1 Introduction
AXI has become increasingly important in the field of nondestructive testing, especially for the inspection and evaluation of MEMS and MOEMS devices and packaging inspection such as CSPs and BGAs. Therefore, stable DC high-voltage power supplies used to generate X-rays have attracted great interest in the field of power electronics. However, due to the special volt-ampere characteristics of X-ray tubes (i.e., voltage and current are basically independently regulated), the X-ray power supply must provide a highly stable and widely adjustable voltage under different X-ray tube currents (hereinafter referred to as tube currents), because high voltage can produce high-intensity X-rays, while relatively low voltage produces low-intensity X-rays for different applications.
Therefore, the control of tube voltage is very important, and many articles have been introduced on how to make the power supply output voltage achieve good steady-state and transient response. However, it can be seen from some past articles that due to several serious problems with high-frequency transformers: high transformation ratio, troublesome high-voltage insulation, especially leakage inductance and parasitic capacitance, huge switching losses and snubber losses will result. The leakage inductance will cause unwanted voltage spikes at the output, which will damage the safety of the X-ray tube and the stability of the generated rays, while the parasitic capacitance will generate spike currents, making the transient response of the voltage very poor.
This paper introduces the latest series-parallel resonant high-voltage power supply based on high-frequency transformer. This power supply can realize ZCS (zero current) soft switching and can easily adjust the output voltage. The peak voltage and current are avoided by utilizing the parasitic parameters of the high-frequency transformer. Another feature of the power supply is that the voltage doubler circuit is used to replace the traditional diode rectifier circuit, thereby reducing the transformation ratio and parasitic parameters of the high-frequency transformer; especially the control strategy of the main circuit - the combination of the BUCK circuit and the inverter circuit - the BUCK circuit can be used to adjust the voltage very conveniently and flexibly, and the fixed-frequency and fixed-width inverter circuit can use the parasitic parameters of the high-frequency transformer to achieve resonant soft switching, and because the duty cycle of the inverter circuit is not used to adjust the voltage, the magnetic properties of the high-frequency transformer can be fully utilized. The control circuit of the power supply adopts a real-time digital PI regulator based on DSP to realize the steady-state and transient characteristics of the circuit.
2 Introduction to the power supply system
2.1 Circuit topology
Figure 1 is a schematic diagram of a high-voltage DC power supply, which mainly has several modules: EMI (anti-interference) module, PFC module, BUCK module, inverter module, resonant transformer module, voltage doubler circuit module and a resonant compensation capacitor. The resonant compensation capacitor can be used to adjust the resonant frequency; the resonant transformer is specially wound; the high-voltage cable in the figure can be used as a smoothing capacitor, and the filament current of the lamp tube is controlled by another filament circuit. The control system is a PI regulator based on TMS320LF2407, and it implements overvoltage, overcurrent, overtemperature protection, and communication functions.
Figure 1 Schematic diagram of high voltage DC power supply
2.2 Voltage Regulation Strategy
From the literature, we know that there are several ways to regulate the voltage across the X-ray tube as shown below:
Figure 2 Comparison of several voltage regulation methods
The first method is to adjust the firing angle of the SCR (thyristor) to adjust the rectified DC base voltage to achieve voltage regulation. The disadvantages of such voltage regulation are obvious. It will generate a large harmonic current to the power grid, and because the control of the thyristor is relatively complex, it makes the control circuit more difficult to implement. The second method is phase-shift voltage regulation, which is essentially to adjust the voltage by changing the fundamental voltage amplitude of the inverter output. This voltage regulation has two disadvantages: first, it will reduce the utilization rate of the subsequent transformer when the duty cycle is very small; second, it will cause great damage to the switch tube of the inverter circuit under low voltage and high current conditions, and the loss is very large; and there are many high-frequency disturbances; the third method is the voltage regulation method adopted by the latest developed power supply. First, because PFC makes the power factor of the circuit close to 1, it greatly reduces the harmonic pollution to the power grid; second, it is very simple and easy to implement to adjust the voltage by changing the duty cycle of the BUCK circuit; at the same time, because the inverter circuit can be fixed frequency and fixed width, it provides good conditions for realizing soft switching, and can make full use of high-frequency transformers, and also avoid damage to the inverter circuit under low voltage and high current conditions;
[page]2.3 Voltage doubling
circuit With the rapid development of semiconductor technology, ultra-high frequency diodes and high frequency capacitors have become very mature, and these two provide support for the application and development of voltage doubling circuits. The most common voltage doubling circuit is the half-wave capacitor diode voltage doubling circuit as shown in Figure 3. The voltage doubling circuit is based on the CW circuit. Other voltage doubling circuits are developed based on the CW circuit as shown in Figure 4. For odd and even voltages, you only need to find a suitable reference ground. To get a negative voltage, just reverse the diode. The basic principles of voltage doubling are as follows:
Figure 3 Half-wave capacitor diode voltage doubler circuit
Figure 4 Various CW circuits
Table 1 shows the dropout voltage and ripple voltage of various CW voltage doubler circuits. As shown in Table 1, the dropout voltage and ripple voltage are directly related to the input frequency f; low switching frequency will produce large dropout voltage and ripple voltage, and high input frequency will reduce dropout voltage and ripple voltage. Therefore, the introduction of high-frequency resonant inverter can improve the control characteristics of the ray power supply.
