A GaN-Based 250-kHz LLC Converter with Hybrid Analogue-Digital Control

Abstract:
A high-frequency GaN-based LLC converter, the GaNSTar, is driven by a microcontroller and is the object of this paper. Unlike conventional approaches in which the microcontroller is conveniently located on the isolated output, the processor is purposely placed on the non-isolated part of the circuit. In this position, the microcontroller can directly access the high-voltage drivers and other instantaneous variables for monitoring purposes. However, regulation becomes a challenge considering an output variable located on the other side of the isolation barrier. The simplest solution to give the microcontroller access, involves a fully-isolated amplifier which cannot often be considered for cost reasons. We purposely departed from this option and used a hybrid analogue-digital solution in which the error voltage is processed on the secondary side via an analogue integrator and transmitted through a simple opto-isolator to the microcontroller ADC input. A discrete pole-zero pair is then applied as part of the compensation strategy, thus offering the flexibility of digital control for shaping part of the loop gain response. To optimize this approach further, a simple alteration to the opto-isolator circuit was implemented to increase signal-path bandwidth.