1. Technical Field
The present disclosure relates to wireless power transmission systems.
2. Discussion of Related Art
Most wireless power systems use voltage fed power stages, as is illustrated in FIG. 1. The present power processing stages that are used in the WPC qi standard as well as other standards are voltage fed architectures, with either half or full-bridge technologies. FIG. 1 illustrates a standard full-bridge wireless power transmitter (TX)/receiver (RX) system.
As is shown in FIG. 1, system 100 includes a transmitter (TX) 132 and a receiver (RX) 134. Transmitter 132, as shown in FIG. 1, includes a DC power source 102 that is coupled across a full bridge formed by transistors 104, 106, 110 and 112. The gates of transistors 104, 106, 110 and 112 are driven by a controller 134 such that transistors 104 (Q2) and 112 (Q3) are turned on when transistors 106 (Q1) and 110 (Q4) are turned off and transistors 106 (Q1) and 110 (Q4) are turned on when transistors 104 (Q2) and 112 (Q3) are turned off. The bridge formed by transistors 104, 106, 110 and 112 is driven at a frequency determined by controller 134, and may be at or near a set frequency. TX coil 114 transmits the power to a RX coil 118 in RX 134.
RX 134 is similarly configured to TX 132. Power received in RX 118 is rectified by the bridge circuit formed by transistors 120 (Q2r), 122 (Q1r), 126 (Q4r), and 128 (Q3r). The gates of transistors 120, 122, 126, and 128 are activated by controller 136 in order to produce DC voltage Vout across capacitor 130. As such, transistors 120 and 128 are turned on/off at the same time that transistors 122 and 126 are turned off/on and at a frequency that substantially matches the frequency set by controller 134.
Current path 134, which indicates both the resonant and load currents, includes the switching currents and consequently flows through two series connected switching field-effect transistors (FETs). Similarly, current path 132 flow through two series connected FETs as well. These, therefore, represent large “lossy” circulating currents through the switching FETs when near resonance operation, near full load, and/or during low TX/RX coupling characteristics. Further, controllers 134 and 136 represent may include highly complex gate drive circuitry. In some cases, the switching is further complicated such that zero voltage switching (ZVS) can be achieved.
Consequently, a large shortcoming in conventional wireless power transmission circuits is the circulation of the resonant currents of the TX coil 115 through the power FETs 104, 106, 110 and 112. At less than ideal transmitter-receiver (TX-RX) coupling placements, these currents can be very high, even at power transmissions of 5 W, and can produce large losses in the power FETs. These losses are proportional to the square of the root-mean-square (RMS) of the resonant currents. This results in very high losses in the power FETs. In addition, control circuitry can be very complex because the power FETs are not ground referenced. Additionally, even though the power FETs 104, 106, 110 and 112 are capable of being zero-voltage switched, special steps have to be taken and additional parts have to be used to exploit the beneficial properties of Zero Voltage Switching.
Therefore, there is a need for better transmitters and receivers to affect a wireless transmission system.