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Electromagnetic Induction: The Basic Principle Behind Wireless Power

June 11, 2015
Simple low-power wireless-charging systems, which often operate in an “open-loop” configuration, is acceptable for low output-power levels but becomes inefficient at higher levels, often generating lots of heat.
1. When ac excitation is applied to the transmitter (lower) coil, it generates a magnetic field. When the receiver (upper) coil is placed in proximity and aligned with the transmit coil, that magnetic field causes a current to flow in the receiver coil. In a real-world application, shielding material is used below the transmit coil, and above the receive coil to contain the field and avoid interference with other nearby circuits.

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).

2. A closely coupled, intelligent wireless-power system requires the coils to be well-aligned laterally and typically within a few millimeters of each other. Power transfers from the transmitter (TX) side to the receiver (RX) side, while the receiver circuit sends feedback in the form of digital pulses back across the magnetic coupling path.

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.


  1. Sengupta & Johns, “Universally compatible wireless power using the Qi protocol,” Low-Power Design, October 1, 2011
  2. Tahar Allag, “Test and troubleshoot a wireless power receiver,” Application Report (SLUA724), August 2014 Instruments
  3. Johns, Antonacci, and Siddabatula, “Designing a Qi-compliant receiver coil for wireless power systems,” Analog Applications Journal (SLYT479), 3Q 2012
  4. Tahar Allag, “Layout Guidelines for wireless power receiver,” Application Report (SLUA710), June 2014
  5. Ilya Kovarik, “Building a wireless power transmitter,” Application Report (SLUA635A), August 2012
  6. Jing Ye, “NVDC charging design considerations,” Video Tutorial.
  7. Norelis Medina, 10W Wireless power system lab demo, Video

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