Without adding components or increasing size, we will teach you how to improve your wireless charger!

This article describes a method to close the control loop between the receiver and transmitter without increasing the number of components (and precious overall size) on the receiver board. We demonstrate this concept by building a closed-loop controlled wireless charger prototype using the LTC4125 AutoResonant™ transmitter and the LTC4124 wireless Li-Ion charger receiver.

The LTC4125 is a single-chip full-bridge AutoResonant wireless power transmitter designed to maximize the power available to the receiver, improve overall efficiency, and provide comprehensive protection for wireless charging systems.

The LTC4125 uses an AutoResonant converter to drive a series LC resonant circuit consisting of a transmit coil (L _TX ) and a resonant capacitor (C _{TX )} . The AutoResonant driver uses a current zero-crossing detector to match its drive frequency to the resonant frequency of the LC resonant circuit. The SW1 and SW2 pins are the outputs of the two half-bridges inside the LTC4125. When the SWx pin detects that the direction of its output current is zero-crossing from negative to positive, SWx is turned on to V _IN with a duty cycle proportional to its corresponding PTHx pin voltage. When the SWx pin is turned on to V _IN , the amount of current flowing through the transmitter resonant circuit increases. Therefore, the duty cycle of each bridge driver controls the amplitude of the transmitter resonant circuit current, which is proportional to the transmitted power. Figure 1 shows the resonant circuit current and voltage waveforms for duty cycles below 50%. The absolute value of the resonant circuit current amplitude is determined by the overall circuit impedance, including the referenced load impedance from the wireless receiver.

In conventional operation, the LTC4125 uses an internal 5-bit DAC to scan the SWx duty cycle; the DAC sets the PTHx voltage to search for a valid load. If some form of voltage change occurs on the FB pin, the scan stops and the duty cycle remains constant for an adjustable scan period (typically set to about 3 to 5 seconds). A new scan period then begins, repeating the same steps. If the load condition changes during the scan period, the LTC4125 responds at the start of the next scan period.

To form a closed loop, the transmit power of the bridge driver should be adjustable according to the control input. The LTC4125 has several features, among which the PTHx pin can not only be used to indicate the bridge driver duty cycle, but also can be driven as an input to set the duty cycle. The internal 5-bit DAC of the chip uses an internal pull-up resistor to set the voltage target value of the PTHx pin. However, as shown in Figure 2, an external pull-down resistor can be connected in series with the FET to discharge the capacitor on the PTHx pin, thereby reducing the average voltage of the PTHx pin. The duty cycle of the PWM signal at the gate of this pull-down FET can control the average voltage of the PTHx pin, thereby controlling the output power.

The LTC4125 is designed to deliver over 5 W to a suitable receiver. When paired with the LTC4124 receiver, the transmit power can be reduced by disabling one of the half-bridge drivers. This can be accomplished by leaving the SW2 pin open and PTH2 shorted to GND. A transmit resonant circuit can then be connected between the SW1 pin and GND. This makes the LTC4125 a half-bridge transmitter, allowing for lower gain at the PTH1 pin, increasing the range of effective control voltages at the PTH1 pin.

The LTC4124 is a highly integrated 100 mA wireless Li-Ion charger designed for space-constrained applications. It contains an efficient wireless power manager, a pin-programmable full-featured linear battery charger, and an ideal diode PowerPath™ controller.

The wireless power manager in the LTC4124 connects to a parallel resonant circuit via the ACIN pin, allowing the linear charger to wirelessly receive power from the alternating magnetic field generated by the transmit coil. When the LTC4124 receives more power than is required to charge the battery at a set rate, the excess energy charges the input capacitor of the linear charger at the VCC pin. When the VCC _{pin voltage} rises to the battery voltage, V _{BAT +} 1.05 V, the wireless power manager shunts the receiver resonant tank to ground until VCC _drops to V _BAT + 0.85 V. In this way, the linear charger is very efficient because its input is always just above its output.

Shunting the receiver resonant tank to ground by the LTC4124 also reduces the referred load impedance on the transmit resonant tank, causing the transmit resonant tank current and voltage amplitude to increase. Because the shunting means that the receiver has received enough power from the transmitter, the increase in the transmitter resonant tank peak voltage can be used as a feedback signal for the transmitter to regulate its output power.

