Research & Implementation of AC—DC Converter with High Power Factor & High Efficiency

In this paper, we design and develop a high power factor, high efficiency two-stage AC—DC power converter. This paper proposes a two-stage AC—DC power converter. The first stage is boost active power factor correction circuit. The latter stage is near constant frequency LLC resonant converter. In addition to traditional LLC high efficiency advantages, light- load conversion efficiency of this power converter can be improved. And it possesses high power factor and near constant frequency operating characteristics, can significantly reduce the electromagnetic interference. This paper first discusses the main structure and control manner of power factor correction circuit. And then by the LLC resonant converter equivalent model proceed to circuit analysis to determine the important parameters of the converter circuit elements. Then design a variable frequency resonant tank. The resonant frequency can change automatically on the basis of the load to reach near constant frequency operation and a purpose of high efficiency. Finally, actually design and produce an AC—DC power converter with output of 190W to verify the characteristics and feasibility of this converter. The experimental results show that in a very light load (9.5 W) the efficiency is as high as 81%, the highest efficiency of 88% (90 W). Full load efficiency is 87%. At 19 W ~ 190 W power changes, the operating frequency change is only 0.4 kHz (AC 110 V) and 0.3 kHz (AC 220 V).


Introduction
Due to the rapid development of the electronics industry and the rising awareness of energy saving and carbon reduction, the high conversion efficiency and compact size of the volume is an inevitable trend. The power converter will develop towards newer and more advanced switching power converter. Energy conservation and the use of a large number of electronic products and frequency conversion equipment make electromagnetic interference and harmonic a growing problem [1][2], thereby affecting the overall efficiency and peripheral products. So in the former stage of converter circuit add power factor correction technology, which will be able to effectively improve the energy conversion efficiency [3][4][5][6].
When switching power converter switch is turned on or cut off, it instantly generates a very high voltage and current stress, easy to cause serious switching loss when the power switch is switched [7][8], from which also electromagnetic interference problems are derived. This drawback can use soft switching with zero voltage switching, ZVS and zero current switching technology, ZCS technology to improve [9][10][11]. In 1988 C.Q. Lee and R. Liu presented LLC series resonant converter, LLC-SRC [12][13][14][15][16][17][18]. Because the LLC resonant converter has the advantage of soft switching, the maximum efficiency can be up to 90% or above [19][20]. Today LLC resonant converter is receiving attention gradually. So far the literature has still continued to be raised for discussion [21][22][23][24][25], but on account of the frequency conversion control, EMI filter design difficulties and low light-load efficiency become the main drawback.
To overcome these shortcomings, this paper has developed a high power factor and high efficiency AC -DC power converter [26]. The first stage achieves high power factor purpose by power factor correction architecture. The latter stage is a LLC resonant converter with variable frequency resonant trough, to achieve near constant frequency control and improve the shortcomings of inverter, which can improve low-loss effect of light-load efficiency [14].

Boost power factor corrector
Power factor corrector of this paper employs boost converter, the structure shown in Fig. 1. And use fixed off time, FOT control method to reach power factor correction. Fig. 2 is a fixed off time control method's inductance current waveform. This method uses the inductance current to follow the peak reference current. When the inductance current rises to the reference current, it switches off. After a fixed off time it starts the next cycle. This control method simultaneously has continuous and In this paper, we design and develop a high power factor, high efficiency two-stage AC -DC power converter. This paper proposes a two-stage AC -DC power converter. The first stage is boost active power factor correction circuit. The latter stage is near constant frequency LLC resonant converter. In addition to traditional LLC high efficiency advantages, lightload conversion efficiency of this power converter can be improved. And it possesses high power factor and near constant frequency operating characteristics, can significantly reduce the electromagnetic interference. This paper first discusses the main structure and control manner of power factor correction circuit. And then by the LLC resonant converter equivalent model proceed to circuit analysis to determine the important parameters of the converter circuit elements. Then design a variable frequency resonant tank. The resonant frequency can change automatically on the basis of the load to reach near constant frequency operation and a purpose of high efficiency. Finally, actually design and produce an AC -DC power converter with output of 190W to verify the characteristics and feasibility of this converter. The experimental results show that in a very light load (9.5 W) the efficiency is as high as 81%, the highest efficiency of 88% (90 W). Full load efficiency is 87%. At 19 W ~ 190 W power changes, the operating frequency change is only 0.4 kHz (AC 110 V) and 0.3 kHz (AC 220 V). discontinuous mode. When the voltage is near zero, it is the discontinuous mode. The output diode has zero current switching characteristics, which can reduce the loss of diodes reverse recovery time. The rest are continuous mode, which can reduce switching loss. Therefore, this paper uses a fixed off time control method. Fixed off time allows the majority of the power factor correction circuit to operate in continuous conduction mode, resulting in high performance. But it can use a simple peak control method, does not require a proprietary controller, and just add some passive components to achieve the purpose. In a typical fixed-frequency mode, when duty cycle is more than 50%, it will generate subharmonic oscillation, causing system instability and slope compensation must be made. Theoretically fixed off time control method duty cycle can reach 100%, while we do not have to do slope compensation, reducing material costs, as shown in Fig. 3.
In addition, a fixed off time also has the advantage of dynamic performance uplift. The internal current control loop responds sensitively to shocks arising from changes of load in the inductive current. External voltage control loop is not affected, and thus the power factor is effectively improved.

