Low-power high-voltage power supply based on BUCK voltage regulation

　　The main contents of the research include the analysis and design of BUCK circuit, the analysis and design of half-bridge inverter circuit, the design of voltage doubler circuit, the design of control circuit, and the use of PSPICE software to simulate and optimize the parameters of the corresponding parts.

　　The main performance achieved in this study is: given an input voltage of 220V AC, the output voltage is required to be adjustable in a wide range of 0~15KV, the power is 15W, and the output ripple is less than 1%.

　　introduction

　　High voltage power supply generally refers to power supply with output voltage above 5,000 volts. Generally, the output voltage of high voltage power supply can reach tens of thousands of volts, or even hundreds of thousands of volts or higher. High voltage power supply is widely used in material modification, metal smelting, environmental protection, high-power laser and microwave and other application fields. Traditional high voltage power supply adopts industrial frequency power supply and LC resonance mode. Although the circuit is simple, its volume and weight are large, and the low-frequency working state, ripple and stability are not satisfactory. With the development of power electronics, high-frequency high voltage power supply has become a development trend.

　　With the continuous emergence of new electronic components, new electromagnetic materials, new power conversion technologies, new control theories and new professional software, and their continuous application in switching power supplies, the performance of switching power supplies has been continuously improved, and the characteristics have been continuously updated, with new characteristics such as high frequency, high efficiency, high power density and high reliability emerging.

　　In the 1970s, a revolution took place in the history of world power supply, that is, the application of 20Hz switching frequency combined with pulse width modulation technology (PWM) in the field of power supply. So far, the frequency of power supply has reached hundreds of Hz, and the application of advanced quasi-resonance technology can even reach the megahertz level. Increasing the output frequency of the oscillator can reduce the basic performance requirements and structural volume of electronic devices such as high-voltage transformers, reactors, smoothing capacitors, and high-voltage capacitors, thereby reducing the volume of high-voltage power supply. High frequency greatly reduces the volume of high-voltage power supply, making it light and portable, and significantly improving its practicality and ease of use.

　　In recent years, with the development of electronic power technology, the application of new generation power devices such as MOSFET, IGBT, and the gradual maturity of high-frequency inverter technology, high-voltage switching DC power supplies have emerged. Compared with linear power supplies, high-frequency switching power supplies have the following outstanding features: high efficiency, small size, light weight, fast response, less energy storage, and short design and manufacturing cycles. Due to its superior characteristics, it has gradually replaced the traditional high-voltage linear DC power supply.

　　With the gradual application of high-tech, new technical problems have also emerged, mainly in that high frequency can improve power performance and reduce the size and ripple factor of transformers. However, since high-frequency and high-voltage transformers have both high frequency and high voltage, new technical difficulties have emerged:

　　① As the volume of high-frequency and high-voltage transformers decreases and the frequency increases, the distributed capacitance becomes smaller, and the insulation problem becomes particularly prominent;

　　② The large voltage change ratio makes the transformer nonlinear more serious, and the leakage inductance and distributed capacitance increase, so it must be isolated from the inverter switch, otherwise the spike pulse will affect the normal operation of the inverter circuit and even break down the power device;

　　③ High frequency leads to an increase in the skin effect of the transformer, which reduces the efficiency of the transformer.

　　In view of the above situation, how to design high-frequency and high-voltage transformers is a difficult and hot issue in current research.

　　The main contents of the research include the analysis and design of BUCK circuit, the analysis and design of half-bridge inverter circuit, the design of voltage doubler circuit, and system simulation research. The circuit includes input rectifier filter circuit, BUCK pre-regulator circuit, half-bridge inverter circuit, voltage doubler circuit and output rectifier filter circuit. The input AC power is converted into DC through the rectifier filter circuit, the voltage is stabilized by the BUCK pre-regulator circuit, and then the DC voltage is converted into AC voltage through the half-bridge inverter circuit, and then the voltage is increased through a voltage doubler circuit, and finally the rectifier filter outputs a stable high voltage.

