Single-inductor multiple-output (SIMO) DC-AC resonant inverters for multi-coil wireless power transfer

Abstract

Multi-coil wireless power transfer (MC-WPT) technology is highly conducive to simultaneous charging of multiple consumer electronic devices such as mobile phones, tablets, wearable devices, and implanted medical devices. Nowadays, one of the most popular inverter topologies used in a typical WPT system is the full-bridge (or half-bridge) inverter. In an MC-WPT system, each of the transmitting coils is powered separately by a dedicated bridge-type inverter. Therefore, the total number of inverters (and also that of power switches) is proportional to the targeted number of output channels, which renders the whole system bulky, costly and inefficient. In view of this, a new series of single-stage, low-cost single-inductor multiple-output (SIMO) resonant inverter is proposed and then thoroughly investigated in this thesis. This research work begins with a complete review of the existing multiple outputs inverter topologies for practical WPT applications. To address their limitations, a SIMO-based boost DC-AC resonant inverter with cascaded blocking diode operating in the discontinuous conduction mode (DCM) of operation, which generates multiple independent AC outputs at the desired frequency from a single DC input, is proposed. Fundamentally, the proposed SIMO inverter is formulated through the proper integration of a conventional SIMO DC-DC converter and multiple parallel resonant blocks. By adding a resonant inductor in parallel with each output capacitor of the SIMO DC-DC converter, a single-stage, scalable and low-cost SIMO DC-AC resonant inverter can be realized. Also, with DCM operation, each output channel can be operated independently without cross-regulation, which makes precise and independent control feasible. An experimental prototype for a single-inductor three-output (SITO) inverter is built to demonstrate the effectiveness of the proposed circuit topology. It is experimentally verified that this SITO resonant inverter can achieve precise and independent peak voltage control across all three sinusoidal output with no noticeable cross-regulation. In our subsequent investigations, we discovered that the blocking diode together with the MOSFET in series in the SIMO boost resonant inverter can actually be replaced by a back-to-back connected MOSFETs. This is advantageous because the overall voltage drop on the switches due to the turn-on resistance of the MOSFET, is much smaller than the forward voltage drop of the Schottky diode when operating at low power. This improves the overall efficiency. Since the proposed SIMO inverter is targeted for real MC-WPT applications, it is important to study its power distribution. A practical design approach of the SIMO inverter, which illustrates that this compact and cost-effective SIMO inverter can potentially be turned into a commercial product, is presented. Wireless power transfer (WPT) is undoubtedly a highly promising technology. However, there exists more than one standard which govern the range of operating frequencies. In view of this, a SIMO-based boost inverter with different output frequencies, which benefits multi-band WPT applications by allowing interoperability of different WPT standards, is investigated. Various switching sequences are proposed to enable different output frequencies across the output channels. Also, due to the recent advances in the semiconductor technology, the hardware prototype of the SIMO boost inverter with multiple frequencies is implemented with the latest GaN transistor. Unlike its silicon counterpart, there is no parasitic body diode inside the GaN transistor. As a result, the cascaded blocking diodes (or back-to-back connected MOSFETs) are no longer needed, provided that an isolated driver is employed to prevent reverse conduction. Thus, a highly-compact and simplified circuit structure can be achieved. To further increase the power rating of the SIMO-based inverter, a SIMO buck-boost resonant inverter is also proposed. Unlike the originally-proposed SIMO inverter operating in DCM, this inverter operates in pseudo-continuous conduction mode (PCCM). By maintaining a positive DC offset of the main inductor current, the input (or output) power can be increased, hence enabling the SIMO buck-boost inverter to operate in the medium power range of 15 W or above, as stated in the Extended Power Profile (EPP) of the Qi standard. This allows quick and simultaneous charging of a larger number of portable electronic devices. In the previously-proposed SIMO inverter, no closed-loop control has been adopted and the relationship between the on-time duty ratio of the main switch and the output voltage magnitude is still unknown. In view of this, a new state partition method for the SIMO-based boost resonant inverter is introduced. A closed-form expression of the sinusoidal-like output voltage as a function of the duty ratio of the main switch is also analytically derived. Based upon the theoretical analysis, a closed-loop system is designed which enables the precise and independent regulation of the output voltage. A single-inductor three-output (SITO) inverter prototype with an analog-based controller is constructed for experimental verification. The experiment results show that the output voltage regulation can be achieved with very high accuracy while no cross-interference across the output channels is observed.