Is the wireless battery charger design too cumbersome? …

There are many ways of wireless power supply. Usually capacitive or inductive coupling is used at a distance of less than a few inches. This article discusses solutions using inductive coupling.

In a typical inductively coupled wireless power system, an AC magnetic field is generated by the transmitter coil, and then an AC power is induced in the receiving coil, just like a typical transformer system. The main difference between the transformer system and the wireless power system is that the transformer system separates the transmitter and the receiver by an air gap or a gap formed by other non-magnetic materials. In addition, the coupling coefficient between the transmitting coil and the receiving coil is usually very low. The coupling coefficient of the transformer system is usually 0.95 to 1, while the coupling coefficient of the wireless power system is 0.8 to 0.05.

Basic knowledge of wireless battery charging

The wireless power system is composed 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).

When designing a wireless power battery charging system, the amount of power that can actually increase battery energy is a key parameter. The received power depends on many factors, including:

● Transmission power value

● The distance and alignment between the transmitting coil and the receiving coil are usually expressed by the coupling coefficient between the coils

● Tolerance of transmitting and receiving components

The main goal of any wireless power transmitter design is to enable the transmitting circuit to generate a strong field to ensure that it provides the required received power under the worst power transmission conditions. However, it is equally important to avoid receiver overheating and electrical overload in the best case. This is especially important when the output power requirement is low and the coupling performance is excellent. For example, the battery uses a battery charger when the receiving coil close to the transmitting coil is fully charged.

Simple and complete transmitter solution using LTC4125

The LTC4125 transmitter IC is designed to be paired with a battery charger IC in the Power by Linear™ product family to use the latter as a receiver; for example, LTC4120—Wireless Power Receiver and Battery Charger IC.

The LTC4125 has all the functions required for a simple, powerful and safe wireless power transmitter circuit. In particular, the output power of the receiver can be adjusted according to the load requirements of the receiver, and the presence of conductive foreign objects can be detected.

As mentioned earlier, the transmitter in the wireless battery charger system needs to generate a strong magnetic field to ensure the transmission of power under the worst power transmission conditions. To this end, LTC4125 uses a proprietary AutoResonant technology.

The LTC4125 AutoResonant drive ensures that the voltage on each SW pin is always in phase with the pin input current. Referring to Figure 2, when current flows from SW1 to SW2, switches A and C are opened, while switches D and B are closed, and vice versa. In this way, the driving frequency is cyclically locked to ensure that the LTC4125 always drives the external LC network at its resonant frequency. This is true even if continuously changing variables affect the resonant frequency of the LC resonator, such as temperature and the reflected impedance of nearby receivers.

Using this technology, the LTC4125 continuously adjusts the driving frequency of the integrated full-bridge switch to match the actual resonant frequency of the series LC network. In this way, the LTC4125 can effectively establish a large-amplitude AC current in the transmitter coil, neither requiring a high DC input voltage nor a high-precision LC value.

LTC4125 also adjusts the pulse width of the waveform on the series LC network by changing the duty cycle of the full-bridge switch. By increasing the duty cycle, more current will be generated in the series LC network, thereby providing higher power to the receiver load.

The LTC4125 scans this duty cycle periodically to find the best operating point for the load conditions at the receiver. This kind of optimal power point search allows larger air gap tolerance and coil misalignment tolerance, while avoiding overheating and electrical overloading of the receiver circuit in all cases. The scan interval can be easily programmed using a single external capacitor.

The system shown in Figure 1 has a high tolerance for misalignment. When the coil is obviously misaligned, the LTC4125 can adjust the intensity of the magnetic field generated to ensure that the LTC4120 receives the full-load charging current. In the system shown in Figure 1, a distance of up to 12 mm can transmit up to 2 W.

Conductive foreign body detection

Another basic feature of any feasible wireless power transmission circuit is the ability to detect the presence of conductive foreign objects in the magnetic field generated by the transmitter coil. The transmitter circuit is designed to provide hundreds of milliwatts or more of power to the receiver, and it needs to be able to detect the presence of conductive foreign objects to prevent the formation of eddy currents in the foreign objects and cause heat generation.

The AutoResonant architecture of the LTC4125 allows a unique way for the IC to detect the presence of conductive foreign objects. Conductive foreign objects will reduce the effective inductance in the series LC network. As a result, the AutoResonant driver increases the driving frequency of the integrated full-bridge.

After the frequency limit is programmed through the resistor divider, the LTC4125 can reduce the driving pulse width to zero within a period of time when the AutoResonant drive exceeds this frequency limit. In this way, when the LTC4125 detects the presence of conductive foreign objects, it will stop transmitting power.

Note that by using this frequency shift phenomenon to detect the presence of conductive foreign objects, the tolerance of the resonant capacitor (C) and the transmitter coil inductance (L) can be used to increase the detection sensitivity. For the 5% typical initial tolerance of each L value and C value, the frequency limit can be programmed with a frequency 10% higher than the expected natural frequency of the typical LC value to obtain reasonable foreign body detection sensitivity and reliable transmitter circuit design . However, the tighter tolerance 1% component setting the frequency limit is only 3% higher than the typical expected natural frequency, and higher detection sensitivity can be obtained while still maintaining the robustness of the design.

Power level flexibility and performance

By adjusting the resistance and capacitance values, the same application circuit can also be paired with different receiver ICs to obtain higher charging power.

Since the transmitting circuit has a high-efficiency full-bridge driver, and the receiving circuit uses a high-efficiency step-down switch topology, the overall system efficiency can be as high as 70%. The overall efficiency of the system is calculated based on the DC input of the transmitting circuit and the battery output data of the receiving circuit. Please note that for the overall efficiency of the system, the quality factor of the two coils and their coupling conditions are as important as the rest of the circuit.

All of these features of the LTC4125 are achieved without direct communication between the transmitter and receiver coils. Therefore, a simple application design can be formed, including various power requirements up to 5 W, and many different physical coil configurations.

Figure 6 shows that a typical LTC4125 application circuit has the characteristics of small size and simple layout. As mentioned earlier, most features can be customized using external resistors or capacitors.

in conclusion

The LTC4125 is a powerful new IC that provides all the features needed to create a safe, simple and efficient wireless power transmitter. AutoResonant technology, optimal power search, and the ability to detect conductive foreign objects through frequency shift simplify the design of a full-featured wireless power transmitter and provide excellent distance and alignment tolerance. The LTC4125 is a good choice for a reliable design of wireless power transmitters.