　　Main circuit design

　　1) Topology of the main circuit (Figure 1)

　　Here we mainly introduce a low-power high-voltage power supply based on BUCK voltage regulation. This power supply can achieve zero current soft switching (ZCS) and can easily adjust the output voltage because it utilizes the parasitic parameters of the high-frequency transformer, thereby avoiding peak voltage and current. Another feature of this power supply is that it uses a voltage doubling circuit to replace the traditional diode rectifier circuit, reducing the transformation ratio and parasitic parameters of the high-frequency transformer; in particular, the control of the main circuit adopts a combined strategy of Buck circuit and inverter circuit, that is, Buck can be used to adjust the voltage very conveniently and flexibly; the use of a fixed-frequency and fixed-width inverter circuit can utilize the parasitic parameters of the high-frequency transformer to achieve resonant soft switching.

　　In addition, since the power supply does not need to adjust the voltage by adjusting the duty cycle of the inverter circuit, the magnetic properties of the high-frequency transformer can be fully utilized; and since its control circuit uses a real-time digital PI regulator based on DSP, the steady-state and transient characteristics of the circuit are realized.

　　2) Buck circuit design

　　(1) Working principle of BUCK circuit, Figure 2.

　　When the switch S is closed, the input voltage is completely applied to both ends of the diode D, with the upper positive and the lower negative, and the diode is reverse biased and cut off. Since the initial voltage of the capacitor C is zero at this time (Vc=Vo, the output voltage is zero), the capacitor voltage cannot change suddenly, so the input voltage is completely applied to the inductor L, forming a loop formed by the switch S, the inductor L, the capacitor C and the resistor R to establish the initial current. As the switch closing time increases, the inductor current gradually increases. Part of this inductor current is supplied to the resistor R to become the output current, and the other part charges the capacitor to gradually increase the voltage across the capacitor. Since the capacitor voltage is established from zero, the increment of the inductor current is relatively large during the closing of the switch S, and the load current output to R is proportional to the capacitor voltage. Therefore, the charging current of the capacitor is the largest at the beginning, and the capacitor voltage rises the fastest.

　　When the switch S is turned off, since the inductor current cannot change suddenly, the inductor current that tends to decrease without external excitation generates an induced potential at both ends of the inductor L, which is positive on the left and negative on the right. This induced potential will overcome the capacitor voltage and make the diode D forward-biased and turned on, forming a freewheeling loop of L→C, R→D→L.

　　When the switch is closed, the inductor current increases, and when the switch is open, the inductor current decreases. The average value of the capacitor's charging and discharging current in one cycle is equal to zero, that is, the capacitor charging current is greater than zero.

　　(2) Selection of main switch and freewheeling diode

　　VDMOS tube is a voltage-controlled device, easy to drive, no secondary breakdown phenomenon, good thermal stability, large safe operating area (SOA), fast switching speed, small switching loss. At the current manufacturing level of VDMOS tube, in the high-frequency and medium-power range, especially in high-voltage and low-current or low-voltage and high-current applications, VDMOS tube has a very high performance-price ratio and is worthy of priority.

　　=300V, ILM=1A, the power switch is high voltage and low current operation, the actual power field effect tube model selected is IRF840, its main parameters are as follows:

　　Maximum reverse voltage VDSVDS: 500V

　　Continuous working current ID: 8A

　　Peak current IDM: 32A

　　On-resistance Ron: <0.85Ω

　　Opening timeton：lOns

　　Turn-off time toff: 9ns

　　The forward rated current of the freewheeling diode must be greater than the maximum load current, and the withstand voltage must be greater than the input voltage with a margin. In addition, another important consideration is to reduce the spike voltage caused by leakage inductance and lead inductance. The freewheeling diode should use a Schottky diode (SBD) with a short reverse recovery time and soft recovery characteristics. The actual model used is FR307, which has a reverse voltage of 700V and a forward rated current of 3A.

　　(3)Simulation waveform

　　The BUCK circuit is shown in Figure 3. The circuit adopts a series switch buck structure, where Q is a power field effect tube MOSFET. During the ton period, the control signal turns on Q, the current increases, and the inductor stores energy; during the toff period, Q is turned off, and the inductor current releases energy to the load through the freewheeling diode D. The BUCK part is simulated and the following waveform is obtained:

　　As shown in Figure 4, the output voltage of the Buck circuit is maintained at about 140V, and the inductor current is pulsating. When the switch is closed, the inductor current increases, and when the switch is open, the inductor current decreases. The switching frequency is 100kHz, and the duty cycle is 45%.

