A basic concept of wireless power is shown in Figure 1. Simple low-power wireless-charging systems such as electric toothbrushes often operate in an “open-loop” configuration and may not be well regulated at the receiver output. The charging source simply generates a continuous ac excitation on the primary coil. When the portable device is placed on it, power is applied to the load to charge the internal battery.
An open-loop unregulated system may be acceptable for low output-power levels of, say, less than 1 W. However, at higher power levels, it becomes both wasteful and inefficient, generating lots of heat. Unlike the open-loop approach used in simple wireless-power applications, an intelligent wireless-power system has microcontrollers on both receive (RX) and transmit (TX) side circuits (Fig. 2).
The transmitter doesn’t generate output power unless it knows that a valid receiver is placed on it. Therefore, if a transmit pad is plugged in with nothing sitting on it, very little power is wasted.
When placing a valid (recognized) device on the charging pad (or table), the transmitter only delivers the level of output required, as requested by the receive controller based on the needed load-current level. Feedback is provided from the receiver back to the transmitter back through the same inductive coupling as the forward power transfer (“in-band” communication). Alternatively, it may be in the form of a dedicated separate communication link such as Bluetooth (“out-of-band” communication).
For a more detailed introduction to the basic concepts of wireless power, see reference 1. References 2-7 provide additional system design guidelines for wireless-power implementations.
- Sengupta & Johns, “Universally compatible wireless power using the Qi protocol,” Low-Power Design, October 1, 2011
- Tahar Allag, “Test and troubleshoot a wireless power receiver,” Application Report (SLUA724), August 2014 Instruments
- Johns, Antonacci, and Siddabatula, “Designing a Qi-compliant receiver coil for wireless power systems,” Analog Applications Journal (SLYT479), 3Q 2012
- Tahar Allag, “Layout Guidelines for wireless power receiver,” Application Report (SLUA710), June 2014
- Ilya Kovarik, “Building a wireless power transmitter,” Application Report (SLUA635A), August 2012
- Jing Ye, “NVDC charging design considerations,” Video Tutorial.
- Norelis Medina, 10W Wireless power system lab demo, Video