Monolithic Full-Bridge Transmitter IC Simplifies Wireless Battery Charger Design

By Eko Lisuwandi, Design Leader, Power Products Division, Linear Technology Corporation

Background Information

Batteries are becoming more and more common in everyday devices. In many of these products, charging connectors are difficult or impossible to use. For example, some products require sealed enclosures to protect sensitive electronic components from harsh environments and to allow for easy cleaning or sterilization. Other products may be too small to accommodate a connector, and if the battery-powered application includes moving or rotating parts, then wired charging is no longer a possibility in such applications. In these and other applications, wireless charging is useful and improves reliability and robustness.

There are many ways to transfer power wirelessly. Over short distances of less than a few inches, capacitive or inductive coupling is commonly used. This article discusses solutions using inductive coupling.

In a typical inductively coupled wireless power transfer system, an AC magnetic field is generated by a transmitting coil, which then induces an AC current in a receiving coil, just like a typical transformer system. The main difference between a transformer system and a wireless power transfer system is that in a wireless power transfer system, a gap of air or other non-magnetic material separates the transmitter and receiver. Additionally, the coupling between the transmit and receive coils is typically very weak. Coupling of 0.95 to 1 is common in transformer systems, but in wireless power transfer systems, the coupling factor can range from 0.8 to as low as 0.05.

Fundamentals of Wireless Battery Charging

A wireless power transfer system consists of two components separated by an air gap: the transmit (Tx) circuit, which includes a transmit coil; and the receive (Rx) circuit, which includes a receive coil.

When designing a wireless power transfer battery charging system, the primary parameter is the amount of power that actually adds energy to the battery. This received power depends on many factors, including: the

amount of transmitted power;
the distance and alignment between the transmit and receive coils, often expressed as the coupling factor between the two coils; and
the tolerances of the transmit and receive components. The

primary goal of any wireless power transmitter design is that the transmit circuitry be able to generate a strong magnetic field to ensure that the required received power is delivered under worst-case power transfer conditions. However, it is also important to avoid overheating and electrical stress on the receiver under best-case conditions. This is particularly important when the output power requirements are low and the coupling is strong. An example is a battery charger when the battery is fully charged and the Rx coil is placed close to the Tx coil.

Simple but Complete Transmitter Solution with the LTC4125

The transmitter IC is designed to work with a variety of different battery charger ICs in the Linear Technology portfolio as receivers, such as the LTC4120, which is a wireless power receiver and battery charger IC.


AIR GAP: Air Gap
SINGLE CELL Li-Ion BATTERY PACK: Single Cell Li-Ion Battery Pack

Figure 1: In a wireless power system using the LTC4120-4.2 as a 400mA single cell Li-Ion battery charger on the receiver, the LTC4125 drives a 24μH transmit coil at 103kHz and uses a 1.3A input current limit, 119kHz frequency limit and 41.5ºC transmit coil surface temperature limit

The LTC4125 provides all the functions required for a simple, robust and safe wireless power transmitter circuit. In particular, the device is able to regulate output power based on the receiver load requirements, as well as detect the presence of conductive foreign objects.

As mentioned earlier, the transmitter in a wireless battery charger system needs to generate a strong magnetic field to ensure that the required received power is delivered under worst-case power transfer conditions. To achieve this goal, the LTC4125 uses Linear Technology's proprietary AutoResonant technology.


Figure 2: LTC4125 AutoResonant Drive Circuit


The LTC4125 AutoResonant drive circuit ensures that the voltage at each SW pin is always in phase with the current entering that pin. See Figure 2: When current flows from SW1 to SW2, switches A and C are turned on and switches D and B are turned off, and vice versa. Locking the drive frequency cycle by cycle in this way ensures that the LTC4125 always drives the external LC network at the resonant frequency. This is always guaranteed, even with continuously changing variables that affect the resonant frequency of the LC tank circuit, such as temperature and the reflected impedance of a nearby receiver.

Using this technique, the LTC4125 continuously adjusts the drive frequency of the integrated full-bridge switching circuit to match the actual resonant frequency of the series LC network. In this way, the LTC4125 can efficiently generate a large AC current in the transmitter coil without requiring a high DC input voltage or a very precise LC value.

By varying the duty cycle of the full-bridge switching circuit, the LTC4125 also adjusts the pulse width of the series LC network waveform. By increasing the duty cycle, the series LC network produces more current, thereby providing more power to the receiver load.


