Analysis, Design & Control of Low-Q LLC DC-DC Resonant Converter for Wide Input Voltage and Load Range Applications
Constant frequency LLC converters reported in literature guarantees optimal efficiency only at nominal operating conditions and lose zero voltage switching (ZVS) at reduced load and high-end of the input voltage range. Traditional variable frequency LLC resonant converter requires wide switching frequency range and a high quality-factor (Q) of resonant circuit for voltage regulation at light loads. A high Q results in large size of the resonant circuit, increased magnetic losses and high circulating current for a wide range of load and input voltage variations. Low-Q LLC resonant circuits may allow efficient operation of the converters due to low voltage stress across the magnetics and capacitors, but they will lose ZVS operation at light loads. This thesis presents the analysis of low-Q LLC resonant converters and also proposes a new control technique. Frequency domain modeling of the low-Q LLC resonant circuit is not possible without considerably sacrificing the accuracy of the design results. This is because for low-Q LLC resonant circuit, the voltage waveform at the output of the circuit is not defined. An accurate time-domain analysis of the resonant converter is, therefore, required and proposed in this thesis. A novel light-load modulation scheme is proposed for achieving ZVS operation from part-load to extremely light-load conditions at high end of the input voltage range. In the proposed scheme, both duty cycle and switching frequency of the full bridge inverter are varied. A time domain model has been developed and resulting non-linear transcendental equations solved using Newton Raphson method. An experimental prototype of 380V/300W LLC converter operating between input voltage 20-40V, is developed for validating the analysis and performance of the proposed LLC converter. An efficiency improvement of upto 6% is observed for the loads below 10% of the full-load.