Cascade control of a two-stage isolated DC/DC converter with wide input voltage range for optimal efficiency

This paper addresses a control concept for a two-stage isolated DC/DC converter with an input voltage range from 400 to 800V and an isolated output voltage of 48 V. The DC/DC converter consists of a buck converter, which is utilized to step down the input voltage, followed by an LLC resonant converter for galvanic isolation. The proposed control strategy makes use of the simple voltage gain function of the LLC resonant converter approximating the converter as a PT1 element in the superimposed voltage control loop. The actuating variable is the duty cycle of the buck converter while the LLC resonant converter switches with fixed frequency. This allows working the LLC resonant converter at the resonant frequency at all times boosting the efficiency of the whole system.