“The wireless power transmission (WPT) system consists of two parts separated by an air gap: the transmitting (Tx) circuit (including the transmitting coil) and the receiving (Rx) circuit (including the receiving coil) (see Figure 1). Very similar to a typical transformer system, the alternating current generated in the transmitting coil generates alternating current in the receiving coil through magnetic field induction. However, unlike a typical transformer system, the degree of coupling between the primary side (transmitting side) and the secondary side (receiving side) is usually very low. This is due to the presence of non-magnetic material (air) gaps.
The wireless power transmission (WPT) system consists of two parts separated by an air gap: the transmitting (Tx) circuit (including the transmitting coil) and the receiving (Rx) circuit (including the receiving coil) (see Figure 1). Very similar to a typical transformer system, the alternating current generated in the transmitting coil generates alternating current in the receiving coil through magnetic field induction. However, unlike a typical transformer system, the degree of coupling between the primary side (transmitting side) and the secondary side (receiving side) is usually very low. This is due to the presence of non-magnetic material (air) gaps.
Figure 1 Wireless power transmission system.
Most wireless power transmission applications currently use wireless battery charger configurations. The rechargeable battery is located at the receiving end, as long as there is a transmitting end, it can be wirelessly charged. After charging is complete, separate the battery from the charger, and the rechargeable battery can supply power to the terminal application. The back-end load can be directly connected to the battery, or indirectly connected to the battery through a PowerPath™ ideal diode, or connected to the output of the battery-powered regulator integrated in the charger IC. In all three cases (see Figure 2), the end application can run on or off the charger.
Figure 2 Wireless Rx battery charger, the back-end load is connected to a) battery, b) PowerPath ideal diode and c) voltage regulator output.
But what if there is no battery at all for a particular application, and instead, only a regulated voltage rail is provided when wireless power is available? Examples of such applications are extremely common in the fields of remote sensors, metering, automotive diagnostics, and medical diagnostics. For example, if the remote sensor does not need continuous power supply, then it does not need a battery, and the battery needs to be replaced regularly (if it is a primary battery) or recharged (if it is a rechargeable battery). If the remote sensor only needs the user to give a reading when it is nearby, it can be wirelessly powered on demand.
Let’s look at the LTC3588-1 nano-power energy harvesting power solution. Although the LTC3588-1 was originally designed for energy harvesting (EH) applications where sensors (such as piezoelectric, solar, etc.) are powered, it can also be used for wireless power applications. Figure 3 shows the complete transmitter and receiver WPT solution using LTC3588-1. At the transmitting end, a simple open-loop wireless transmitter based on the LTC6992 TimerBlox® silicon oscillator is used. In this design, the drive frequency is set to 216 kHz, which is lower than the resonant frequency of the LC resonant circuit at 266 kHz. The precise ratio of fLC_TX to fDRIVE is best determined empirically to minimize the M1 switching loss caused by zero voltage switching (ZVS). Regarding the design considerations of the transmitter coil selection and operating frequency, it is no different from other WPT solutions, that is to say, there is nothing unique about using LTC3588-1 at the receiver.
Figure 3 WPT employing the LTC3588-1 to supply a regulated 3.3 V rail.
At the receiving end, the resonance frequency of the LC resonant circuit is set to be equal to the driving frequency of 216 kHz. Given that many EH applications require AC to DC rectification (just like WPT), the LTC3588-1 has built-in this function, allowing the LC resonant circuit to be directly connected to the PZ1 and PZ2 pins of the LTC3588-1. The rectification is broadband rectification: DC to >10 MHz. Similar to the VCC pin of LTC4123/LTC4124/LTC4126, adjust the VIN pin of LTC3588-1 to a level suitable for powering the back-end output. For LTC3588-1, it is the output of the hysteresis step-down DC-DC regulator rather than the output of the battery charger. Four output voltages can be selected through pins: 1.8 V, 2.5 V, 3.3 V and 3.6 V are selectable, and the continuous output current is up to 100 mA. As long as the average output current does not exceed 100 mA, a suitable output capacitor can be selected to provide a higher short-term burst current. Of course, to fully realize the 100 mA output current capability, it also depends on whether the transmitter has an appropriate size, the coil pair, and whether it is sufficiently coupled.
If the load demand is lower than the supported available wireless input power, the VIN voltage will increase. Although LTC3588-1 integrates an input protection shunt that can provide up to 25 mA of sourcing current when the VIN voltage rises to 20 V, this function is not necessary. As the VIN voltage rises, the peak AC voltage on the receiving coil also rises.
This is equivalent to a drop in the amount of AC that can be provided to the LTC3588-1, rather than just circulating in the receiving resonant circuit. If the open circuit voltage (VOC) of the receiving coil is reached before VIN rises to 20 V, the back-end circuit is protected and no heat will be generated in the receiving end IC to cause energy consumption. Test result: For the application with an air gap of 2 mm shown in Figure 3, the maximum output current that can be provided at 3.3 V is 30 mA, and the VIN voltage measured without load is 9.1 V. When the air gap approaches zero, the maximum output current that can be provided increases to approximately 90 mA, while the VIN voltage at no load only increases to 16.2 V, which is much lower than the input protection shunt voltage (see Figure 4).
Figure 4 The maximum output current that can be provided at various distances at 3.3 V.
For battery-free applications using wireless power supplies, the LTC3588-1 provides a simple integrated solution that can provide a low-current regulated voltage rail with complete input protection.