Is it possible to use wireless power for applications without batteries?

A wireless power transfer (WPT) system consists of two parts separated by an air gap: the transmit (Tx) circuit (including the transmit coil) and the receive (Rx) circuit (including the receive coil) (see Figure 1). Much like a typical transformer system, the AC generated in the transmit coil generates AC in the receive coil through magnetic field induction. However, unlike a typical transformer system, the coupling between the primary (transmitter) and secondary (receiver) sides is usually low. This is due to the presence of a gap of non-magnetic material (air).

Figure 1. Wireless power transfer system.

Most current wireless power transfer applications use a wireless battery charger configuration. A rechargeable battery is located on the receiving end and can be wirelessly charged whenever a transmitter is present. Once charging is complete, the battery is separated from the charger and the rechargeable battery can power the end application. The downstream load can be connected directly to the battery, indirectly to the battery through a PowerPath™ ideal diode, or to the output of a 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 with downstream load connected to

a)Battery

b) PowerPath Ideal Diode

c) Output terminal of the voltage regulator.

But what if a particular application doesn’t have a battery at all, and instead only needs to provide a regulated voltage rail when wireless power is available? Examples of such applications are common in remote sensors, metering, automotive diagnostics, and medical diagnostics. For example, if a remote sensor doesn’t need to be powered continuously, then it doesn’t need a battery, which would need to be replaced (if it’s a primary cell) or recharged (if it’s a rechargeable cell) periodically. If that remote sensor only needs to give a reading when the user is nearby, then it can be powered wirelessly on demand.

Let’s look at the LTC3588-1 nanopower energy harvesting power supply solution. Although the LTC3588-1 was originally designed for energy harvesting (EH) applications powered by sensors (such as piezoelectric, solar, etc.), it can also be used in wireless power applications. Figure 3 shows a complete transmitter and receiver WPT solution using the LTC3588-1. On the transmitter side, 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 tank circuit, 266 kHz. The exact ratio of fLC_TX to fDRIVE is best determined empirically to minimize the switching losses of M1 caused by zero voltage switching (ZVS). The design considerations regarding the transmitter coil selection and operating frequency are no different from other WPT solutions, that is, there is nothing unique about using the LTC3588-1 on the receiver side.

Figure 3. WPT uses the LTC3588-1 to provide a regulated 3.3 V rail.

On the receiving end, the resonant frequency of the LC tank is set equal to the 216 kHz drive frequency. Given that many EH applications require ac-to-dc rectification (just like WPT), the LTC3588-1 has this capability built in, allowing the LC tank to be connected directly to the PZ1 and PZ2 pins of the LTC3588-1. The rectification is broadband: dc to >10 MHz. Similar to the VCC pin of the LTC4123/LTC4124/LTC4126, the VIN pin of the LTC3588-1 is regulated to a level suitable for powering the back-end output. For the LTC3588-1, it is the output of a hysteretic step-down DC-DC regulator rather than the output of a battery charger. Four output voltages are pin-selectable: 1.8 V, 2.5 V, 3.3 V, and 3.6 V are available with continuous output currents up to 100 mA. The output capacitor can be sized appropriately to provide higher short-term burst currents as long as the average output current does not exceed 100 mA. Of course, realizing the full 100 mA output current capability is dependent upon having an appropriately sized transmitter, coil pair, and adequate coupling.

If the load demand is lower than the available wireless input power supported, the VIN voltage will increase. Although the LTC3588-1 integrates an input protection shunt that can source up to 25 mA when the VIN voltage rises to 20 V, this feature is not required. As the VIN voltage rises, the peak AC voltage on the receive coil also increases,

This is equivalent to a reduction in the amount of AC available to the LTC3588-1, rather than just circulating in the receive resonant circuit. If the open circuit voltage (VOC) of the receive coil is reached before VIN rises to 20 V, the downstream circuit is protected and no heat is generated in the receive IC to cause energy loss. Test results: For the application with a 2 mm air gap shown in Figure 3, the maximum output current available at 3.3 V was measured to be 30 mA, and the VIN voltage measured at no load was 9.1 V. When the air gap is close to zero, the maximum output current available 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. Maximum output current available at various distances at 3.3 V.

For battery-free applications using wireless power, the LTC3588-1 provides a simple, integrated solution that provides a low current regulated voltage rail with complete input protection features.