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How to explain the basic principle of wireless charging? Aug 20, 2018

Wireless charging is on fire after the release of the new iPhone! Once, on the subway, I heard a little boy next door said to the little girl: "I bought a wireless charger yesterday and put the phone on it to charge it, so amazing!" As a wireless charging practitioner, I am still a little proud to hear this. So what is the basic principle of wireless chargers on the market? I hope this article can solve some of the confusion that is already using wireless charging products or preparing to use wireless charging.

There are two tips before reading:

This is not a popular science article and requires some basic knowledge of electronics.

If you are a fan in this area, you don't have to pursue the meaning of each noun to understand the general logic.

The length of the article is not long. It only uses the diagram to speak and answers four basic questions:

What are the components of wireless charging?

What is the power transmission path for wireless charging?

What is the control loop for wireless charging?

What is the communication mechanism for wireless charging?

What are the components of wireless charging?

the components of wireless charging.png

The left side is TX, which is the transmitting end, and the corresponding product is the wireless charging transmitter.

The right side is Rx, which is the receiving end, and the corresponding product is a mobile phone with wireless charging function.

TX terminal: MCU, power full bridge, and LC resonance tank composed of inductor and capacitor, in which the inductor is the transmitting coil.

RX: MCU, rectifier bridge, LDO, Charger chip, battery, and LC resonant tank, where the inductor is the receiving coil.

What is the power transmission path for wireless charging?

the power transmission path for wireless charging.png

The wireless charging transmitter inputs a DC voltage.

The DC voltage is passed through the power full bridge, and an AC voltage is generated at the SW1 and SW2 points, which is actually a square wave.

The square wave loading produces an alternating current (AC Current) across the LC Tank.

The alternating current generates a magnetic field through the coil.

The receiving coil senses this magnetic field and generates an alternating current (AC Current) at the receiving end LC Tank.

The alternating current is converted to a DC voltage through a rectifier bridge.

The DC voltage then charges the battery through an LDO and a Charger.

What is the control loop for wireless charging?

the control loop for wireless charging.png

In order to ensure the stability of the entire wireless charging system, VRECT needs to set a target voltage (Target Voltage) and control it through a feedback loop; in order to achieve high conversion efficiency, its value is usually set to be only a little higher than LDO.

The feedback loop first collects the VRECT voltage and subtracts it from the target voltage to generate an error signal (Control Error).

The error signal is transmitted to the MCU of the transmitting end through the communication method specified by the WPC, and the MCU determines whether the receiving end wants to increase the energy or reduce the energy of the transmitting end.

The MCU controls the emitted energy by controlling three signals:

Input voltage

Power full bridge switching frequency

Power full bridge input duty cycle

Note: Why can you control the emission energy by controlling these three signals? The Voda Academy will explain it later.

What is the communication mechanism for wireless charging?

the communication mechanism for wireless charging.png

The essence of conveying information is to make the signal change. Just like I waved with you, you see that my hand is changing, I know I am calling you.

When the receiving end conveys the information, it will access or pick up a set of capacitors on both sides of the LC Tank at the receiving end.

The access or removal of the capacitor causes a change in the equivalent impedance of the LC Tank at the transmitter.

A change in the equivalent impedance causes a change in the current in the LC tank at the emitter and a change in the voltage at the junction of the capacitor and the inductor.

This changed signal is collected and demodulated and passed to the MCU.

The whole change rule is written in the WPC protocol. The MCU uses the WPC protocol to know what information the receiver has told the transmitter.

DIX10.JPG

If you have any questions, you can leave a message and discuss. Please correct me if there is something wrong. We hope to promote the knowledge of wireless charging through communication, so that more people can truly understand wireless charging and learn wireless charging to promote the development of the entire wireless charging industry.