Near constant frequency LLC resonant converter
Although the LLC resonant converter already has many advantages, there are still some shortcomings. E.g., with changes in the load switching frequency varies, causing electromagnetic interference filter design problems, and poor conversion efficiency at light loads. Fig. 4 is a traditional style of LLC resonant converter gain curve. It can be seen from figure that when slowly loading from lighter loads frequency also will slowly decline.  Fig. 6 is its gain curve. It can be seen from the graph that when slowly loading from light load, due to different inductance curve still staggers at the same operating point. The frequency is almost unchanged, while achieving near constant frequency effect. To achieve this effect the architecture designs a variable inductor which can automatically change inductance value so that the resonant tank possesses characteristics of the variable resonant frequency.

Principle of variable resonant tank
The principle of variable resonant tank is mainly to design an inductor whose resonant inductive value can also change accordingly with the changes in the loads. To achieve this goal, we develop a special core gap capable of partial core saturation. When the load increases, the core saturation region increases, the resonant inductive value also changes accordingly. Fig. 7 is a schematic diagram of the core with an air gap of variable resonant inductor. (B) Adjust the turn ratio, and make the gain value rise or fall while approaching a light load and heavy load gain curve intersection.

Design example
This article designs a set of two stage high power factor highefficiency AC -DC 190W power supply, whose specifications are shown in Table 1. The main structure of the circuit is shown in Fig. 8. And actually measure to validate feasibility of the article proposed method. Wherein, the power factor correction circuit specifications are as shown in Table 2. The controller uses ST Microelectronics' L6562. Table 3 shows the LLC resonant converter specifications. The controller uses ST Microelectronics' L6599.

Experimental results and discussion
This paper presents the high power factor high efficiency AC -DC converter. The efficiency of its latter stage employing near constant frequency architecture is compared with that of its latter stage using traditional LLC resonant converter, as shown in Table 4. Fig. 9 and Fig. 10 is efficiency comparison curves diagram between near constant frequency and traditional LLC resonant converter. It can be seen from the figure that in a very light load (5%, 10% load, DC 110V) efficiency reaches more than 81%. Under full load efficiency reaches 87%. The maximum efficiency is 88%, a significant improvement in efficiency.  Fig. 11 and Fig. 12 are from the very light load to full load LLC switching frequency curve graph when the input is 110 V and 220V. The operating frequency change of the converter is less than 0.4 kHz, close to the constant frequency state.

Conclusion
This article combined active power factor corrector with near constant frequency LLC resonant converter to achieve the purpose of high power factor high efficiency, the actual completion of high efficiency high power factor AC -DC 190 W power converter. The former and latter stage circuits have the same characteristics. That is, by the principle of a simple structure without increasing components, it can significantly improve performance. In terms of a power factor correction device, when the measurement of the input AC voltage is 110 V and 220 V, it is able to meet the high power factor. Regarding the latter stage near constant frequency LLC resonant converter from very light load to full load, when the switching frequency is almost unchanged, power switch can also achieve zero voltage switching, reduce the power switch losses, effectively improve the efficiency from light loads to very light load. The near constant frequency control significantly reduces electromagnetic interference.