Introduction: Wireless Charger
Since the industrial revolution in the 19th century, people’s daily life has been totally changed with nearly all aspects characterized with innovative ways of existence. As for the wireless power transfer, it has gone through Maxwell’s equations, Poynting’s theorem and two schemes of wireless signaling put forward by William Henry Ward and Mahlon Loomis.
What struck us the most was Nikola Tesla’s discovery of Tesla coils. He accidentally came up with the idea of an “oscillating transformer” in an experiment of powering a Ruhmkorff coil with a high-speed alternator. This kind of powerful Tesla coil could be used to produce electricity of high-voltage, low-current, and high-frequency alternating-current.
The earlies wireless power technology dated back to inductive power transfer between nearby wire coils, while the mid-range wireless powering and charging technology became popular among portable wireless communication devices such as mobile phones, tablets just in recent decades. Without our realization, technological innovation has brought us to a wireless world.
The advent of the first handheld cellular mobile phone in 1973 especially expedited the life pace on the earth. Digital cellular networks led to the introduction of digital signal processing in wireless communications, accelerating the circulation speed of information in the whole world, in this way, higher working efficiency came along with the less required time.
In the current market, the majority of phone chargers are equipped with cables, which means that quick charging can be realized only when the charger, the cable, and your phone meet specific conditions. However, forgetting cables is not rare especially when you’re outside, which may make your phone in face of low-battery.
With all the above taken into consideration, it’s guaranteed if wireless charger gains ground all over the world. In other words, a mobile phone and a wireless charger will be sold in pairs in the foreseeable future, getting you rid of cable troubles.
Although it has impressed us when the network makes our daily life closer, the smartphone makes our communications quicker, and rapidly developed vehicles make our journey more diversiform, wireless technology will probably innovate human society once again in that simply a magnetic field created by the charger is enough for your phone’s charging.
Chapter 1 Power Supplies
What is a power supply? Briefly speaking, it’s a device that can supply electric power, either with a cord or not. After entering the digital age, humans have been closely connected via websites than ever before, with “Plugging in” being a ubiquitous phrase in daily life. Possibly with the popularity of another phrase “Putting on”, wireless charging devices particularly designed for mobile phones will belong to an unreversible technological trend.
- DC Power Supply
Literally, the DC power supply refers to a device that generates a DC output voltage, despite an AC or DC input. There’re roughly four types of DC power supplies, including linear power supply, switched-mode power supply (SMPS), capacitive power supply, and linear regulator.
A power transformer is needed for the AC input voltage in a linear power supply to pass through before its rectification and filtering to obtain a DC voltage, while the AC mains input in a switched-mode power supply is directly rectified and filtered. In contrast, the reactance of a capacitor in a capacitive power supply is applied to turn the mains voltage into a smaller AC voltage. What differs from the former three types in a linear voltage regulator lies in the conversion of a varying DC voltage to a constant, often specific, and lower DC voltage.
- AC Power Supply
A transformer is required for the AC power supply to convert the voltage from a wall outlet to the desired voltage. In this regard, there’re several kinds of transformers, including isolation transformers and autotransformers. When the source voltage equals the output voltage, it’s called an isolation transformer; when no mains isolation is to be supplied, then it’s an autotransformer. Besides, there’re also other kinds of AC power supplies that provide a nearly constant voltage, while the output voltage may vary. Particularly, it’s worth mentioning that an AC adapter, as a power supply, can be a part of an AC mains power plug, with a variety of names, such as “plug pack”, “plug-in adapter”, or “wall wart”.
- Programmable Power Supply (PPS)
A programmable power supply refers to the power supply whose integral microcomputer aims to control and monitor the power supply operation of voltage, current, and also frequency.
Chapter 2 Important Definitions
In this chapter, we’re going to know something about several definitions which play an important role in wireless chargers. More understanding will give you a hand when you’re picking up a wireless charger for your iPhone’s companion. Now let’s explain them one by one as followed.
- Wireless & Wireless Power Transfer (WPT)
Are you still bothered by cables when charging your phone? Do you often forget to take the physical tools with you when traveling or just walking outside? With the advent of wireless power transfer, cables will never ever be your gnat.
Literally, wireless means no cables; wireless power transfer means the cableless transmission of electrical energy. Specifically speaking, a power source drives electric power to a transmitter device which generates a time-varying electromagnetic field, to transmit power across space to a receiver device. Power will be extracted from the field and then supplied to an electric load.
- Qi Standard
Qi comes from the Chinese word qi and is pronounced “chee”. Comprised of the Base Station and Mobile Devices, the Qi system base itself on Qi, an open interface standard, which denotes the specifications of the wireless power transfer by inductive charging over a distance of up to 4 cm. Concerning the adoption of the system, there’re two types of positioning of the Mobile Device on the Base Station, the so-called guided positioning and free positioning, depending on whether there’s a direct alignment with the transmitter coil.
MagSafe, a magnetic wireless power transfer technology, was first introduced by Apple Inc. The MagSafe charger is designed to align automatically with compatible devices which have magnets surrounding the Qi wireless charging coil. At present, two MagSafe compatible chargers are available: a single charging pad and a double charging mat.
- Power Up Procedure
Generally, there’re four charging phases regarding Lithium-ion batteries: Trickle Charging (Pre-Charging with a relatively low voltage), Constant Current Charging, Constant Voltage Charging, and Charging Completion.
At the very beginning, the voltage value doesn’t reach the threshold, the battery components will be pre-charged first. Once it meets the threshold, the constant current charging will be intrigued, while the voltage value is still rising. At the maximum voltage value, the constant voltage charging begins. Last but not least, the charging completion is also important for the battery’s full charging. Then it’s not advisable to pull out the charger immediately, once the phone battery reaches 100%.
