PFM-PWM Digital Controller for Miniaturized High-Frequency Isolated LLC Converters Integrated in Advanced IoT Devices

This article introduces a highly miniaturized soft-switched power supply suitable for advanced Internet-of-things applications. It incorporates an HB-LLC resonant converter operating in the megahertz range and a new pulse frequency modulation (PFM)-PWM digital controller that facilitates tight output voltage regulation carried out through a simple voltage loop controller hardware. The switching frequency and the duty cycle are adjusted simultaneously to enhance output voltage regulation capabilities while full continuity is achieved through a combined pulsewidth and frequency modulation scheme and the utilization of a new high-resolution digital pulsewidth modulator with time resolution of a single delay element. The controller's dual-loop architecture incorporates two proportional-integral compensators to guarantee smooth recovery from load transients or input voltage aberrations. The controller accounts for the nonmonotonicity attribute of asymmetrically driven LLC resonant converters by carrying out dedicated mitigation sequences for low-Q operation. The operation of the controller is experimentally verified on a 3-7 V input HB-LLC converter, demonstrating excellent voltage regulation capabilities and significant reduction of the passive components' stress during startup.