Two-Stage Automotive DC-DC Converter Design with Wide Voltage-Transfer Range Utilizing Asymmetric LLC Operation

An onboard DC-DC converter connects the high voltage traction battery to the low voltage auxiliary battery of an EV. It has to provide power across a wide range of input and output voltages. This paper presents the design and evaluation of an economical two-stage converter concept consisting of a first-stage boost converter and a second-stage LLC converter. While for low input voltages, the boost converter can supply the second-stage LLC with the optimum bulk voltage, for high input voltages, the boost converter is turned off and the LLC regulates the output voltage on its own. Whereas this is unproblematic for high output currents, for low loads high switching frequencies become necessary. For this purpose, the LLC needs to be designed for a wide gain range. Traditionally, this is achieved through a small magnetizing inductance resulting in increased conduction losses. If an asymmetric duty cycle operation is used to cover the low gains at low output current, the LLC can be optimized for a better efficiency. A prototype design proves that the asymmetric duty cycle operation is feasible to achieve a wide gain range at a high efficiency whereas the conventional design achieves very poor efficiencies.