Chapter 3 Main Standards
- Wireless Power Consortium (WPC)
The Wireless Power Consortium, founded on December 17th, 2008, and based in Piscataway, New Jersey, centers on the creation and promotion of universal market acceptance of its interface standards Qi, Ki Cordless Kitchen, and Qi Medium Power.
- Quick Charge (QC)
Quick charge, a technological protocol developed by Qualcomm, is mainly implemented on devices such as mobile phones and some chargers. It’s designed for the charging of battery-powered devices at power levels exceeding 5 volts at 2 amps. As for the versions, QC 3.0 power supplies are backward-compatible with QC 2.0. Since QC 3.0 supports higher currents at the same voltages, it will deliver more power to QC 2.0 loads. Here are some QC-based protocols: VOOC (OPPO and pre-2020 Realme models), SuperCharge (Huawei), Adaptive Fast Charging (Samsung), Mi Fast Charge (Xiaomi), to name a few.
- USB Power Delivery (PD)
USB Power Delivery, designed to coexist with the previous USB Battery Charging specifications, is intended to deliver increased power (over 7.5 W) to USB-powered devices including phones, laptops, tablets, etc. Greater power demands lie in higher currents and supply voltages from compliant hosts, e.g., up to 2 A at 5 V (power consumption of up to 10 W), optionally up to 3 A or 5 A at either 12 V (36 W or 60 W) or 20 V (60 W or 100 W). As a natural extension of the existing mobile phone charging standards, USB PD is hereby designed to meet expanded power demand. USB PD has gone through its revisions 1.0, 2.0, and 3.0. Despite that Qualcomm’s QC 2.0, VOOC, and others deliver power e.g., exceeding the 7.5 W cap, some of these, such as QC 4.0, were eventually compatible with USB PD again.
Chapter 4 Wireless Technologies
- Electromagnetic Induction
Electromagnetic induction, also called inductive coupling, belongs to near-field techniques. It refers to power transmission between coils of wire by a magnetic field. In this case, a transformer is comprised of the transmitter and receiver coils.
So far, inductive coupling is the most widely used technology for wireless charging. Its applications involve cordless appliances, and biomedical prosthetic devices, and so on. However, the increasingly applicable device is wireless charging pads to recharge mobile phones and other handheld devices such as laptops, tablets, etc.
- Resonant Coupling
At the crossroad of the 20th century, Nikola Tesla discovered resonant coupling in his experiments of wireless power transfer, while its broader applications have just recently been greatly explored.
Specifically, resonant coupling system applies “mid-range” transfer, which, in contrast to the “short-range” transfer of non-resonant inductors, can transmit more power over longer distances, making the most of the weaker magnetic field in the peripheral regions of the near fields. Moreover, the resonant circuits are more powerful due to their mutual interactions, in this way, the power loss caused by absorption is negligible.
However, a drawback of this system lies in its frequency which will reach another peak, if the two resonant circuits are tightly coupled. Despite this, resonant coupling technology now becomes popular in the arena of wireless power systems.
Power can also be transferred in the microwave range via radio waves, and electromagnetic radiation with wavelengths in the electromagnetic spectrum. In the process of power transmission, a rectenna can convert the microwave energy back into electricity. In this regard, microwave, a far-field radiative technique, is now widely applied to wireless high-power transmission.
Chapter 5 Applied Sectors
- Mobile Phones
First of all, mobile phones / cellular phones embrace wireless technology joyfully. Radio waves are underway to transfer signals from signal-transmission towels to various locations worldwide. In this way, the whole world will be connected as a wireless space where information and signal are transmitted at a speed of higher efficiency. Furthermore, people can also get rid of the annoying cables, once wireless charging of mobile phones becomes commonplace just like the daily used salt.
- Energy Transfer
By way of either far-field methods such as radio waves or near-field methods including electromagnetic induction, energy can be transferred wirelessly from the power source to an electrical load. In recent years, the sector of wireless chargers is booming in the industry of mobile phones. Its promising future is seemingly doomed. Thanks to it, people’s daily life will be more convenient, rather expedited, and abound in multiplied and diversified possibilities.
- Data Communications
Wireless technology makes data communications quicker in that the transfer speed has been tremendously updated. For example, Wi-Fi enables portable computing devices easier connection with other devices, peripheries, and also the Internet. No matter at home, in the office, or in public places, Wi-Fi has become the de facto method accessible to the Internet. In addition to mobile phones, wireless technology is also applied in the sectors such as transportation, maritime, aviation, etc.
Chapter 6 Tipps for You
- How does wireless charging work?
The wireless charger is generally parallel with Qi-standard as stated in Chapter 2, which means both the charging device and the gadget you want to charge are equipped with electromagnetic coils. Thus, power will transfer from the device to your gadget via induction. Please pay attention that some kinds of phone cases are incompatible with wireless charging, lest the case is thin enough and its material won’t interfere with the signal. It also works if your phone is protected by a magnetic phone case.
- Make sure the charging watts
Before checking in, you’d better make sure how the specifications of your phone define, especially the charging watts. The charging watts may vary depending on different phones, so it’s of importance for you to make clear the value. According to WPC, the current Qi specification supports up to 15 W.
- Make sure the output watts
Upon buying a wireless charger, you must check the maximum power output which typically ranges from 5 W to 10 W. If your phone supports higher charging watts than the charging device, the charging speed may be lower. In this case, you could choose a charger that supports higher output watts.
- Choose an appropriate design
They’re totally two types of wireless chargers, one is a charging pad, the other is designed as a stand. The former usually has a circular or flat form, while the latter can satisfy your needs of the phone’s juice-up and using it simultaneously. Which one to choose merely depends on your own preference.