses. The BOOST conversion program is used here (Figure 1). In the diagram, R1: 1kΩ; R2: 20Ω; R3: 10kΩ; R4: 5kΩ; R5: 5kΩ; R6: 800kΩ; R7: 10kΩ; R8: 20kΩ; R9: 1MΩ; R10: 0.2Ω; R11: 50kΩ; R12: 10kΩ; C1: 0.01mF; C2: 200pF; C3: 0.1mF; C4: 50mF; C5: 100mF/400V; C6: 2mF/～250V; C7: 2000pF; D1: 12V/0.5W Zener diode; D2: 3A fast recovery diode HER308; D3, D6: 2A/400V rectifier bridge; Q1, Q3: 8050 NPN transistors; Q2: 8550 PNP transistors; Q4: 10A/30V VDMOS; L1: 2mH/1A high-frequency inductor; L2: 2mH/1A high-frequency inductor; IC: UC3852. For detailed operating principles of the UC3852, please refer to the relevant documentation from Texas Instruments. It is important to note that the UC3852 is a switching power supply control chip specifically designed to improve the input power factor. Its basic principle is to operate at frequencies well above 50 Hz, controlling the switching transistors to conduct for a constant time Ton. During conduction, inductor L2 bears the entire AC input voltage Vin. At the end of Ton, the inductor current, i.e., the input current Iin = (Vin / L2) × Ton, therefore Iin is proportional to Vin. Next, the UC3852 turns off the switching transistor, and L2 discharges to the load. Iin decays linearly. When the UC3852 detects that Iin has decayed to zero, it turns the switching transistor back on, entering the next Ton period, as shown in Figure 2. In the figure, the upper part shows the driving waveform of the UC3852; the high level indicates that the switching transistor is on, and the high-level time Ton is constant. The lower part shows the waveforms of the input voltage Vin and the input current Iin. During the Ton period, Iin rises linearly, and the rate of rise is proportional to the current Vin value. Since it always starts from zero, the peak value of Iin is also proportional to Vin. In reality, the operating frequency of the UC3852 is much higher than shown in the figure, approximately 25kHz. Therefore, Vin can be considered constant in each triangular segment, and the average value of Iin is also proportional to Vin. After low-pass filtering by L1 and C6, the input current waveform is consistent with the input voltage, resulting in a high power factor. This is the simple working principle of the UC3852. The circuit parameters in Figure 1 constitute a 100W output circuit. The SITH transistor is the main switch, using the drive circuit (I) scheme, with Q1, Q2, Q3, and Q4 forming the SITH transistor drive circuit; L2 is the BOOST inductor; C5 is the output filter capacitor; R6, R7, R8, R9, and C3 form a feedback circuit for the UC3852 to control the output voltage stability; R12 and C7 are the timing circuit for the UC3852 operating frequency, set to around 25kHz here; L1 and C6 are low-pass filters to prevent high-frequency components in Iin from returning to the mains; R10 is the current sampling resistor, providing the UC3852 with the Iin waveform. As can be seen from the waveform in Figure 2, this is a discontinuous current mode. Its advantage is that the switching transistor always starts conducting from zero current and eventually reaches twice the average current, thus allowing the SITH transistor to avoid the weakness of slow conduction while leveraging its advantages of good high-current performance. It also allows diode D2 to withstand reverse voltage only after the forward current reaches zero, avoiding the problem of reverse recovery loss. The function of D2 is to prevent the output capacitor C5 from discharging to L2 and the SITH transistor. The forward current flowing through it is the load current, while the reverse voltage it withstands is 400V. In the test, a medium-speed fast recovery diode was used for D2, resulting in very low temperature rise. The output voltage of 400V is required for the circuit operation (refer to relevant data from Texas Instruments), and therefore cannot be lowered. The SITH transistor also withstands a 400V reverse voltage when turned off, providing a large margin, which is also an advantage. The main measured specifications are as follows: Input voltage: AC220V 50Hz; Input current: 0.58A; Input power: 111W; Power factor: 0.95; Output voltage: DC410V; Ripple peak-to-peak value: Vpp=20V; Output load resistance: 1610Ω; Output power: 104W; Efficiency: 0.94. Therefore, the SITH device and its driving circuit met the requirements, and the results were satisfactory. However, the SITH transistor had a power dissipation of approximately 3.5W during operation, making it the only device in the circuit with a significant temperature rise. The SITH transistor used in the circuit was a TO-220 package. Initially, with a 50mm × 60mm heatsink, the surface temperature rose to 50°C within 5 minutes of power-on, and the input power increased by 3W. Later, a CPU cooler with a fan was used, with a fan power of 1W. After 15 minutes of power-on, the surface temperature rise still did not exceed 5°C, and the input power remained around 111W. This suggests that, like other power devices, the switching loss of the SITH transistor increases with temperature. Poor heat dissipation can create a vicious cycle. Frequent temperature differences can also cause premature solder embrittlement, deteriorating the soldering quality. Therefore, good heat dissipation is the most effective and important means to ensure reliability and high efficiency. Because SITH transistors have strong overcurrent capability, under good heat dissipation, the traditional method of "reducing capacity" is unnecessary. Devices with current ratings comparable to the operating rating can be selected to reduce costs without affecting reliability. 