Figure 3: LTC4125 Pulse Width Sweep - Voltage and Current in Tx Coil Increase as Duty Cycle IncreasesThe

LTC4125 periodically sweeps the duty cycle to find the best operating point for the receiver load condition. This optimal power point search tolerates large air gaps and large misalignments between coils under all operating conditions while avoiding overheating and electrical stress on the receiver circuitry. The sweep period is easily set with a single external capacitor.

The system shown in Figure 1 can tolerate considerable coil misalignment. When the coils are misaligned significantly, the LTC4125 is able to adjust the strength of the generated magnetic field to ensure that the LTC4120 receives the full charging current. In the system shown in Figure 1, up to 2W of power can be transmitted over distances up to 12mm.

Conductive Foreign Object Detection

Another essential feature of any viable wireless power transfer circuit is the ability to detect the presence of conductive foreign objects in the magnetic field generated by the transmit coil. Transmit circuits used to deliver more than a few hundred milliwatts of power to a receiver must be able to detect the presence of conductive foreign objects to prevent eddy currents from forming in the foreign object, causing undesirable temperature increases.

The LTC4125's AutoResonant architecture allows the IC to detect the presence of conductive foreign objects in a unique way. Conductive foreign objects reduce the effective inductance value in the series LC network. This causes the AutoResonant driver to increase the drive frequency of the integrated full-bridge circuit.

Figure 4: Frequency comparison of the LTC4125 transmitter LC tank voltage with and without a conductive foreign object.

TANK VOLTAGE: Tank voltage

WITHOUT: 40V PEAK TO PEAK 103kHZ: Without Conductive Foreign Object: 40V Peak to Peak, 103kHz
WITH: 4V PEAK TO PEAK 303kHz: With Conductive Foreign Object: 4V Peak to Peak, 303kHz
TIME: Time

The graph shown in Figure 4 compares the frequency of the voltage generated across the transmit coil with and without the presence of a conductive foreign object.

The LTC4125 reduces the drive pulse width to zero during periods when the AutoResonant drive exceeds the frequency limit set by a resistor divider. In this way, the LTC4125 stops delivering power when it detects the presence of a conductive foreign object.

Note that by using this frequency shift phenomenon to detect the presence of a conductive foreign object, a direct trade-off can be made between detection sensitivity and the component tolerances of the resonant capacitor (C) and the transmit coil inductance (L). With a typical initial tolerance of 5% for each L and C value, this frequency limit can be set 10% above the natural frequency expected from typical LC values ​​to achieve reasonably sensitive foreign object detection and a reliable transmitter circuit design. However, tighter 1% tolerance components can be used with the frequency limit set only 3% above the expected typical natural frequency to achieve higher detection sensitivity while still maintaining a reliable robust design.

Flexibility and Performance for Power Variations

By simply changing the values ​​of the resistors and capacitors, the same application circuit can be paired with different receiver ICs to achieve higher wattage charging.

Figure 5: In this wireless power transfer system, the LTC4125 drives a 24μH transmit coil at 103kHz, the frequency is limited to 119kHz, the transmit coil surface temperature is limited to 41.5ºC, and on the receiver side, the LT3652HV is used as a 1A single-cell LiFePO4 (3.6V floating voltage) battery charger.

AIR GAP: Air Gap
SYSTEM LOAD: System Load
SINGLE LiFePO4 CELL: Single LiFePO4 Battery

Due to the use of high efficiency full-bridge drivers on the transmit circuit and high efficiency buck switching topology on the receive circuit, an overall system efficiency of up to 70% can be achieved. This overall system efficiency is calculated using the DC input of the transmit circuit and the battery output of the receive circuit. Note that the quality factor of the two coils and their coupling are as important to the overall efficiency of the system as the rest of the circuit.

All of these functions of the LTC4125 can be achieved without any direct communication between the transmitter and receiver coils. This allows for simple application designs covering a wide range of power requirements up to 5W and many different practical coil arrangements.


Figure 6: Typical and Complete Wireless Power Transmitter Board Using LTC4125

Figure 6 shows that the overall size of a typical LTC4125 application circuit is small and simple. As mentioned earlier, most functions can be customized with external resistors or capacitors.

Conclusion

The LTC4125 is a powerful new IC that provides all the functions required to form a safe, simple and efficient wireless power transmitter. AutoResonant technology, optimal power search and frequency-variation based conductive foreign object detection ease the design burden of a full-featured wireless power transmitter with excellent distance and misalignment tolerance. The LTC4125 is an easy and excellent choice for reliable wireless power transmitter design.