Design of 3KW power factor corrector and its application in Xray power system

The current single-phase APFC technology has been fully mature, applied to switching power supply can improve the power factor to o. Above 98, it has become a necessary pre-stage for many switching power supplies, and is increasingly widely used. Designing an APFC that meets the system requirements quickly and efficiently has become a problem that engineers and technicians must face. The powerful signal analysis and processing capability of MATLAB brings great convenience to the efficient design of APFC and parameter tuning of each link. This paper uses MATLAB to design and implement a 3KW power factor corrector, gives the SI MULINK simulation circuit and waveform, and successfully applied to the Xray power system developed.

Authors: Mou Shujia, Jiang Xuedong

1.introduction

The current single-phase APFC technology has been fully mature, applied to switching power supply can improve the power factor to o. Above 98, it has become a necessary pre-stage for many switching power supplies, and is increasingly widely used. Designing an APFC that meets the system requirements quickly and efficiently has become a problem that engineers and technicians must face. The powerful signal analysis and processing capability of MATLAB brings great convenience to the efficient design of APFC and parameter tuning of each link. This paper uses MATLAB to design and implement a 3KW power factor corrector, gives the SI MULINK simulation circuit and waveform, and successfully applied to the Xray power system developed.

2. Brief introduction of APFC control principle

In the traditional power factor corrector, the main circuit generally adopts the B00ST boost circuit, and the control strategy adopts the average current method to control. The basic control idea is to detect the flat current of the circuit, make it follow the network voltage, and have the same waveform and phase as the network voltage, so that the power factor of the input terminal is approximately 1. As shown in Figure 1, Fcn(qk) is the network voltage attenuation link, and the network voltage signal is obtained as part of the standard reference value of the current; Fcn(bk) is the feedback voltage correction link to maintain the stability of the output voltage; Fcn(I) is the current The correction link realizes the sinusoidal correction of the current.

Design of 3KW power factor corrector and its application in Xray power system

3. HATLAB design of APFC

Here, take the design of a 3 Kw active power factor corrector as an example to describe. Assume that the input voltage is 2 2 0 Vac and the output voltage is 4 OOV dc. The output capacitor is 9 4 ou F, and the energy storage inductance is 1 m H. Based on this, MATLAB simulation design is carried out for the control part of APF c. The design of the APF c control circuit is to adjust the parameters of the three links of Fcn(qk), Fcn(bk) and Fcn(i) reasonably, so that the circuit can obtain good steady-state and dynamic response performance.

The network voltage and output voltage are respectively multiplied by the multiplier through the feedforward link Fcn(qk) and the feedback link Fcn(bk) as the reference quantity of the current loop. In this way, in order to ensure the sinusoidal waveform of the loop current, the output of the multiplier must be a standard sinusoidal waveform, so F cn (qk) and F cn (bk) should attenuate as much as possible various harmonics and phase shifts that may cause current waveform distortion. factor. The output amplitude of the multiplier determines the size of the average current. In order to achieve a stable output voltage under a wide range of input voltages, the output amplitude of the multiplier is inversely proportional to the network voltage.

●The design and training of the feedforward voltage link (Fcn(qk)).

The output voltage of the APFC circuit is stable under a wide range of input voltage. According to the APFC control theory, the amount of network voltage after Fcn (qk) must be inversely proportional to the network voltage. At the same time, the effect of the second harmonic on the input current distortion needs to be attenuated to the greatest extent. In this regard, a single-pole filter with a very low cut-off frequency can be designed to obtain the average input voltage, but the system also has higher requirements on the response speed of the input voltage. A second-order filter is chosen here as a choice for a balance trade-off. Also, the second order filter will cause the second harmonic phase to shift by 180 degrees, so that the phase shift of the generated third harmonic current from the input current becomes the same as the voltage. Based on this, a trial-and-error design was made for the filtering link. The filter link of the feedforward link is not counted, and the main purpose is to determine the position of the two poles. Using the MATLAB self-control design toolbox, the pole position can be easily adjusted to obtain good light attenuation performance and fast response. As shown in Figure 2, the second wave is almost impossible to pass, and the system also has good response performance. After many trial and error experiments, the two open-loop poles are finally set as:

Design of 3KW power factor corrector and its application in Xray power system

p1=23.4 p2=-1 O. 1

Suppose the rectified voltage is

Design of 3KW power factor corrector and its application in Xray power system

Known by Fourier decomposition

Design of 3KW power factor corrector and its application in Xray power system

Design of 3KW power factor corrector and its application in Xray power system

Here, CO and is the average value of the network pressure

Design of 3KW power factor corrector and its application in Xray power system

At the end of the above description, the obtained feedforward link is shown in Figure 4.

