Exploitation of MOSFET‐based AC switches in capacitive impedance matching networks in inductive wireless power transfer systems

Abstract

This study investigates the exploitation of MOSFET-based AC switches in capacitive impedance matching networks (IMNs) for inductive wireless power transfer (WPT). The IMN optimum capacitance has been chosen for a 200 kHz resonant frequency. The activation of the tuning capacitor on the tuning branch is achieved with the use of MOSFET AC switches in order for the WPT system to achieve maximum power transfer efficiency. The MOSFET AC switch is modelled as an internal parasitic resistor and capacitor connected in parallel. A WPT analytical model is developed to study the effects of the MOSFET's parasitic elements on the WPT system's efficiency and is verified experimentally. Various MOSFET switches and relays have been implemented as the IMN switching elements and compared when tested under the same conditions. It is concluded that MOSFET AC switches which have low on-resistances and small parasitic capacitances are desirable as they have a smaller impact on efficiency. Additionally, parasitic capacitances of MOSFET AC switches need careful consideration for different resonant frequencies as they can affect the overall IMN tuning capacitance especially when they are turned off. Comparing to commonly used relay switches, MOSFET-based AC switches have similar switching performance but are significantly smaller in size.

Comparisons of MOSFET and Relay Switches in Impedance Matching Networks for Wireless Power Transfer

“…From the theoretical analysis of the switch mentioned in Section II.D, it is known that the on-resistance of the MOSFET has an important role. Previous study [5] showed that the CSD18533KCS MOSFET has minimum effect on the WPT efficiency due to its low on-resistance of 5 mΩ. This MOSFET is also characterized by a parasitic capacitance of 2420 pF.…”

“…Fig. 2(b) illustrates the switch equivalent model which is represented by its parasitic resistor, R p and capacitor, C p connected in parallel [5]. Based on this model, the equivalent impedance formula of a MOSFET AC switch is derived in Eq.…”

“…when the switch is on and off. It was found that when the switch is on, a small onresistance is desirable as it shorts the parasitic capacitor and does not influence the equivalent impedance of the network [5]. In contrast, when the switch is off, the off-resistance is large and can be treated as open circuit.…”

“…In contrast, when the switch is off, the off-resistance is large and can be treated as open circuit. It was concluded that in order to isolate to branch from the circuit, it is preferred for the MOSFET to have a small parasitic capacitance so as not the affect the equivalent impedance of the IMN and thus, the efficiency [5]. Considering that in the experiment, it was infeasible to determine the value of C p , the MOSFET with low on-resistance was selected.…”

“…In complex designs constituting of multiple capacitors using EMR switches offer the disadvantage of an increased IMN size if compared to MOSFET switches or a non-switchable IMN [5]. Therefore, the MOSFET switch is an alternative which currently in literature has not been thoroughly investigated for this application.…”