31.5 A 750mW, 37% Peak Efficiency Isolated DC-DC Converter with 54/18Mb/s Full-Duplex Communication Using a Single Pair of Transformers

Power and signal isolation are crucial to guarantee system safety and reliability in harsh industry environments. Conventionally, separate transformers or high-voltage capacitors are used for power and signal isolation [1]. To obtain a compact design, previous works used high-frequency power transfer that reduced the size of the transformers, while fulfilling power and data transfer using a single pair of transformers, as shown in Fig. 31.5.1. A primary-side LC-oscillator improved the efficiency due to the charge recycling of parasitic capacitance. Then [2] used amplitude shift keying (ASK) to transmit bidirectional data, i.e., forward data through primary-side supply modulation and backward data through secondary-side capacitance-amplitude (C-Amp.) modulation. However, the small modulation depth of the backward ASK (5 % in measurement) makes it susceptible to voltage noise. Conversely, frequency shift key (FSK) is preferable for the applications that demand high-quality data. A straightforward implementation is to drive a power amplifier (PA) with a frequency-modulated signal [3], allowing a large modulation depth. However, it exhibits a high driving loss at the 100-MHz carrier frequency. Meanwhile, [3] only supported forward data transfer, and hence needs another transformer for backward data. To fulfill a high-efficiency FSK, an oscillator with frequency modulation is preferred. In [4], a backward FSK facilitates the frequency splitting effect of the LC oscillator. It selects one of the two splitting resonant frequencies by modulating the secondary capacitance (C-f Mod.) with C₁ and S₁. However, before each data bit transmission, the damping resistors R₁ and R₁^ (controlled by switches S₂ and S₂^) should quench the existing oscillation. This results in an increased settling time and hence lower data rate (e.g., oscillating frequency up to 30 MHz, and data rate 5 Mbps in [4]). Furthermore, [4] did not include the forward data transmission.