Charging made easy

In many of applications today, a connector for charging purposes is difficult or impossible to use. For example, some products require sealed enclosures to protect sensitive electronics from harsh environments. Others may simply be too small to include a connector. And in products where the battery-powered application includes movement or rotation, it is virtually impossible to have charging with wires.

Therefore an alternative method must be employed to deal with these circumstances; clearly one that eliminates the connector. Increasingly, wireless charging solutions are being used; adding value, reliability and robustness in applications where connectors cannot.

Power transmission

If wireless charging is a good solution in those instances where a connector cannot be used, what is it and how is it accomplished? A simple and straightforward definition might be the transmission of electrical energy from a power source to an electrical load without the use of man-made conductors. However, this tends to be a little too simplistic to demonstrate the challenges involved in performing this process; here a more in-depth explanation is offered, along with a discussion of the constraints and the methods being developed to overcome them.

At a basic level, an electric current flowing through a conductor, such as a wire, carries electrical energy. When an electric current passes through a circuit (or wire) there is a magnetic field in the area surrounding the conductor. In a circuit with alternating current, there exists a time-varying magnetic field in the vicinity of the wire. And if a conductor is placed into this time-varying field, a current is induced.

One common occurrence in electronic systems is electrical transients such as lightning strikes from an external source or capacitor discharge, which could be an internal repetitive disturbance such as in the condenser discharge of an ignition system.

Figure 1: Wireless power transfer from a primary transmit coil (Tx) to a secondary receive coil (Rx), including the LTC4120

The magnetic field intensity is proportional to the magnitude of the current flowing in the conductor. Energy is transferred from a conductor that produces the fields (the primary) to any conductor on which the fields impinge (the secondary) via the magnetic coupling defined above. In a loosely coupled system where the coupling coefficient is low, a high frequency current does not pass for long distances along a conductor but rapidly loses energy because of the impedance mismatch along the cable, which causes the energy to be reflected back to the source or radiated into the air. See the illustration in Figure 1 for a graphical representation of loosely coupled windings connected via a magnetic field. It should be noted that this figure also shows the LTC4120, which will be discussed in more detail later in this article.

Charging with wireless power

When designing a wireless power charging system, a key parameter is the amount of charging power that actually adds energy to the battery. This received power depends on many factors, including the amount of power being transmitted, the distance and alignment between the transmit coil and the receive coil also known as the coupling between the coils, and finally the tolerance of the transmit and receive components.

The primary goal in any wireless power design is to guarantee delivery of the required power under worst-case power transfer conditions. However, it is equally important to avoid thermal and electrical over-stress in the receiver during best-case conditions. This is especially important when output power requirements are low. Take for example when the battery is fully charged or nearly fully charged and the coils are in close proximity to each other. In such a scenario, available power from the wireless system is high, but demanded power is low. This excess power typically leads to high rectified voltages or a need to dissipate the excess power as heat.

There are several ways to deal with excess power capacity when the demanded receiver power is low. The rectified voltage can be clamped with a power Zener diode or transient voltage suppressor. However, this solution is typically large and generates considerable heat. The transmitter power can be reduced, but this will either limit the available received power or it will reduce the transmit distance. It is also possible to communicate received power back to the transmitter and adjust transmit power accordingly. This is the technique used by wireless power standards such as the Wireless Power Consortium Qi standard. However, it is also possible to solve this issue in a compact and efficient manner without resorting to complicated digital communication techniques. Techniques that communicate via small variations in the transmitted power level require a minimum amount of power transmission and may not work for systems with variable transmit distances.

To meet these goals, Linear Technology’s LTC4120 wireless power receiver and battery charger integrates technology by PowerbyProxi, Linear’s technology partner. PowerbyProxi’s patented Dynamic Harmonisation Control (DHC), technique enables high efficiency contactless charging without thermal or electrical over-stress concerns in the receiver. Using this technology, up to 2W can be transmitted at a distance of up to 1.2cm. However, for single-cell Li-Ion batteries, the maximum charge voltage of 4.2V and maximum charge current of 400mA will limit this value to 1.7W. Similarly, the 2W maximum will limit 2 Series Li-Ion batteries (8.4V maximum charge voltage) to 240mA of charge current.

The metrics of power, efficiency, range and size determine system performance and so the LTC4120-based wireless power system was designed to receive up to 2W at the battery up to a distance of 1.2cm when used with one of several transmitter options. Efficiency calculations vary tremendously based on the technique and components used. Typically, the battery will receive 45% – 55% of the DC input power fed to the transmitter in an LTC4120-based system.

PowerbyProxi’s DHC tuning technology embedded in the LTC4120 provides significant advantages over other wireless power solutions. In response to environmental and load changes, DHC dynamically varies the resonant frequency of the resonant tank circuit on the receiver. DHC achieves greater power transfer efficiency, enabling smaller receiver sizes, even as it allows greater transmission range. Unlike other wireless power transfer technologies, DHC enables intrinsic power level management as part of the inductive power field, eliminating the need for a separate communication channel to validate receivers or to manage variation in load demand during the battery charge cycle.

Figure 2: LTC4120 application schematic illustrates a complete wireless battery charging circuit

It is clear that DHC solves a problem fundamental to all wireless power systems. Every system must be designed to receive a certain amount of power at a given maximum transmit distance. Every system must also be designed to survive a no-load condition at minimum transmit distance. The competitive alternatives solve this problem with a complicated digital communication system that adds complexity and cost, limiting power transmission distance. The LTC4120-based wireless power system easily solves this problem by incorporating PowerbyProxi’s DHC technology.

PowerbyProxi has been working to deliver wireless power solutions for industrial customers since 2007. Rather than invest a great deal of money into its own marketing efforts, they decided to develop and improve their technology and to partner with industry leaders such as Linear Technology to bring the technology to market. With significant success and a broad portfolio of technology embodiments to its credit, they are now gaining recognition as the leading global wireless power company.

One of the key reasons why Linear partnered with PowerbyProxi was due to their significant IP portfolio and solution design know-how that offers customers the leading technology in the industry and also assurance that the technology is fully backed by IP. PowerbyProxi has more than 30 patents in process and many more in the review and filing stages, making them the leading innovator and IP leader in the wireless power field.

Outside of the mass consumer market there is a class of portable industrial and medical products where the ability to wirelessly charge their internal batteries across air gaps or through non-ferrite materials up to a centimetre or so, is a ‘must have’ requirement for their deployment. Up until now, a design engineer’s options have had limitations, which have hindered their end products’ success and viability. The recent introduction of the LTC4120 from Linear Technology could change that; this highly integrated IC, which can wirelessly receive power transmitted from a coil of up to 1.2cm apart, as well as charge a battery, provides a simple and effective solution. It is the embodiment of wireless charging made easy — no connectors necessary!