PLL based bridge synchronization as an alternative to digital isolators for Dual Active Bridge DC-DC converters

Digital isolators are widely used in isolated power converters for transmitting high-frequency gating pulses. Conventional opto-isolators suffer from low reliability and poor delay matching, while emerging RF based isolators are either expensive or power-hungry. In this work, a Phase-Locked-Loop (PLL) based synchronization scheme is demonstrated to operate a bi-directional Dual-Active-Bridge (DAB) dc-dc converter without any digital isolators. The converter is targeted to renewable energy applications with integrated storage. The reference clock is generated on the secondary-side controller and sensed on the primary-side using the main power transformer. A synchronized clock within the primary-side controller is generated using the PLL by modulating the supply voltage of a voltage controlled oscillator. The proposed method is implemented and demonstrated on a 150 W, 94.2% efficient DAB converter prototype. With this synchronization scheme in place, low-frequency communication across the isolation transformer is also demonstrated by modulating the switching frequency.