A Digital Pulse Frequency Modulation Technique for a Series Resonant Converter-Based LED Driver

Pulse frequency modulation (PFM)-based controllers are frequently employed for resonant converter-based light-emitting diode (LED) driver applications. Most of the PFM controllers rely on multiple sensors/control loops and exploit multiple proportional integral (PI) controllers. Thus, straightforward digital implementation of these controllers will not be simple and cost-effective. This article proposes a simple digital PFM control scheme for the series resonant converter (SRC)-based LED driver. The slope of the frequency change of the proposed PFM scheme is implemented using a hysteresis current regulator. The hysteresis current regulator utilizes an envelope signal and the rising and falling of the envelope signal are controlled by a hysteresis band. A sawtooth wave is compared with the envelope signal to produce variable frequency pulses. Thus, the proposed scheme can implement PFM without using any PI controller. Furthermore, the proposed technique exploits only one sensor and one control loop. Therefore, the proposed scheme is simple, cost-effective, and does not impose a huge computational burden on the digital platform. Guidelines for designing different components of the proposed control scheme are presented. Moreover, considering current output, the gain of the whole SRC LED system is derived and the design steps of the output filter are discussed. A 170W SRC LED driver prototype is developed and the proposed scheme is validated using a field-programmable gate array (FPGA)-based digital platform. The proposed digital control scheme can be extended further for other resonant converter-based LED drivers.