Research on the Application of Static Induction Thyristor in Power Circuit

1 Introduction

In the previously published article "Application Research of Static Induction Thyristor (SITH)", we studied the basic characteristics of domestic SITH devices and developed four driving circuits. Driven by these four circuits, SITH devices achieved a switching speed of less than 0.2 ms. Now we further expand the driving circuit and SITH devices into an actual switching power supply application circuit. After testing, we have obtained relatively advanced performance indicators. In this way, the application research of SITH devices is more comprehensive, laying a solid foundation for its promotion and application.

2 Circuit Study

2.1 Application Circuit (I)

This application circuit is a switching power supply, which is designed based on the following considerations: (1) For high-power applications such as motor speed regulation and temperature control, it is determined to be AC-DC conversion, where AC specifically refers to the 50-cycle single-phase voltage 220V industrial frequency power grid; (2) The power input must have a high power factor of more than 0.95 and must also be low in noise, meeting or exceeding national standards; (3) The power efficiency is required to be more than 90%; (4) It can reflect the advantages of SITH tubes and avoid their weaknesses.

Here, the BOOST conversion program is used (Figure 1). In the figure, 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 voltage regulator diode; D2: 3A fast recovery diode HER308; D3, D6: 2A/400V rectifier bridge; 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; IC: UC3852.

For the detailed working principle of UC3852, please refer to the relevant information of Texas Instruments. It should be pointed out that UC3852 is a switching power supply control chip specially used to improve the input power factor. Its basic principle is to work at a frequency far higher than 50 cycles, control the switch tube to conduct at a constant time Ton, and when it is turned on, the inductor L2 bears the entire AC input voltage Vin. At the end of Ton, the inductor current, that is, the input current Iin=(Vin/L2)×Ton, so Iin is proportional to Vin. Then, UC3852 turns off the switch tube, L2 discharges to the load end, and Iin decays linearly. When UC3852 detects that Iin decays to zero, it controls the switch tube to conduct again and enters the next Ton, as shown in Figure 2.

In the figure, the upper part is the driving waveform of UC3852. The high level is to drive the switch tube to conduct, and the high level time Ton is constant; the lower part is the waveform 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 Vin value at that time. Since each time starts from zero, the peak value of Iin is also proportional to Vin. In fact, the operating frequency of UC3852 is much higher than that shown in the figure, about 25kHz. Therefore, it can be considered that Vin is constant in each triangle 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, and the power factor must be very high. This is the simple working principle of UC3852.

The circuit parameters in Figure 1 form a circuit with an output of 100W. The SITH tube is the main switch. The drive circuit (I) is adopted, and the drive circuit of the SITH tube is composed of Q1, Q2, Q3 and Q4; L2 is the BOOST inductor; C5 is the output filter capacitor; R6, R7, R8, R9 and C3 form a feedback circuit for UC3852 to control the output voltage stability; R12 and C7 are the timing circuit of the UC3852 operating frequency, which is set at about 25kHz here; L1 and C6 are low-pass filters to prevent the high-frequency components in Iin from being transmitted back to the power grid; R10 is the current sampling resistor, which provides the waveform of Iin to UC3852.

As can be seen from the waveform in Figure 2, this is a current discontinuous mode. Its advantage is that the switch tube always starts to conduct from zero current and eventually reaches twice the average current, so that the SITH tube avoids the weakness of slow conduction and develops its advantage of good high current performance. It also makes the diode D2 withstand reverse voltage 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 tube. The load current flows through it in the forward direction, and the voltage it withstands in the reverse direction is 400V. In the test, D2 uses a medium-speed fast recovery diode with very low temperature rise. The output voltage of 400V is required by the circuit operation (see the relevant information of Texas Instruments), so it cannot be reduced. The SITH tube also withstands a reverse voltage of 400V when it is turned off, which has a large margin for it and is also an advantage.

The main indicators are as follows: input voltage: AC220V 50Hz; input current: 0.58A; input power: 111W; power factor: 0.95; output voltage: DC410V, ripple peak value: Vpp=20V; output load resistance: 1610Ω; output power: 104W; efficiency: 0.94. From this, it can be seen that the SITH device and its driving circuit meet the requirements and the results are satisfactory.