Design of 3KW power factor corrector and its application in Xray power system

This feedforward eases the amount into the multiplier as follows:

Design of 3KW power factor corrector and its application in Xray power system

It is known from formula 3-4 that the feedforward link entering the multiplier is a sinusoidal quantity that is inversely proportional to the network pressure

●Design of voltage feedback link (Fcn(bk))

The basic low frequency model of the power output stage is a current source driving a capacitor, forming an integrator with a gain characteristic that rolls off 20dB for every tenfold increase in frequency. Because the bandwidth of the voltage loop is relatively narrow compared to the switching frequency, the main consideration in the design of the voltage loop is to ensure that the input distortion is minimized, not stability. First, the bandwidth of the voltage loop must be narrow enough to attenuate the second harmonic on the output capacitor and keep the modulation of the input current relatively small. Secondly, the voltage loop must have sufficient phase shift, so that the modulated signal can be kept in phase with the input voltage, and a higher power factor can be obtained. The second harmonic of the output voltage can be obtained from 2-5.

Design of 3KW power factor corrector and its application in Xray power system

Vopk is the peak value of the output ripple voltage, fr is the ripple frequency Co is the output capacitor, and Vo is the output DC voltage.

Assuming that APFC requires 3% THD, from the APFC design principle, O. 75% of THD is allocated to the voltage loop, so the voltage loop output ripple voltage should be limited to 1.5%. Based on this, the gain of the voltage loop at the second harmonic frequency is determined. The design principle is similar to the design of the feedforward voltage loop. The final feedback link of the voltage loop is as follows:

Design of 3KW power factor corrector and its application in Xray power system

●Current loop (Fcn(I)) compensation

After compensating the feedforward voltage and the feedback voltage double loop, an ideal reference current waveform is generated by the multiplier. Compensates the current loop to provide a flat gain close to the switching frequency. Among them, the zero point of the mid-low section of the amplifier provides high gain, and the average current mode control can work. Amplifier gain near the switching frequency is determined by matching the rate of decline of the Inductor current. Among them, the change of the current feedback signal of the power circuit:

Design of 3KW power factor corrector and its application in Xray power system

RS is the main circuit detection resistance. (3-7)

The gain of the amplifier at the switching frequency (here, assuming a switching frequency of 40K):

Design of 3KW power factor corrector and its application in Xray power system

Where VS is the current loop output (3-8)

The current loop is compensated using the PID modulator as follows:

Design of 3KW power factor corrector and its application in Xray power system

From the above, the visual interface of MATLAB’s automatic control toolbox can easily adjust the zero and pole positions, and can intuitively observe the steady-state and transient response performance of each link, which is convenient for real-time adjustment of design. MATLAB is a fast and efficient method. Designing an APFC that meets the needs provides great convenience.

4. SIMUL INK simulation circuit and waveform of APFC

Finally, according to the design compensation of the previous three links, the following SIMULINK simulation circuit is obtained. Figure 7.

Design of 3KW power factor corrector and its application in Xray power system

Design of 3KW power factor corrector and its application in Xray power system

Design of 3KW power factor corrector and its application in Xray power system

The triangular wave generating circuit is composed of a clock, a sample-and-hold device, a composite device and Fcn4 to generate a triangular waveform with a frequency of 40K and an amplitude of 2.5; the feedback voltage link is composed of a constant Constant, an adder A dd 2, and a transfer function T ransfer F cn 2; the current link is composed of a transfer function, forming a PI regulator; the addition of two saturators, saturationl and saturation2, limits the maximum output of the voltage loop and the current loop. The input AC voltage waveform detection part is composed of a sine wave generator Sine Wave 1 and an absolute valuer A bs 1, that is, a general expression F cn 1, to simulate the obtained grid voltage.

Design of 3KW power factor corrector and its application in Xray power system

After building the simulation circuit, select the appropriate algorithm for simulation. Among them, the solution options are: variable step size, maximum compensation 1e-6, relative accuracy 1e-3, algorithm selection ode23.

The following are the measured waveforms of the simulated circuit.

5.in conclusion

The control circuit of the APFC is a double-loop coupling control circuit, and the parameters need to be debugged repeatedly in order to finally obtain good results. In this paper, the automatic control toolbox and signal processing toolbox of MATLAB are used to quickly and efficiently set the APFC circuit parameters that meet the circuit requirements, which greatly reduces the difficulty of actual circuit debugging, and is successfully applied to the developed Xray power supply system.

The Links:   6MBP50VAA060-50 BSM10GP60