Investigation Of Bifrequency Con-Trol Technique For Switching DC-DC Converters

Pulse width modulation (PWM) and pulse frequency modulation (PFM) are two most widely used techniques in switching dc-dc converters. PWM benefits from constant switch-ing frequency and simple implementation, but suffers from low efficiency at light load, slow transient response speed, large electromagnetic interference (EMI) noise and so on. PFM ef-fectively improves the efficiency at light load, but suffers from varying switching frequency depending on line or load variation, which makes the design of feedback loop and EMI filter difficulty.To solve the problems of PWM and PFM, a novel control technique, called as bifre-qunecy (BF) control, for switching dc-dc converters is proposed and investigated in this dis-sertation. It achieves the control of switching converters by the adjusting combination of two different control pulses with different frequencies. Unlike PWM and PFM, which adjust the duty ratio of control pulse cycle by cycle continuously, BF control technique achieves the control of switching converters by regulating the combination of high and low frequency control pulses, which provids a novel way for the control of switching dc-dc converters. The converter with BF control technique has advantages such as simple implementation, fast transient response speed, low EMI noise, and high efficiency at light load, and so on.BF controlled switching converter operating in discontinuous conduction mode (DCM) have been investigated in detail in this dissertation. Before studying BF control technique with control pulses of the same on-time but different frequencies, the conventional constant on-time (COT) controlled buck and boost converters are investigated. The study results in-dicate that the equivalent series resistance (ESR) of the output filter capacitor is the key fac-tor to produce pulse bursting phenomenon, which can be effectively avoided by increasing the vaule of ESR. The investigation results provide a useful guideline for the application of BF control technique to switching dc-dc converters operating in continuous conduction mode (CCM).Two different BF control techniques, i.e. voltage mode and current mode bifrequency control, are analyzed in detail. Their implementations, operation principle, characteristics, stabilities, control rules, combination of control pulses in the repetition cycle, output voltage ripple characteristics, steady-state and transient characteristics, and spectrum features of control signal are presented. The differences between voltage mode and current mode bifre-quency control, as well as their operation characteristics and internal relations of control rules, are investigated. Research results indicate that the current mode bifrequency control technique is superior to that with voltage mode.To extend the operation region of BF control technique, BF control technique is applied to switching dc-dc converters operating in CCM. The differences between CCM and DCM applications are investigated. The steady-state and transient problems existing in BF con-trolled CCM converter are revealed. Corresponding methods are provided to improve the control performances of BF controlled switching dc-dc converters operating in CCM.In order to verify the theoretical analysis results, a lot of simulation results are provided, and corresponding prototypes are set up. Simulation and experimental results have a good agreement with the theoretical analysis presented in this dissertation.