　　(1) Working Principle of Half-bridge Inverter Circuit The schematic diagram of half-bridge inverter circuit is shown in Figure 5. It has two bridge arms, each of which consists of a controllable device and an anti-parallel diode. Two sufficiently large capacitors are connected in series on the DC side, and the connection point of the two capacitors becomes the midpoint of the DC power supply . The load is connected between the midpoint of the DC power supply and the connection point of the two bridge arms.

　　Assume that the gate signals of the switching devices V1 and V2 are positively biased for half a cycle and reversely biased for half a cycle in one cycle, and the two are complementary. When the load is inductive, its working waveform is shown in Figure 6. The output voltage uo is a rectangular wave with an amplitude of Um=Ud/2. The waveform of the output current io varies with the load. Assume that before time t2, V1 is in the on state and V2 is in the off state. At time t2, a shutdown signal is given to V1 and an on signal is given to V2, then V1 is turned off, but the current io in the inductive load cannot change direction immediately, so VD2 is turned on for continuous flow. When t0 drops to zero at time t3, VD2 is turned off, V2 is turned on, and io begins to reverse. Similarly, at time t4, a shutdown signal is given to V2, and after a turn-on signal is given to V2, V2 is turned off, VD1 is turned on for continuous flow first, and V1 is turned on at time t5. The names of the devices turned on in each period are marked at the bottom of Figure 6.

　　When V1 or V2 is on, the load current and voltage are in the same direction, and the DC side provides energy to the load; when VD1 or VD2 is on, the load current and voltage are in opposite directions, and the energy stored in the load inductor is fed back to the DC side, that is, the load inductor feeds back the reactive energy absorbed by it to the DC side. The fed-back energy is temporarily stored in the DC side capacitor. The DC side capacitor acts as a buffer for this reactive energy. Because diodes VD1 and VD2 are channels for the load to feed back energy to the DC side, they are called feedback diodes; and because VD1 and VD2 play the role of making the load current continuous, they are also called freewheeling diodes.

　　When the controllable device is a thyristor without gate turn-off capability, a forced commutation circuit must be added for normal operation.

　　The advantages of the half-bridge inverter circuit are simplicity and the use of fewer components. Its disadvantages are that the amplitude of the output AC voltage Um is only Ud/2, and two capacitors are required in series on the DC side, and the voltage balance of the two capacitors must be controlled during operation. Therefore, the half-bridge inverter circuit is often used for small power inverters below a few kW.

　　(2) Selection of switching devices

　　In voltage regulation and inverter circuits, switching devices play a core role. There are many kinds of switching devices. For example, according to the power level, there are micro-power devices, low-power devices, high-power devices, etc.; according to the manufacturing material, there are germanium tubes, silicon tubes, etc.; according to the conductive mechanism, there are bipolar devices, unipolar devices, hybrid devices, etc.; according to the control method, they can be divided into three types of devices: uncontrollable devices, semi-controllable devices and fully controllable devices: uncontrollable devices include rectifier diodes, fast recovery diodes, Schottky diodes, etc.; semi-controllable devices include ordinary thyristors, high-frequency thyristors, bidirectional thyristors, photo-controlled thyristors, etc.; fully controllable devices include power transistors (BJT), power field effect tubes power field effect tubes (Power MOSFET), insulated gate bipolar transistors (IGBT),

　　Static induction transistor (SIT), gate turn-off thyristor (GTO), static induction thyristor (SITH), etc. When selecting a switching device, the performance characteristics of the electrical device can be mainly examined from the following five aspects: ① conduction voltage drop, ② operating frequency, ③ device capacity, ④ impact resistance, and ⑤ reliability.

　　In this system, a fully controllable (self-shutdown) switching device is required. IGBT is a power semiconductor device that has the advantages of both the high-speed switching characteristics of power MOSFET and the low on-voltage characteristics of bipolar transistor. It can switch at high speed and withstand high voltage and large current, so this design selects MOSFET as the switching device.