Table 1 Comparison of dropout voltage and ripple voltage of various voltage doubler circuits
3. Working Principle of Series-Parallel Resonance Circuit and Soft Switching
The equivalent circuit of this converter is shown in Figure 5. The series-parallel resonant circuit has two operating modes: discontinuous current mode (DCM) and continuous current mode (CCM). In the DCM mode, the inductor current is zero for a period of time. This operating mode occurs when the input voltage frequency Fs of the input resonant circuit is less than half of the natural frequency Fr ( ); in the CCM mode, it occurs.
Figure 6 shows the control signal of the inverter circuit and the simulation waveform of the resonant circuit. Since the half cycle of the inverter circuit is greater than the resonant cycle (), the current waveform flowing through the leakage inductance is shown in Figure 6-a. Whether the switch devices S1, S2, S3, S4 or the diodes D1 D2 D3 D4 are turned on and off, zero current is achieved; at the same time, as shown in Figure 6-b, the switch device can achieve zero current shutdown, but there is a certain turn-on current; and the reverse diode is also zero current shutdown but has a turn-on current.
It can be seen from the figure that as long as we can find the inherent resonant frequency of the circuit or control the switching frequency to less than half of the resonant frequency by adding suitable inductors and capacitors externally, this can not only reduce the switching frequency, but also achieve zero current and greatly reduce the switching loss.
[page]4 Test and discussion
The parameters of the test system are as follows:
Input voltage: 220V AC
Output voltage: 0-160KV DC adjustable
Output current: 0-1.2mA adjustable
High-frequency high-voltage transformer turns ratio: 22.5:720
Inverter switching frequency: 16.7KHZ
The resonant frequency changes with the change of the external capacitance;
As shown in the test results, it is basically consistent with the simulation results of theory and simulation. 5 Conclusion: The series-parallel transformer resonant high-voltage DC power supply for generating X-rays introduced in the article adopts a voltage doubling circuit to reduce the transformer ratio, which is feasible in process and manufacturing, and can achieve zero current soft switching under certain conditions, greatly reducing switching losses; in addition, this topology is innovative, and the introduction of PFC greatly reduces the harmonic pollution to the power grid. At the same time, it can work under different voltages of 110V and 220V at home and abroad, opening up domestic and foreign markets; it is worth noting that such a topology is simple to control, and this power supply also uses DSP to realize digital PID real-time control so that it can work well and realize remote communication. 6 References [1] Koki Ogura: Inductor Snubber-Assisted Series Resonant ZCS-PFM High Frequency Inverter Link DC-DC Converter with Voltage Multiplier IEEE 2002. [2] Yong Ju Kim: Comparative Evaluations of Phase-Shifted PWM Resonant Inverter-fed DC-DC Converter with High-Voltage High-Frequency Transformer Link, IEEE Catalog No. 9. 5TH8025 1995. [3] Junming Sun: Series Resonant High-Voltage ZCS-PFM DC-DC Converter for Medical Power Electronics, IEEE 2000. [4] Shyh-Shin Liang and Ying-Yu Tzou: DSP control of a Resonant Swithing High-Voltage Power Supply for X-ray Generators, IEEE 2001. [5] Belgin Tǔrkay: Harmonic Filter Design and Power Factor Correction in a Cement Factory, IEEE Porto Power Tech Conference, 2001 [6] JSun: Series resonant ZCS-PFM DGDC converter with multistage rectified voltage multiplier and dual-mode PFM control scheme for medical-use high-voltage X-ray power generator, IEE Pinc.-Elecli.. Poaw Appl., Vol. 147, No. 6, November 2001.
