Application of single-phase power factor correction module based on MC56F8323

　　0 Preface

　　Power electronic conversion technology has developed along with the development of power semiconductor devices. With the rapid development of computer and information technology, digital signal processing technology has emerged and has developed rapidly. Digital control has become an important research direction of power electronics due to the continuous improvement of its control theory and implementation methods, and because of its highly integrated control circuit, precise control accuracy, and stable working performance. In addition, digital control is also an effective means to ultimately realize the modularization, integration, digitization, and greening of power supplies.

　　Power factor correction, as a typical application of power electronic power conversion, has a wide range of engineering application value. This paper studies the power factor correction application module of digital control based on the digital control hardware composed of Moturola's MC56F8323, and applies digital control to the field of power factor correction of high-frequency switches.

　　l Single-phase power factor correction technology

　　The definition of power factor PF (Power Factor) is the ratio of AC input active power P to input apparent power S, and its expression is:

　　Where: Vrms is the effective value of the grid voltage;

　　Irms is the effective value of the grid current;

　　V1rms is the effective value of the fundamental voltage of the power grid. In the following discussion, the power grid voltage is considered to be an ideal sine wave, that is, Vrms=V1rms.

　　I1rms is the effective value of the fundamental current of the power grid; cosΦ is the phase shift factor (displacement factor) of the fundamental voltage and current:

　　γ=I1rms/Irms is the distortion factor (distonlon factor) of the grid current.

　　Therefore, the power factor PF can be defined as the product of the current distortion factor and the phase shift factor.

　　The single-tube Boost PFC circuit is the most widely used active power factor correction circuit in actual engineering applications. Its circuit working block diagram is shown in Figure 1. The main circuit consists of an uncontrolled rectifier circuit, an inductor, a switch tube and a filter capacitor. There is an energy storage inductor L on its input side, which can reduce the input current ripple, prevent the high-frequency transient impact of the power grid on the main circuit, and reduce the requirements for the input filter, presenting a current source load characteristic to the rectifier; there is a filter capacitor on its output side, which can reduce the ripple of the output voltage and present a voltage source characteristic to the load.

　　From the previous analysis, we can see that the PFC circuit mainly completes two tasks:

　　1) Control the inductor current, try to make the input current close to the sine wave, ensure that its γ is close to 1, and make the input current fundamental wave follow the input voltage phase;

　　2) Control the output voltage to keep it constant.

　　Therefore, two control loops are required for control. The voltage loop is the outer loop, which samples the output voltage and keeps the output voltage constant; the current loop is the inner loop, which samples the inductor current, forces the inductor current to track the given current and reduce the input current harmonics.

　　2 System Framework Based on Digital Control Power Factor Correction Module

　　Digitalization makes power electronic conversion control more flexible. When the CPU computing speed allows, complex control algorithms that are difficult to achieve with analog control can be implemented. Users can easily change controller parameters according to their system requirements. Even if the control object changes, there is no need to modify the controller hardware. Just change some software parameters, which greatly enhances the hardware compatibility of the system. On the other hand, digital circuits are not easily disturbed by the external environment, which enhances the reliability of the system.

　　However, the CPU calculation speed used by digital control determines the applicable occasions of the digital control system. At present, digital control is mostly used in occasions where the calculation speed requirements are not too stringent, such as UPS and inverter control, and the calculation frequency is generally less than 20 kHz. High-frequency power conversion with a control frequency greater than 100 kHz is still mainly controlled by analog devices, which is mainly limited by the CPU calculation speed. This paper uses Motorola's new DSP chip MC56F8323 to introduce digital control into the control of high-frequency active power factor correction, completes the application of power factor correction module based on digital control, and achieves good control effect.

　　The system block diagram of the PUC module based on MC56F823 is shown in Figure 2. The main circuit adopts the traditional single-tube Boost power topology, which consists of the main power tube S, boost diode D, energy storage inductor L and output capacitor C. The input side also includes input EMI filtering, input relay and diode full-wave rectification circuit. The three analog variables of full-wave rectified voltage Vrect, input current Iin, and output DC bus voltage Vbus are sent to DSP for analog-to-digital conversion. The digital regulators in this article all use the PI algorithm. As can be seen from Figure 2, the digital PFC

　　Dual-loop control is adopted, and the outer voltage loop is slower. The output DC bus voltage is sampled and compared with the given value of the output voltage, and then passes through the voltage loop PI regulator G1, and the output is expressed as a.