　　(3) Calculation of main parameters and simulation waveform

　　Generally, when the output power is below 500W, consider using a half-bridge simulation inverter circuit as shown in Figure 7.

　　The simulation waveform is shown in Figure 8. The two MOSFETs are controlled by a given pulse signal, one is turned on and the other is turned off, and there is a dead time. After passing through the half-bridge inverter circuit, the voltage output to the high-frequency transformer is about AC 70V.

　　3) Design of high frequency transformer

　　The advantages of high-frequency high-voltage power supply are small-sized device, fast dynamic response of the system, high efficiency of power supply device, and effective suppression of environmental noise pollution. However, the obstacles to the development of high-frequency high-voltage power supply are mainly reflected in high-frequency high-voltage transformers. The main problems are: 1. The volume of high-frequency transformers is reduced, but the insulation problem is prominent. 2. The higher the voltage output, the higher the transformation ratio of the transformer. The large transformation ratio will inevitably make the transformer nonlinear, which will greatly increase its leakage inductance and distributed capacitance.

　　Figure 9 is a simplified model of the equivalent circuit of a high-frequency high-voltage transformer, which consists of leakage inductance LD, secondary distributed capacitance Cp and an ideal transformer. When the leakage inductance is the same, the inductive reactance working at high frequency fs increases by fs/50 times compared with the power frequency, which seriously limits the power output; when the distributed capacitance is the same, the capacitive reactance at high frequency is reduced to 50/fs compared with the power frequency, resulting in large no-load current, low power factor, and prominent no-load heating problems. The solution to the above problems is to vacuum immerse the transformer in oil (limited by experimental conditions, this design did not adopt this method), and use a large magnetic core to ensure sufficient insulation distance to reduce the distributed capacitance Cp and its influence, but the reduction of Cp will inevitably increase LD.

　　4) Design of voltage doubler circuit

　　(1) Voltage doubler circuit

　　This design uses a voltage doubler circuit to boost the voltage after the output of the step-up transformer, which can reduce the size of the transformer and improve efficiency. Voltage doubler rectification can not only convert AC into DC (rectification), but also does not require the addition of filter capacitors. It can obtain a DC voltage several times higher than the given voltage (voltage doubler). As long as the total volume of the capacitors used in the voltage doubler circuit is not very large, the volume of the entire power supply device can be reduced.

　　Now let's analyze the four-fold voltage rectifier circuit shown in Figure 10. In the analysis process, it is assumed that the charging speed of each capacitor is much faster than the discharging speed, and the conducting diode is replaced by a short circuit.

　　After starting to work, in the positive half cycle of the first cycle, the voltage u charges the capacitor C1 to um through the diode VD1, and in the negative half cycle, u and the voltage on C1 are connected in series to charge C2. In the positive half cycle of the next cycle, while the voltage u charges C1, since VD1 is turned on and there is no voltage on C3, C3 will charge C3 through VD1 and VD3; in the negative half cycle, u and C1 charge C2 while C3 also charges C4 which has no voltage. The working process of the quadruple voltage circuit in the positive and negative half cycles of this cycle is shown in Figure 11.

　　It can be seen that in this voltage-doubling rectifier circuit, the energy is transferred gradually from front to back, and one step is transferred backward every half cycle. After four and a half cycles, that is, two cycles, part of the energy of the four-fold voltage rectifier circuit is transferred to the final capacitor C4. In the subsequent cycles, the positive half cycle repeats the process of Figure 11 (a), and the negative half cycle repeats the process of Figure 11 (b). After more than a certain number of cycles, except for the voltage on capacitor C1, which is um, the voltages on the remaining capacitors are all 2um. The voltage obtained on the load RL is the sum of the voltages on C2 and C4, which is 4um. By analogy, the same conclusion can be obtained for the four-stage (eight-fold voltage) rectifier circuit. The eight-fold voltage rectifier circuit used in this design is shown in Figure 12:

　　(2) Simulation waveform

　　The AC voltage input from the high-frequency transformer to the voltage doubler circuit is about 2 kilovolts. After the voltage doubler rectification by the eight-fold voltage rectifier circuit, the final output DC voltage can reach about 15 kilovolts. The following figure is a simulation circuit that combines the half-bridge inverter circuit, high-frequency transformer, and voltage doubler circuit. See Figure 13.

