The Design Of The Control Mode And The Key Techniques About One-Cycle BCM PFC Converter

With the rapid development of various electric devices and products, as well as the successive advancement of the integrated circuit manufacturing process, the switching power device has become one of the research focuses on the electric devices, and furthermore it has decided the success of the electronic product design. As one of the main control techniques of the switching power, the power factor correction (PFC) technology is also undergoing constant improvement and developing toward the direction of high efficiency, low cost, low power, light weight and small size. PFC technology can effectively restraint the input current waveform distortion at the side of the grid, and minimize the total harmonic distortion (THD), thereby meeting the increasingly stringent requirements of power factor and the harmonic content for the switching power supply by the grid, especially meeting the demands of the low power application for the portable electronic products.In this paper, the operating principle and control pattern of the single-stage active power factor correction (APFC) circuit are discussed completely at first. For its advantages of low cost and low power consumption, the single-stage APFC is widely used in the moderate and low-power portable electronic products. Then the topological structure, operating principle and the stability of the single-stage Boost PFC converter are discussed more in depth, mainly including the current continuous conduction mode (CCM), the current discontinuous conduction mode (DCM), and the boundary conduction mode (BCM) Boost PFC converter.According to the discussion of the nonlinear controlling mechanism, the operating principle, the modulation pattern and the stability of different operating modes, involving CCM, DCM, and BCM, have been analyzed and discussed. At last the stability working condition of the one-cycle PFC converter is proposed. Using the voltage and current double closed-loop feedback control technique achieves the one-cycle boundary conduction mode PFC converter with the rectification and stability functions. According to the loop stability conditions the compensation networks of the voltage control loop and the current control loop have been built respectively. Based on Simulink tool the high-level behavior model of the one-cycle PFC converter has been established. The simulation and test results verify the correctness and feasibility of this model. A one-cycle control BCM variable frequency PFC converter circuit is implemented using SinoMos 1.0μm BiCMOS process. Based on the double-loop optimal control mode, the circuit introduces the multi-vector error amplifier, the zero-crossing detection and the programmable sawtooth oscillator in order to achieve the variable frequency control, which effectively reduces the power loss of the entire power system at light loads and heavy loads, as well as the conduction loss and noise, and removes the complex analog multiplier and so simplifies the conventional PFC control circuit. The key design techniques used in the proposed PFC converter are as follows:1) A high-order curvature-compensated bandgap reference is studied. Based on the first-order temperature compensation, utilizing the characteristics of a bipolar transistor current gainβexponentially changing with temperature changes, a high-order temperature-compensated bandgap reference is implemented. In addition, an over-temperature protection circuit with thermal hysteresis function to prevent thermal oscillation is proposed, which greatly improves the performance of stability of the bandgap reference.2) A multi-vector error amplifier and a programmable sawtooth oscillator are introduced in this paper in order to modify the PWM switching frequency according to the external loads by the system, thus reducing the power loss of the entire power system at light loads and heavy loads effectively and increasing the useful power. The simulation and test results show that the PFC converter could change the work frequency with the load changes, and transform between the variable frequency mode and the interval mode to meet the requirements of low-power electric-saving PFC converter.3) A zero-crossing detection circuit is applied to realize the current control loop, which control the switching turn-on time to achieve the one-cycle BCM operating mode and to reduce the conduction loss and noise, as well as avoiding large current gap and improving power factor. A hysteresis voltage amplifier with 350mV hysteresis voltage is employed to strengthen the anti-jamming capability of the system. A peak current detection network and a leading edge blanking circuit are used for the current distortion as small as possible.4) The periodic self-starting timer circuit, with the shunt resistor Rz attached between the oscillator and the auxiliary winding, enables the system to accurately regulate the duty cycle of the PWM switching signals in terms of the input ac line voltage timely, thereby reducing the zero-crossing distortion in the vicinity of the AC input voltage zero-crossing points significantly.Based on SinoMos 1.0μm BiCMOS process, the whole layout of the chip is realized. At the same time, some issues which should be considered about the overall layout routing are discussed in detail, including the matching of devices and unit circuits, the routing of critical paths, the isolation of power supply and ground, the crosstalk noise, the latch-up effect, and the ESD protection, and so forth. The active die area is 1.61mm×1.52mm.Based on SinoMos 1.0μm BiCMOS process, the entire circuit has been verified. The test results are as follows:the designed system has a good power factor correction function, its start current is only 36μA, the stable operating current is 2.43mA, the normal operating frequency is 5～6kHz, power factor is 0.988, the linear adjusting rate is less than 1%, the load regulation rate is 3%, THD is 3.8%, and the system efficiency is 97.3%. All these data indicate that the proposed one-cycle BCM PFC converter could meet the requirements for low power saving mode and low zero-crossing distortion, as well as a simple structure and high efficiency. The circuit could adjust the PWM switching frequency automatically according to the load changes, and reduce required power consumption of the system chip efficiently.