Reference address：Innovative X-ray power system based on soft switching
AXI has become increasingly important in the field of nondestructive testing, especially for the inspection and evaluation of MEMS and MOEMS devices and packaging inspection such as CSPs and BGAs. Therefore, stable DC high-voltage power supplies used to generate X-rays have attracted great interest in the field of power electronics. However, due to the special volt-ampere characteristics of X-ray tubes (i.e., voltage and current are basically independently regulated), the X-ray power supply must provide a highly stable and widely adjustable voltage under different X-ray tube currents (hereinafter referred to as tube currents), because high voltage can produce high-intensity X-rays, while relatively low voltage produces low-intensity X-rays for different applications.
Therefore, the control of tube voltage is very important, and many articles have been introduced on how to make the power supply output voltage achieve good steady-state and transient response. However, it can be seen from some past articles that due to several serious problems with high-frequency transformers: high transformation ratio, troublesome high-voltage insulation, especially leakage inductance and parasitic capacitance, huge switching losses and snubber losses will result. The leakage inductance will cause unwanted voltage spikes at the output, which will damage the safety of the X-ray tube and the stability of the generated rays, while the parasitic capacitance will generate spike currents, making the transient response of the voltage very poor.
This paper introduces the latest series-parallel resonant high-voltage power supply based on high-frequency transformer. This power supply can realize ZCS (zero current) soft switching and can easily adjust the output voltage. The peak voltage and current are avoided by utilizing the parasitic parameters of the high-frequency transformer. Another feature of the power supply is that the voltage doubler circuit is used to replace the traditional diode rectifier circuit, thereby reducing the transformation ratio and parasitic parameters of the high-frequency transformer; especially the control strategy of the main circuit - the combination of the BUCK circuit and the inverter circuit - the BUCK circuit can be used to adjust the voltage very conveniently and flexibly, and the fixed-frequency and fixed-width inverter circuit can use the parasitic parameters of the high-frequency transformer to achieve resonant soft switching, and because the duty cycle of the inverter circuit is not used to adjust the voltage, the magnetic properties of the high-frequency transformer can be fully utilized. The control circuit of the power supply adopts a real-time digital PI regulator based on DSP to realize the steady-state and transient characteristics of the circuit.
2 Introduction to the power supply system
2.1 Circuit topology
Figure 1 is a schematic diagram of a high-voltage DC power supply, which mainly has several modules: EMI (anti-interference) module, PFC module, BUCK module, inverter module, resonant transformer module, voltage doubler circuit module and a resonant compensation capacitor. The resonant compensation capacitor can be used to adjust the resonant frequency; the resonant transformer is specially wound; the high-voltage cable in the figure can be used as a smoothing capacitor, and the filament current of the lamp tube is controlled by another filament circuit. The control system is a PI regulator based on TMS320LF2407, and it implements overvoltage, overcurrent, overtemperature protection, and communication functions.
2.2 Voltage Regulation Strategy
From the literature, we know that there are several ways to regulate the voltage across the X-ray tube as shown below:
The first method is to adjust the firing angle of the SCR (thyristor) to adjust the rectified DC base voltage to achieve voltage regulation. The disadvantages of such voltage regulation are obvious. It will generate a large harmonic current to the power grid, and because the control of the thyristor is relatively complex, it makes the control circuit more difficult to implement. The second method is phase-shift voltage regulation, which is essentially to adjust the voltage by changing the fundamental voltage amplitude of the inverter output. This voltage regulation has two disadvantages: first, it will reduce the utilization rate of the subsequent transformer when the duty cycle is very small; second, it will cause great damage to the switch tube of the inverter circuit under low voltage and high current conditions, and the loss is very large; and there are many high-frequency disturbances; the third method is the voltage regulation method adopted by the latest developed power supply. First, because PFC makes the power factor of the circuit close to 1, it greatly reduces the harmonic pollution to the power grid; second, it is very simple and easy to implement to adjust the voltage by changing the duty cycle of the BUCK circuit; at the same time, because the inverter circuit can be fixed frequency and fixed width, it provides good conditions for realizing soft switching, and can make full use of high-frequency transformers, and also avoid damage to the inverter circuit under low voltage and high current conditions;
[page]2.3 Voltage doubling
circuit With the rapid development of semiconductor technology, ultra-high frequency diodes and high frequency capacitors have become very mature, and these two provide support for the application and development of voltage doubling circuits. The most common voltage doubling circuit is the half-wave capacitor diode voltage doubling circuit as shown in Figure 3. The voltage doubling circuit is based on the CW circuit. Other voltage doubling circuits are developed based on the CW circuit as shown in Figure 4. For odd and even voltages, you only need to find a suitable reference ground. To get a negative voltage, just reverse the diode. The basic principles of voltage doubling are as follows:
[]
Table 1 shows the dropout voltage and ripple voltage of various CW voltage doubler circuits. As shown in Table 1, the dropout voltage and ripple voltage are directly related to the input frequency f; low switching frequency will produce large dropout voltage and ripple voltage, and high input frequency will reduce dropout voltage and ripple voltage. Therefore, the introduction of high-frequency resonant inverter can improve the control characteristics of the ray power supply.