However, the SITH tube has a tube power consumption of about 3.5W when working, and is the only device in the circuit with a significant temperature rise. The SITH tube used in the circuit is a TO-220 package. Initially, a 50mm×60mm heat sink was added. The surface temperature rose by 50℃ within 5 minutes after the power was turned on, and the input power increased by 3W. Later, a CPU heat sink with a fan was used instead, and the fan power was 1W. After 15 minutes of power on, the surface temperature still did not exceed 5℃, and the input power was always around 111W. It can be considered that the switching loss of the SITH tube increases with the increase of temperature, just like other power devices. If the heat dissipation is not good, a vicious circle will be formed. Frequent high temperature difference changes will also make the solder brittle prematurely, and the welding quality will deteriorate. Therefore, good heat dissipation is the most effective and important means to ensure reliability and high efficiency. Since the SITH tube has a strong overcurrent capacity, under good heat dissipation, it is not necessary to adopt the traditional method of "reduced capacity use". Devices with current ratings equivalent to working ratings can be selected to reduce costs without affecting reliability.

2.2 Application Circuit (II)

The output voltage of application circuit (I) is fixed at 400V DC, which cannot be adjusted flexibly and cannot be isolated from the input, so a post-stage circuit is often required. This application circuit (II) is modified based on circuit (I), with output isolation and voltage adjustment, as shown in Figure 3. In the figure: 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: 10kΩ; R13: 20kΩ/3W; W; 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 voltage-stabilizing 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.

The parameters of this circuit are 48V/100W output DC power supply. Its working principle is the same as circuit (I), except that the output part uses capacitor C10 to cut the DC square wave of SITH tube drain into AC square wave, sends it to the primary of transformer Tr, and then couples it to the secondary to change the voltage, and then outputs it after rectification and filtering. The secondary is divided into two coils, which pass through different rectification and filtering links respectively. This is because the AC square wave passing through C10 is asymmetric. The positive part is actually the discharge of inductor L2, which is a current source and needs to be rectified by the upper half of the secondary side; the negative part is actually the discharge of C10, which is a voltage source and needs to be rectified by the lower half of the secondary side.

Changing the turns ratio of Tr can get the required output voltage, and it is also isolated from the input power grid. Optocoupler 521-1 is an isolation element for negative feedback. D2, C5, C8, R13, and R14 form an overshoot absorption circuit. Due to leakage inductance, when the transformer receives the current source pulse, the overshoot is very strong, which is very unfavorable to the device. At the same time, it also causes a lot of high-frequency noise, which must be attenuated. The absorption circuit can effectively attenuate the overshoot, but a large number of tests have shown that the energy consumption of this attenuation is quite large, at the expense of reduced efficiency. In this test, the absorption link consumes about 6W of power, and the overshoot still accounts for 25% of the main wave, which reduces the efficiency from 91% to 84%. There is an obvious heat source on the circuit board. Therefore, before 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 load and has a wide range of applications. It was originally composed of thyristors or bidirectional thyristors, but is now replaced by two SITH tubes. It has the functions of zero-crossing opening and zero-crossing closing to reduce the impact on the power grid, and can also be shut down immediately when needed. Because of the use of SITH tubes, it has the function of forced shutdown, that is, fast shutdown. This is something that thyristors or bidirectional thyristors cannot do, and it can be used in conjunction with a single-chip microcomputer. Here only the schematic diagram of the high-voltage part is provided (Figure 4). In the figure: 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: optocoupler 521; Tr1: power transformer 220V/9V×2～3W.

In the figure, two SITH tubes are connected in anti-series, and each is anti-parallel with a diode. The two SITH tubes are turned on or off at the same time, forming an AC switch. The IC1 comparator generates a zero-crossing pulse signal as the clock signal of the IC2 D flip-flop, so IC2 can only change the output when the AC voltage passes through zero. Its pair of complementary outputs controls the gate injection of the SITH tube and the VDMOS tube respectively, which controls the zero-crossing conduction and shutdown of the AC switch. The forced shutdown signal directly controls the R reset terminal of the D flip-flop to immediately turn off the AC switch. The control signal is input through an optocoupler to prevent interference.