2.2 Application Circuit (II) The output voltage of Application Circuit (I) is a fixed DC 400V, which cannot be flexibly adjusted and is not isolated from the input, often requiring additional stage circuitry. This Application Circuit (II) is a modification of Circuit (I), isolating the output while allowing voltage adjustment, as shown in Figure 3. In the picture: R1: 1kΩ; R2: 20Ω; R 3: 10kΩ; R4: 5kΩ; R5: 5kΩ; R6: 800kΩ; R7: 10kΩ; R8: 20kΩ; R9: 1MΩ; R10: 0.2Ω; R11: 50kΩ; R12: 10k Ω; R13: 20kΩ/3W; R14: 40Ω/10W; C 1: 0.01mf; C2: 200pf; C3: 0.1mf; C 4: 50mf; C5: 2000pf / 400V; C6: 2mf / ~250V; C7: 2000pf; C8: 0.1mf / 600V; C9: 3000mf/50V; C10: 2mf/ 400V; D1: 12V / 0.5W Zener diode; D2, D7, D8, D9: 3A fast recovery diode HER308; D3, D4, D5, D6: 2A / 400V rectifier bridge; D10: 46V / 1W Zener diode; Q1, Q3: NPN transistor 8050; Q2: PNP transistor 8550; Q4: VDMOS 10A / 30V; L1: 2mH / 1A high-frequency inductor; L2: 2mH / 1A high-frequency inductor; L3: 500mH / 2A high-frequency inductor; Tr: high-frequency transformer; turns ratio: 2:1×2; IC1: UC3852; IC2: optocoupler 521-1. This circuit parameter is for a 48V / 100W DC power supply output. Its working principle is the same as circuit (I), except that the output section uses capacitor C10 to cut the DC square wave from the drain of the SITH transistor into an AC square wave, which is then sent to the primary winding of transformer Tr. This is then coupled to the secondary winding, changing the voltage, and finally rectified and filtered before being output. The secondary winding is divided into two coils, each passing through different rectification and filtering stages. This is because the AC square wave passing through C10 is asymmetrical. The positive part is actually the discharge of inductor L2, which is a current source and requires rectification using the upper half of the secondary winding; the negative part is actually the discharge of C10, which is a voltage source and requires rectification using the lower half of the secondary winding. Changing the turns ratio of Tr can obtain the required output voltage and also isolate it from the input power grid. Optocoupler 521-1 is a negative feedback isolation component. D2, C5, C8, R13, and R14 form an overshoot absorption circuit. Due to leakage inductance, the transformer experiences a very strong overshoot when receiving current source pulses, which is detrimental to the components and also causes significant high-frequency noise, which must be attenuated. The absorption circuit can effectively attenuate the overshoot, but extensive testing has shown that this attenuation consumes a considerable amount of energy, at the cost of reduced efficiency. In this experiment, the absorption circuit consumed approximately 6W of power, and the overshoot still accounted for 25% of the main wave, causing the efficiency to drop from 91% to 84%, resulting in a noticeable heat source on the circuit board. Therefore, until the problem of efficient absorption is solved, this circuit should be considered for low-power applications. 2.3 Application Circuit (III) This is an AC switch that can switch 1000W loads and has a wide range of applications. Originally composed of thyristors or bidirectional thyristors, it is now replaced by two SITH transistors. It has zero-crossing turn-on and zero-crossing turn-off functions to reduce the impact on the power grid and can also be turned off immediately when needed. Due to the use of SITH transistors, it has the function of forced turn-off, i.e., rapid turn-off. This is something that thyristors or bidirectional thyristors cannot do, and it can be used in conjunction with a microcontroller. Only the schematic diagram of the high-voltage part is provided here (Figure 4). In the diagram: R1, R2: 1kΩ; R3: 10kΩ; R4: 200Ω; R5, R6, R7: 5.1kΩ; C1: 300mF/16V; D1, D2: 10A/400V rectifier diodes; D3, D4, D5: 1N4001; Q1, Q2: VDMOS (Ron=0.05W); IC1: LM393 dual comparator; IC2: CD4013 D flip-flop; IC3, IC4: 521 optocoupler; Tr1: 220V/9V power transformer (2-3W). In the diagram, two SITH transistors are connected in anti-series connection, and each is connected in anti-parallel connection with a diode. The two SITH transistors can be turned on or off simultaneously, forming an AC switch. The comparator IC1 generates a zero-crossing pulse signal, which serves as the clock signal for the IC2 D flip-flop. Therefore, IC2 can only change its output when the AC voltage crosses zero. Its complementary outputs control the gate injection of the SITH transistor and the VDMOS transistor respectively, thus controlling the zero-crossing turn-on and turn-off of the AC switch. The forced turn-off signal directly controls the R reset terminal of the D flip-flop, immediately turning off the AC switch. The control signal is input through an optocoupler to prevent interference.