Nonlinear Load Analysis and Design of Resonant dc-dc Converters

Abstract

Modern electronic systems demand high power density, low EMI and high efficiency power processors. The choice of power supply topology in particular design is significantly based on the needs of the application itself. Resonant dc-dc converter is more and more popular because it has the major advantage of easy to achieve zero voltage switching. However, in resonant dc-dc converters, the load of resonant circuit is a nonlinear component, a rectifier, which makes system analysis complex. Traditional dc-dc converter analyses treat the rectifier load as an equivalent resistive load, then, the analysis can follow linear resonant circuit analysis. But in some cases, like burst mode control systems, the equivalent impedance of rectifiers can’t be got by linear analysis. Moreover, the behaviors of dc-dc resonant converters can be derived only by system dc operating point. Involving load impedance complicates system analysis in some cases. Two resonant systems, PT dc-dc converters and a single-stage LLC dc-dc converter are involved in this thesis. A novel analysis based on energy equilibrium is presented for burst mode control PT dc-dc converters. The PT input current and system efficiency of PT dc-dc converters are derived in time domain. The results show that the system behaviors only depend on system operating point and the PT input current contains the information of output voltage. Then, a primary control PT dc-dc converter is built for verification. A conventional power supply was designed with two stages, the former stage function as a power factor corrector (PFC), and the latter stage is a dc-dc converter which regulates the output voltage of the system. However, the cost and energy conversion efficiency of the two-stage system is higher than a single-stage one. For simplification, one control IC is used to drive two power converters, PFC regulator and resonant converter, in a single stage LLC dc-dc converter. However, the bulk capacitor between two converters becomes unpredictable because the power of PFC regulator and resonant converter are not balance. In this thesis, the relationship between operating frequency and output power of two converters are derived. Actually, the load impedance is removed to simplify the analysis. The results help to design single stage LLC dc-dc converter. In summary, the rectifier load of resonant circuit is treated as a voltage source. The system behaviors can be derived by given system dc operating point. Thus, the load impedance can be removed from the system analysis which simplifies system analysis and helps system design.