Understanding the Interchangeability of Transmitter and Receiver Coils for Optimal Design

If you’ve shopped for off-the-shelf wireless power coils, you’ll noticed they are usually segregated into transmitter (Tx) coils and receive (Rx) coils.? Have you ever wondered what the difference is?? Rather than the obvious “throw and catch” example, the answer may surprise you.

To understand the difference, let’s start by looking at the Qi (pronounced “chee”) standard.?Qi is intended to ensure compatibility of wireless chargers for handheld electronics, such as smartphones.? The standard defines the construction of various wireless charging coils in detail.? While this is great for compatibility and interchangeability, it doesn’t always lead to the most efficient designs.? Consider the economics involved: receiver coils are produced by the hundreds of millions and need to be packaged into consumer devices with extreme cost and size pressures.

Transmitter coils on the other hand see more limited production as they can support multiple devices per household, and are often viewed as a supplement to plug-in chargers.? As such, the wireless charging pads that use the transmitter coils do not have the size and cost pressure of the receiver coils.? Additionally, as a plug-in household device, efficiency is a concern.? Because of this, Qi transmitter coils are typically larger and use more expensive materials than Qi receiver coils.

Since many off-the-shelf coils are based on Qi designs, most manufacturers differentiate transmitter and receiver coils in their catalog.? But what about using these coils for industrial and medical applications, where many of the consumer device paradigms don’t apply?

Most of the time you can just ignore the “transmit” and “receive” labels.? The coils are fully interchangeable (i.e. a coil listed as “transmitter” can be used a receiver coil, and vice-versa.) ?Rather than making the coil selection process more confusion, this actually gives you more design freedom, as well as a powerful tool to optimize the cost and efficiency of your design.

Here are the main parameters for coil selection:

• Current rating
• Inductance
• Size (footprint and thickness)
• Lead length and termination

You may notice that some commonly specified items such as resistance and quality factor aren’t listed here. While optimizing those items can improve efficiency, they’re not inherently important to the proper functioning of wireless coils.

Coupling, defined by the following formula, is critical to efficiency in a wireless power system:

From this we can see that coupling can be improved by decreasing leakage inductance.? Leakage inductance is caused by flux that’s generated in the transmitter coil that doesn’t couple to the receiver coil.? There are several ways to minimize leakage inductance and improve coupling:·???????

• Reduce the distance between Tx and Rx coils
• Improve x-y alignment between coils
• Ensure coils are parallel to each other
• Use matched coils (same size, turn count, etc.)

Let’s examine the last item, using matched coils.? Choose the best coil for your application while ignoring the “Tx” and “Rx” catalog labels, then buy two of them to use as a matched pair.? Using the same coil part number for both the transmit and receive coil is especially important for lower volume applications, as it could help ease minimum order quantity issues, and possibly push your chosen coil into a lower price tier.

When would it make sense to use different coils for the Tx and the Rx?? While designers usually have quite a bit of freedom with the form factor of the transmitter, the receiver coil is often constrained by the packaging of the device, especially in the case of small rechargeables such as wearable medical devices.? Many of these devices require small or uniquely shaped receiver coils, so using different coils for the Tx and Rx may be necessary.

Another area that confuses designers is coil power rating.? Since the Qi standard prioritizes interchangeability, many of the Qi coil designs are actually capable of transmitting quite a bit more power than their Qi rating.? Wireless power coil suppliers rate their coils based on the current carrying capacity, which is a function of the winding wire size, the applied frequency and the available cooling medium.

For example, look at coil part number 760308110, which is a Qi A10 type coil from Wurth Electronics.? This coil is used in 15W Qi transmitters, but is rated at 6A at 125kHz by the manufacturer.? So in a 12V application with good cooling, it would be possible to transmit up to 72W.

In summary, the distinction between transmitter (Tx) and receiver (Rx) coils lies not in their construction but in their intended application. By understanding the flexibility of these coils, designers can optimize for cost, efficiency, and performance across various applications, whether consumer, industrial, or medical. Using identical coils for both Tx and Rx can simplify the design process and reduce costs, especially in lower-volume projects. However, specific use cases, such as devices with constrained form factors, may require different coils for each role. Additionally, understanding a coil's current rating and coupling efficiency enables designers to exceed the standardized Qi ratings and achieve tailored power solutions. With this knowledge, designers can confidently navigate coil selection, leveraging the full potential of wireless power technology.

Click here to browse all Wireless Power Transmission components available from Würth Elektronik.