Wireless battery charging is a technique for supplying power to devices in difficult to reach locations . It allows products to be charged while locked within sealed enclosures, or in moving or rotating equipment, or where cleanliness or sanitation is critical. Linear Technology’s LTC4120 (Fig. 1) is one component of a complete wireless power transfer system consisting of transmit circuitry, transmit coil, receive coil and receive circuitry, as well as a constant-current/constant-voltage battery charger. Fig. 2 shows a simplified wireless power transfer system with battery charging that employs the LTC4120.
Applications include handheld instruments, industrial/military sensors and similar devices in harsh environments, portable medical devices, physically small devices and electrically isolated devices. These systems offer solutions that are much simpler than those implementing the Qi standard, with additional benefits, including greater transmission distance and no software required.
The circuit in Fig. 3 is a fully functional wireless power transfer system using a basic current-fed resonant converter for the transmitter and an LTC4120 to control a series resonant converter for the receiver. The receive coil provides a rectified 12 V to 40 V input to the LTC4120, whose synchronous step-down (buck) powers the battery charger. Its charging features include:
· Constant-current/constant-voltage 400 mA output
· Programmable charge current
· Programmable 3.5 V to 11 V battery float voltage with ±1% accuracy
· Programmable float voltage accommodates several battery chemistries
· Battery preconditioning with half-hour timeout
· Precision shutdown/run control
· Bad battery fault detection
· NTC thermal protection
· Output flags indicate state of charge and fault status
· Two-hour safety termination timer
After terminating charging , the IC signals end-of-charge and enters a low current sleep mode. Its auto-restart feature starts a new charging cycle if the battery voltage drops by 2.5%.
The LTC4120 works with Linear Technology’s discrete resonant transmitter reference design or with an advanced off-the-shelf transmitter designed and manufactured by PowerbyProxi, a New Zealand-based leader in wireless power solutions. PowerbyProxi transmitters offer advanced features, including simultaneous charging of multiple receivers with a single transmitter and foreign object detection to prevent excessive heating during transmit faults.
Wireless Power Transfer
The push-pull current-fed resonant converter, shown in Fig. 3, is an example of a basic power transmitter that may be used with the LTC4120. In operation, the transmitter generates an alternating magnetic field, and the receiver inductively collects power from that field. The transmitter efficiently generates a large alternating current in the transmitter coil, LX. Typically, this transmitter operates at approximately 131kHz; although the actual operating frequency depends on the load at the receiver and the coupling to the receiver coil. For LX = 5.0 μH, and CX = 0.3µF, the transmitter frequency, fO, is:
This transmitter typically generates an AC coil current of about 2.5 ARMS. The receiver consists of a coil, LR, configured in a resonant circuit followed by a rectifier and the LTC4120. The receiver coil presents a load reflected back to the transmitter through the mutual inductance between LR and LX. The reflected impedance of the receiver may influence the operating frequency of the transmitter. Likewise, the power output by the transmitter depends on the load at the receiver. The resonant coupled charging system, consisting of both the transmitter and LTC4120 charger, provides an efficient method for wireless battery charging as the power output by the transmitter varies automatically based on the power used to charge a battery.
The LTC4120 includes dynamic harmonization control (DHC), PowerbyProxi’s patented technique that provides optimal wireless power transfer across a variety of conditions while providing thermal management and overvoltage protection. DHC is an efficient method for regulating the received input voltage in a resonant coupled power transfer application.
DHC operates by modulating the resonant frequency of the receiver to regulate the voltage at the input to the LTC4120. When the input voltage to the LTC4120 is below the VIN(DHC) set point, the IC allows more power to appear at the receiver by tuning the receiver resonance closer to the transmitter resonance. If the input voltage exceeds VIN(DHC), the IC tunes the receiver resonance away from the transmitter, which reduces the power available at the receiver. The amount that the input power increases or decreases is a function of the coupling, tuning capacitor, C2P, receiver coil, LR, and operating frequency.
Fig. 4 illustrates the components that implement the DHC function. Capacitor C2S and inductor LR serve as a series resonator. Capacitor C2P and the DHC pin of the LTC4120 form a parallel resonance when the DHC pin is low impedance, and disconnects when the DHC pin is a high impedance. C2P adjusts the receiver resonance to control the amount of power available at the input of the LTC4120. C2P also affects power dissipation in the LTC4120 due to the AC current being shunted by the DHC pin.
DHC results in significant power savings, as the power required by the transmitter automatically adjusts to the requirements at the receiver. Furthermore, DHC reduces the rectified voltage seen at the input of the LTC4120 under light load conditions when the battery is fully charged.
Battery Charging Features
During the charging cycle, a negative temperature coefficient (NTC) thermistor allows the LTC4120 to monitor battery temperature. If the battery temperature moves outside a safe charging range, the IC suspends charging and signals a fault condition until the temperature returns to the safe charging range. Two comparators monitor the voltage at the NTC pin to determine this charging range. NTC qualified charging is disabled if the NTC pin is pulled below about 100 mV (VDIS).
When the battery voltage reaches its float voltage, a safety timer begins counting down a 3-hour timeout. If charge current falls below one-tenth of the programmed maximum charge current, the charge status flag rises, but top-off charge current continues to flow until the timer finishes. Once the charge status flag rises, the LTC4120 stops monitoring the battery temperature via the NTC pin. After the timeout, the LTC4120 enters a low power sleep mode.
In sleep mode, the IC continues to monitor battery voltage. If the battery falls 2.5% from the full-charge float voltage, the LTC4120 engages an automatic recharge cycle. Automatic recharge has a built-in filter of about 0.5ms to prevent triggering a new charge cycle if a load transient causes the battery voltage to drop temporarily.
The LTC4120 automatically preconditions heavily discharged batteries with 10% of the full-scale charge current. When the battery rises above the trickle charge threshold, the IC applies the full-scale charge current. If the battery remains below the trickle charge threshold for more than 30 minutes, charging terminates and the fault status flag asserts to indicate a bad battery.