　　As shown in FIG14 , the voltage input to the high-frequency transformer is 70V AC. The output voltage is increased to about 2kV through the high-frequency transformer with a transformation ratio of 30, and then passes through an eight-fold voltage rectifier circuit, and finally the output voltage is about 15kV.

　　Control circuit

　　According to the conventional closed-loop design concept, the feedback voltage of the closed loop should be taken from the output voltage, but the output voltage of the high-voltage power supply in the project is as high as 15KV. Therefore, when the feedback voltage is taken from the output voltage, it is bound to put forward higher insulation requirements for the sampling isolation circuit, which will be difficult to achieve in practice and will also increase the production cost of the power supply. Considering the above situation, the sampling voltage of the closed-loop design in the project is taken from the output voltage of the BUCK circuit.

　　According to experience, the operational amplifier of the PI regulator is LM7131B/NS, and the comparator is LM339. R1=20K, R2 =100K, C=5n.

　　The closed-loop circuit schematic in PSPICE is shown in Figure 15.

　　The waveform shown in Figure 16 is obtained through PSPICE simulation:

　　As shown in the figure, when the reference voltage Vref = -2V, the output DC voltage is about 12V, and when the reference voltage Vref = -2.5V, the output DC voltage is about 15V. It can be seen that by adjusting the value of the reference voltage Vref, the design can be adjusted in a wide range of 0-15KV. As shown in the figure, the waveform of the output voltage under the voltage closed-loop drive control meets the design technical parameter requirements.

　　It can be seen from the previous waveform that the closed-loop circuit can work normally, and after adding input disturbance, it can basically achieve zero net error in regulation. So far, the principle design of the high-voltage power supply in the project design has been basically completed, and the chip control drive in the actual circuit is given below.

　　in conclusion

　　This article introduces a low-power high-voltage power supply based on BUCK voltage regulation, which has the following characteristics: ① The voltage doubling circuit is adopted to reduce the transformer ratio, making it possible in process and manufacturing, and can achieve zero-current soft switching under certain conditions, thereby greatly reducing switching losses; ② The power supply can work at different voltages of 110V and 220V, thus opening up domestic and foreign markets; ③ The topology is simple and easy to implement; ④ The power supply uses DSP to realize real-time control of digital PI, so it can work well and realize remote communication.

　　The project design was mainly completed in PSPICE software. First, the basic working principles and simulation optimization of each link of the high-voltage power supply system were analyzed. Secondly, a systematic closed-loop design was carried out based on the open-loop design. The various parameters of the circuit were adjusted so that all indicators of the closed-loop system met the requirements, and the closed-loop system could be adjusted without net difference in the presence of disturbances.

　　Through the design process of the high-voltage power supply of the subject , the following conclusions can be drawn:

　　① In view of the system requirements of output voltage of 0-15KV and output power of 15W, the BUCK voltage regulation circuit is combined with the bridge inverter circuit to obtain high-frequency pulse voltage, which is then boosted and rectified through a high-frequency transformer and a voltage doubler circuit.

　　②The BUCK closed-loop link uses the photocoupler HCNR201 for voltage sampling isolation, and the MOSFET isolation drive is completed using HCPL4504 and UCC27321 to ensure the effectiveness and safety of the drive circuit.

　　③The control circuit of the inverter circuit is completed by the chip SG3535 and IR2110. The SG3525 controller integrates overvoltage protection, overcurrent protection, soft start, undervoltage lockout, breakdown short circuit protection and other functions to ensure the accuracy of the control signal. The PWM signal output by SG3525 drives the two bridge arms of the inverter circuit after passing through two IR2110 chips, which ensures the dead time between the drive signals and prevents the direct pass phenomenon of the bridge arm.

　　④ The circuit design abandons the traditional industrial frequency transformer boost mode, and adopts high-frequency transformer and voltage doubling circuit to complete the boost function, which plays an outstanding role in reducing the system volume.