Research Of The PSM And PFM Controlled LLC Full-bridge Resonant Converter

In recent years, the switching power supply has been widely used in computers, home appliances, electricity, communications and other fields. High efficiency development of small volume, low switching losses, low voltage stress, low EMI noise, high power density, high reliability power supply system has become the development trend of switching power supply. Range of input voltage and load changes in the full-bridge LLC resonant converter can achieve wide and primary power zero-voltage(ZVS) opening and secondary rectifier zero current(ZCS) off with a suitable power, high efficiency, strong anti-interference performance obvious advantages. Full-bridge LLC resonant converter has become an aroused general interest in research field.Usually there are two ways in controlling full-bridge LLC resonant converter: phase shift control method and frequency control method. However, the phase shift control is easy to implement but narrow input voltage range; high effectiveness performance in frequency control method except the difficulty of transformer magnetic component design. In this paper, The two control methods can be integrated to form a hybrid control used in the full-bridge LLC resonant converter: using phase shift control at high input voltage, low voltage variable frequency control input. Not only retains the advantages of the phase shift control, but also take into account the excellent characteristics of frequency control. The main work summarized as follows:(1) Comparative analysis of the advantages and disadvantages of different resonant converter and its control method, selected as the object of this paper phase-shifted full-bridge inverter control LLC resonant converter. Time domain analysis derived phase shift control when the duty cycle of the converter and inverter control- voltage gain characteristic equation of state, and its duty cycle is given under different quality factor- voltage gain curve; the use of the fundamental component method deduced frequency ratio- voltage gain characteristic equation of state, and gives a different frequency than the quality factor under- voltage gain curve.(2) Analysis of the operating mode of input voltage and switching point of the resonant converter efficiency, but also by analyzing the main circuit loss and calculation to verify the selected voltage switching point accuracy.(3) Analysis of the PWM control circuit transitions from the PWM signal to send out a signal PFM circuit principle, given by UCC3895 phase shift PWM control chip peripheral circuits to achieve its transformation can be achieved at different input voltages and phase-shifted frequency two control specific circuit pattern. Analysis of the conditions for the composite controller is working properly and to determine the limits of its operating frequency.(4) Using full-bridge resonant converter software for system simulation test, the results show a wide input voltage range of circumstances can achieve zero-voltage primary side of the power tube(ZVS) opening and secondary rectifier zero current(ZCS) is turned off, and it performs low converter loss, wide input voltage.