After the feedback signal from the receiver is obtained on the resonant transmitter side, the feedback signal needs to be converted and fed to the control input of the transmitter to close the control loop. As shown in Figure 6, the peak circuit voltage signal can be obtained from the half-wave rectifier composed of a diode and capacitor C _FB1 . This voltage signal is further divided by resistors R _FB1 and R _{FB2 . In order to detect the change in peak voltage, a low-pass filter composed of a resistor (R AVG} ) and a capacitor (C _AVG ) is used to filter the peak voltage signal to obtain the average value of the voltage signal. By comparing this average signal with the original peak voltage signal, a square wave pulse can be generated. This pulse is then fed to the duty cycle control input of the LTC4125 to adjust the transmitter output power.

When the receiver is not receiving enough power, the LTC4125 should increase its output power. This can be achieved by setting an internal voltage target for the PTHx pin. The internal voltage target can be set via the PTHM pin, which sets the initial 5-bit DAC voltage level before starting the LTC4125 search cycle. A 1V reference voltage can be connected to the IMON pin to disable the search, so that the PTHx pin target voltage always remains at the initial value during operation. If the LTC4124 receiver requires more power, the shunting will stop and the FET that discharges PTHx will not turn on. The LTC4125 will charge the PTHx voltage with reference to the internal voltage target until the LTC4124 receives enough power to enable the shunting.

When the receiver outputs the preset maximum charging current at the worst coupling coefficient position in the application, the required maximum transmit power can be determined by measuring the PTHx voltage. When setting the PTHM pin voltage, the maximum transmit power requirement should be met.

Figure 7 shows the complete schematic of a closed-loop controlled transmitter based on the LTC4125 and a 100mA receiver based on the LTC4124. As shown, the number of components required on the receiver side is minimal, which reduces the cost and size of the receiver. Only a few additional components are required on the transmitter side to achieve closed-loop control compared to a typical LTC4125 application. Most of the features of the LTC4125 are retained, including the AutoResonant switch, multiple foreign object detection methods, overtemperature protection, and resonant circuit overvoltage protection. For more information on these features, refer to the LTC4125 data sheet.

A closed-loop wireless transmitter based on the LTC4125 can dynamically adjust its output power to match the power requirements of the receiver. Figure 8 shows the response of this wireless charger when the receiver coil is offset from the center of the transmitter coil and then quickly returns to the original position. The output power of the LTC4125 transmitter is represented by the peak transmit circuit voltage V _{TX_PEAK} , which responds smoothly to changes in the coupling coefficient between the two coils to keep the charging current constant.

During the rising charge current transient, the LTC4124 shunting stops, allowing the LTC4125 to charge its PTH1 pin internally. As a result, the LTC4125 increases its half-bridge driver duty cycle to increase transmit power. Once transmit power is high enough for the LTC4124 to regulate its charge current, shunting resumes and the duty cycle remains at the efficiency-optimal level. During the falling charge current transient, the LTC4124 shunts more frequently. The LTC4125’s external circuitry quickly discharges the capacitor on its PTH1 pin to reduce the duty cycle and reduce the LTC4125’s transmit power.

Because the transmit power always matches the receiver's needs, the overall efficiency is greatly improved compared to the typical configuration of wireless chargers based on LTC4124 and LTC4125 without closed-loop control. Since the original LTC4125 best power search mode is not used, there is no DAC stepping effect, and the efficiency curve of this configuration is smoother. Because the power loss is greatly reduced, the LTC4124 charger and battery are always kept close to room temperature during the entire charging period.

The LTC4125 can be configured as a power-adjustable transmitter with a control input. The feedback signal can be provided to the transmitter by shunting the LTC4124 wireless charger receiver. The feedback signal can be demodulated through a half-wave rectifier, a voltage divider, a low-pass filter, and a comparator. The processed signal is fed into the power-adjustable transmitter based on the LTC4125 to close the control loop. We have built a prototype to verify this concept. This prototype is able to respond quickly and smoothly to changes in the coupling coefficient and charging current. With this approach, the end user can allow for greater deviations when placing the receiver above the transmitter without worrying about whether the receiver can obtain the required power. In addition, this closed-loop approach allows the transmitter output power to always match the receiver's power requirements, thereby improving overall efficiency and making the entire charging cycle safer and more reliable.