The equivalent circuit of this converter is shown in Figure 5. The series-parallel resonant circuit has two operating modes: discontinuous current mode (DCM) and continuous current mode (CCM). In the DCM mode, the inductor current is zero for a period of time. This operating mode occurs when the input voltage frequency Fs of the input resonant circuit is less than half of the natural frequency Fr ( ); in the CCM mode, it occurs.
Figure 6 shows the control signal of the inverter circuit and the simulation waveform of the resonant circuit. Since the half cycle of the inverter circuit is greater than the resonant cycle (), the current waveform flowing through the leakage inductance is shown in Figure 6-a. Whether the switch devices S1, S2, S3, S4 or the diodes D1 D2 D3 D4 are turned on and off, zero current is achieved; at the same time, as shown in Figure 6-b, the switch device can achieve zero current shutdown, but there is a certain turn-on current; and the reverse diode is also zero current shutdown but has a turn-on current.
It can be seen from the figure that as long as we can find the inherent resonant frequency of the circuit or control the switching frequency to less than half of the resonant frequency by adding suitable inductors and capacitors externally, this can not only reduce the switching frequency, but also achieve zero current and greatly reduce the switching loss.
[page]4 Test and discussion
The parameters of the test system are as follows:
Input voltage: 220V AC
Output voltage: 0-160KV DC adjustable
Output current: 0-1.2mA adjustable
High-frequency high-voltage transformer turns ratio: 22.5:720
Inverter switching frequency: 16.7KHZ
The resonant frequency changes with the change of the external capacitance;
As shown in the test results, it is basically consistent with the simulation results of theory and simulation. 5 Conclusion: The series-parallel transformer resonant high-voltage DC power supply for generating X-rays introduced in the article adopts a voltage doubling circuit to reduce the transformer ratio, which is feasible in process and manufacturing, and can achieve zero current soft switching under certain conditions, greatly reducing switching losses; in addition, this topology is innovative, and the introduction of PFC greatly reduces the harmonic pollution to the power grid. At the same time, it can work under different voltages of 110V and 220V at home and abroad, opening up domestic and foreign markets; it is worth noting that such a topology is simple to control, and this power supply also uses DSP to realize digital PID real-time control so that it can work well and realize remote communication. 6 References [1] Koki Ogura: Inductor Snubber-Assisted Series Resonant ZCS-PFM High Frequency Inverter Link DC-DC Converter with Voltage Multiplier IEEE 2002. [2] Yong Ju Kim: Comparative Evaluations of Phase-Shifted PWM Resonant Inverter-fed DC-DC Converter with High-Voltage High-Frequency Transformer Link, IEEE Catalog No. 9. 5TH8025 1995. [3] Junming Sun: Series Resonant High-Voltage ZCS-PFM DC-DC Converter for Medical Power Electronics, IEEE 2000. [4] Shyh-Shin Liang and Ying-Yu Tzou: DSP control of a Resonant Swithing High-Voltage Power Supply for X-ray Generators, IEEE 2001. [5] Belgin Tǔrkay: Harmonic Filter Design and Power Factor Correction in a Cement Factory, IEEE Porto Power Tech Conference, 2001 [6] JSun: Series resonant ZCS-PFM DGDC converter with multistage rectified voltage multiplier and dual-mode PFM control scheme for medical-use high-voltage X-ray power generator, IEE Pinc.-Elecli.. Poaw Appl., Vol. 147, No. 6, November 2001.
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