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Implement compact wireless battery charging

Posted: 04 Feb 2014 ?? ?Print Version ?Bookmark and Share

Keywords:connector? Wireless charging? LTC4120? PowerbyProxi? DC-AC transmitter?

When choosing a transmitter, several factors should be considered. Is transmitter standby power (when a receiver is not present) important? Does the transmitter need to differentiate between a valid receiver and unrelated foreign metal objects? How sensitive is surrounding circuitry to EMI?

The basic transmitter is a very simple and inexpensive solution. Due to passive resonant filtering, the spectrum of EMI is well controlled at the fundamental transmitter frequency (about 130kHz). However, it transmits at full power whether an LTC4120-based receiver is present or not. Therefore, its standby power is relatively high. It also does not differentiate between an LTC4120 and foreign metal objects and can cause unrelated metal objects to warm up through induced eddy currents.

Two off-the-shelf production transmitters can be purchased from PowerbyProxi: Proxi-Point and Proxi-2D. Transmit distance and alignment tolerance performance of these transmitters is virtually identical to that of the basic transmitter. However, these transmitters can detect whether a valid LTC4120-based receiver is present or not. This feature allows them to reduce standby power if no receiver is present and terminate power transmission if unrelated foreign metal objects are nearby.

Due to the buck switching topology of the LTC4120 charger as well as the DHC technology, overall system efficiency is about 50 C 55%. To calculate this value, divide the battery charge power by the DC input power to the transmitter. Overall efficiency is very dependent on coupling and load. When charging a single-cell Li-Ion battery at 400 mA, the components on the LTC4120-based receiver board stay within 10§C of ambient temperature.

Other system considerations
The LTC4120-based wireless charging system can charge a battery at 400 mA across an impressive gap. Lithium-based rechargeable batteries power many handheld applications and both 1S (nominally 3.7V) and 2S (nominally 7.4V) Li-Ion packs are common. Extended cycle life and improved safety features are creating significant market space of LiFePO4 batteries as well. Furthermore, there is a wide variety of target charge voltages within these battery packs as engineers fine-tune the trade-offs between initial battery capacity, cycle life and retained capacity over time. To accommodate this variety, the LTC4120 does not need any additional circuitry to charge 1- and 2-cell Li-Ion batteries as well as 1-, 2- and 3-cell LiFePO4 batteries. Charge current can be programmed from 50 mA to 400 mA while charge voltage can be programmed from 3.5V to 11V.

In addition to a built-in constant-current / constant-voltage charge algorithm, the LTC4120 includes multiple battery safety features. A termination timer safely ends a charge cycle, while an NTC input provides battery temperature monitoring and automatically suspends charging during unsafe temperature conditions. Two charge status pins provide charge cycle and fault status information.

Conclusion
Wireless charging can add value, reliability and robustness in many different types of applications. It is important to consider how much power is required in the application as well as how far that power must be transmitted and with what kind of alignment tolerance. Determining how to deal with the worst-case condition of maximum load power with minimum coupling between the transmitter and receiver is often the easy part. Managing additional available power during low load or no load conditions with maximum coupling can be challenging.

About the author
Trevor Barcelo is Design Manager for Battery Management Products at Linear Technology.

To download the PDF version of this article, click here.


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