　　The transfer function of G1 is

　　Where: Kpv is the voltage loop proportional coefficient;

　　kiv is the voltage loop integration coefficient.

　　a is multiplied by the other two quantities b and c to be used as the given Iref of the inner current loop, that is,

　　That is, the inverse of the square of the average value of the input full-wave rectified voltage Vrect, c is the input full-wave rectified voltage, so that the output a of the voltage loop PI regulator determines the amplitude of the current loop setting, the sampling value c of the input full-wave rectified voltage determines the shape of the current loop setting, and the introduction of feedforward voltage control b ensures that the input power is constant and is not affected by the change of the input grid voltage. The speed of the inner loop current loop is faster, and the input current sampling value is compared with the current loop setting, and the duty cycle parameters are generated by the current loop PI regulator G2, and finally the main power switch tube control waveform is given through PWMO.

　　The transfer function of G2 is

　　Where: Kpi is the current loop proportional coefficient;

　　Kii is the current loop integral coefficient.

　　After MATLAB simulation, the control coefficients of the voltage loop and current loop can be obtained. Based on the initial simulation values, a large number of experimental adjustments are carried out, and the final control parameters are listed in Tables 1 and 2. In order to ensure that the system performance always reaches the best state when the input voltage changes over a large range, different PT parameters are used for the current loop when the effective value of the input voltage is 110V and 220V, which is also impossible for analog control.

　　Since DSP control is a discrete digital control, it can only calculate the control quantity based on the deviation value at the sampling time. Therefore, the above formula must be discretized and a series of sampling time points k are used to represent the continuous time t. The discrete PI control algorithm expression is:

　　Where: k=0, 1, 2... represents the sampling sequence;

　　u(k) represents the output value of the PI regulator at the kth sampling moment;

　　e(k) represents the deviation value input at the kth sampling moment;

　　Ts represents the sampling period;

　　TI represents the integration time constant;

　　Kp is the proportionality coefficient;

　　Ki is the integration coefficient.

　　The digital control program is composed of the main program and the interrupt service subroutine. The main functional modules include voltage loop calculation, current loop calculation, PWM output refresh and fault protection interrupt modules. The software system structure is listed in Table 3. [page]

　　3 System Experiment

　　This paper experimentally verifies a 500W PFC circuit module prototype on a digital platform based on MC56F8323, proving that in high-frequency power conversion applications, the use of digital control can not only complete the traditional analog control functions, but also maintain a higher power factor over the entire input range, resulting in better system performance.

　　The basic characteristics and resource utilization of MC56F8323 are listed in Table 4. The input voltage range of the prototype is a universal AC input, that is, the input voltage range is designed to be AC85~265V. Figure 3 shows the input voltage and input current waveforms when the input voltage is 110V and the output is full-load, where channel 1 is the voltage waveform and channel 2 is the current sampling waveform. The sampling ratio of voltage is 1:500, and the sampling ratio of current is 1:10. At this time, the input current THD is 8.6% and the input power factor is 0.994; Figure 4 shows the input voltage and current waveforms when the input voltage is 220V and the output is full-load. The channel description and sampling ratio are the same as before. At this time, the input current THD is 10.5% and the input power factor is 0.994. Experiments show that when the output full-load power remains unchanged and the input voltage changes in the range of AC 85~265V, the input current tracks the input voltage waveform in terms of both waveform and phase, and digital PFC control can always keep the circuit at a very high power factor.

　　Table 5 and Table 6 are the experimental data when the output load changes when the input voltage is 110V and 220V respectively. From these data, it can be seen that when the load changes from full load to no load, the output voltage remains constant and the input power factor is always maintained at a high level. The experiment shows that the digitally controlled power factor correction system has good performance in a large load change range.

　　4 Conclusion

　　Digital control has become an important development direction in the field of power electronics research. The application of DSP-based control technology in the field of power electronics has gradually become popular and fully recognized by the market. The application research of digital control in power factor correction module not only provides a complete DSP control solution in power factor correction, but also combines DSP control with power electronics professional applications more closely, providing a new idea for power electronics design. This paper first gives the control principle and design method of power factor correction application based on MC56F8323, and finally makes a 500W digital power factor correction module prototype, and verifies the excellent performance of the digital control system by experiment.