Research on Electromagnetic Compatibility of Boost Power Factor Corrector

introduction

In order to reduce harmonic pollution to the AC power grid, some standards for limiting current harmonics have been introduced, such as the IEC100032 Class D standard, which requires that measures must be taken to reduce the current harmonic content of the input power grid and improve the power factor.

When traditional diodes and capacitors rectify and filter the input signal, the input current is only available at the peak part of the input AC voltage, resulting in a large current harmonic content, which seriously interferes with the power grid and is far from meeting the standard requirements. In order to make the input current harmonics meet the requirements, power factor correction (PFC) must be added. The more mature and widely used are two-stage solutions, which have their own power devices and control circuits. The PFC stage makes the line current follow the line voltage, making the line current sinusoidal, which is easy to achieve a high power factor and reduce the harmonic content. Especially in recent years, with the rapid development of power electronics technology, the application of a large number of power electronic devices has caused serious harmonic interference to the power grid, bringing serious harm. Therefore, all countries have proposed corresponding EMC (electromagnetic compatibility) standards to strictly stipulate the allowable level of harmonic interference of equipment connected to the power grid. The 3C certification standard promoted by my country requires that all electrical products must pass this certification before they can be sold, and a very important part of this standard is the EMC standard.

1 Power supply parameters

A large number of electrical devices connected to the power grid are supplied to the load by rectifying the mains power into DC. The traditional method is voltage-type uncontrolled rectification, which is a diode bridge rectifier connected to a large capacitor for wave smoothing. This rectifier circuit is a combination of a nonlinear device and an energy storage element. Although the input AC voltage is sinusoidal, the diode conduction angle is very small, and the input current is severely distorted and pulsed, as shown in Figure 1.

PFC technology is to add a DC/DC switching converter to the uncontrolled rectifier circuit and apply current feedback technology to make the input current waveform track the AC input voltage waveform, so that the input current is close to sinusoidal. This article discusses the electromagnetic compatibility issues in the design of a typical Boost PFC circuit.

The technical parameters of the PFC circuit are:

Input AC 150~270V, 50~60Hz;

Output DC 380~400V, ripple <5%; power 600W;

Switching frequency 100kHz;

After correction, the power factor is >0.99.

The basic schematic diagram of the circuit is shown in Figure 2.

2 PFC working principle based on UC3854

This design is a Boost circuit working in the continuous current mode (CCM) of the inductor, and uses the dedicated PFC chip UC3854 from Unitrode. The core of the chip is an analog multiplier, whose output current Imo is determined by the voltage loop output, and whose waveform is determined by the input voltage sampling Iac. When the circuit is stable, ImoRmo is proportional to IiRs. Because Imo is a sine wave in phase with the input voltage, Ii is also a sine wave, thus achieving PFC.

The basic parameters of the main circuit are: input Boost inductor L=1mH, C=470μF, maximum input current effective value is 4A, the switch tube is IRF460, and the diode is a fast recovery diode RHRP1560.

3 Electromagnetic compatibility issues of Boost PFC

3.1 Sources of electromagnetic interference

There are many main electromagnetic interference sources in this circuit, the most important of which is the electromagnetic noise caused by the switching power devices and the converter circuit during the switching process. Whether it is the power semiconductor devices of the main circuit or the high-speed integrated circuits of the control circuit, the power electronic devices have very high di/dt during the switching process of the devices. They cause transient electromagnetic noise through the lead inductance of the lines or components. The frequency can be as high as tens of kHz or even hundreds of kHz, which is a noise source that cannot be ignored. The interference sources are analyzed one by one below.

IRF460 is a power field effect transistor (MOSFET), which is a multi-sub device and does not have the problem of reverse recovery . However, its switching speed is very high, and the di/dt (dv/dt) generated during the switching process can reach a very high value. Acting on the parasitic inductance (capacitance) in the circuit, it will generate very high transient voltage and current and cause oscillation. If the switching time is 10ns, the lead inductance is 500nH, and the maximum current during the switching process can reach 6A, then the voltage generated on the lead is

500×10－9×（6/10×10-9）=300V

Such a large pulse voltage (current) will cause serious electromagnetic interference.

Noise is also generated during the diode switching process. When the diode is turned on, the current increases rapidly, but its tube voltage drop does not drop immediately, but a rapid upsurge occurs, resulting in a broadband electromagnetic noise. When it is turned off, a large number of excess minority carriers in the long base region of the PN junction need to recombine, resulting in a large reverse recovery current, which is proportional to the turn-off current and the turn-off speed. In the case of high speed and high current, the reverse current will be quite large, and it will be superimposed on the switching current when it is turned on, and in severe cases, the switching device will be burned. Therefore, a diode with fast recovery characteristics must be selected to minimize the reverse recovery current.

3.2 Coupling Paths of Electromagnetic Interference

There are two ways for the electromagnetic noise caused by high-frequency switching power supplies to couple to the interfered object: conduction and radiation. According to the characteristics of electromagnetic noise coupling, conduction coupling can be divided into three types: direct conduction coupling, common impedance coupling and transfer impedance coupling. In this circuit, direct conduction coupling, common impedance coupling and radiation coupling should be considered as the key points.

Direct conduction coupling means that the noise is directly coupled to the interfered object through the wire or parasitic elements, such as Ldi/dt can be coupled through the wire. Therefore, in the experimental circuit, the length of the wire should be shortened as much as possible. Of course, the best method is to apply zero current switching (ZCS) soft switching technology.

Common impedance conduction coupling is the common ground impedance coupling generated by noise through the common ground wire of the equipment and the common impedance in the ground network. If the ground wire is not arranged properly, the ground wire will be greatly disturbed, and burrs with an amplitude of up to several V can usually be detected, and the circuit will not work properly. Therefore, the grounding should be arranged reasonably, and the ground wire should be arranged as short as possible, and the power ground and signal ground should be separated. After this treatment, the burrs on the ground wire will be significantly suppressed.

Radiated coupling refers to the energy of electromagnetic noise, which is transmitted through space radiation in the form of electromagnetic field energy and coupled to the interfered device (circuit). In this circuit, switches and diodes are the largest sources of electromagnetic noise, and electromagnetic noise will radiate to other parts of the circuit. The energy of electromagnetic noise received by the interfered circuit is proportional to the area of ​​the circuit loop, so the area of ​​the circuit loop must be reduced as much as possible when arranging circuit components.

4 Experimental Results

The circuit of this experiment is a Boost type power factor corrector based on UC3854. The working mode is current continuous mode, the output is 380~400V DC voltage, and the output power is 600W.

In the experiment, it is necessary to arrange the layout of components and ground wires reasonably, shorten the lead length and reduce the area of ​​the main circuit loop as much as possible, and arrange the main circuit and the control circuit separately. In this way, the electromagnetic compatibility problem can be greatly improved. From the experimental waveforms in Figures 3 and 4, the PFC function is basically realized, and the waveforms are less interfered.

5 Conclusion

This paper analyzes the electromagnetic compatibility issues of Boost PFC circuits, such as interference sources and coupling paths, and proposes solutions in actual experiments, which are verified through experiments.