ELECTRONIC DEVICE CHARGING Charging solutions for electronic devices
The IoT, and now 5G, has accelerated the growth of mobile electronic devices including smartphones and smartwatches. However, these devices’ performance is always limited by that of their batteries and charging circuits. This article looks at how modern charging circuits address the diverse power challenges of high-performance 5G smartphones and of wearables.
The advent of the IoT and 5G has meant a huge growth in the number of electronic devices available, with entire new categories appearing. Two categories already familiar to every consumer – smartphones and wearable devices, particularly smartwatches – are particular beneficiaries of these trends. 5G’s significantly enhanced performance over any earlier standards enables far more powerful smartphones, while wearable devices thrive on IoT connectivity.
However, while ever-evolving computing and communications technology ushers in these more powerful devices, their ability to deliver on their potential is always limited by how well they can supply power to their internal hardware – reliably, safely, and for sufficiently extended periods.
Smartphones and wearables, like any other mobile or remotely located electronics, cannot be tethered to an external power supply, so they depend mostly - although not always – on internal batteries. While recognizing that power performance depends very much on the batteries’ chemistry, this article focuses on its other key factor - the charging technology and associated strategies used.
There is no single ‘ideal’ charging technology available: the best solution depends on the type of device to be powered. To highlight this dependency, we are going to look at the diversity in power requirements and charging solutions between 5G smartphones and wearable devices.
5G smartphone charging
As consumers seek to use the greater computational power and functionality enabled by 5G, their usage patterns also change, with more connections and longer periods on standby: the power solution provided must be longer-lasting, lower-cost and safer. However, as smartphone manufacturers have moved from nickel-cadmium to lithium-ion batteries to achieve the necessary power density, safety considerations have become as important as charging speed. Accordingly, much effort has gone into developing innovative charging techniques that help to satisfy both these objectives by companies such as OPPO.
For example, voltage, current and charging speed can be boosted without degrading the battery cell by more precisely controlling voltage and current. Conventional lithium-ion batteries are charged in three phases: trickle charging, constant current, and constant voltage. The battery’s internal resistance and reactance can cause long charging times. To overcome this, the charging system applies more precise control over the voltage and current, so that the system can accurately compensate for the drop in current due to the battery’s internal resistance and reactance.
Pulse charging is another useful technique: one that enables much smaller, simpler charging systems that are longer-lived and more robust. It minimizes the impact of the charger on the electrical supply system, and smooths out the battery’s capacitive reactance. OPPO, for example, uses both a high-power system and a low-power system. The low-power system uses trapezoidal or square wave pulses; the high-power system has sine wave pulses.
As mentioned, safety concerns for lithium-ion batteries are significant – and the greatest concern relates to excessively high voltage during charging. Conventional surge protection devices use MOSFETs to cut current when it reaches dangerous levels. However, these devices are not sufficiently reliable. One alternative is a flash charge technology that incorporates fuses alongside a triggering circuit in an innovative design that is compact and has low internal resistance. This offers much more reliable circuit protection. Today, fuses are included in the overcurrent protection devices on many smartphones as an important part of safe charging technology.
Battery charging can be made smart to improve safety and reliability. If, instead of being ‘dumb’, the charger communicates with the battery’s battery management system (BMS), it can obtain feedback about the charging process and see how well its delivered power is being taken up: is a sufficient proportion being absorbed by the battery electrolyte rather than being converted to damaging heat?
If impedance rises, but remains within acceptable parameters, the power output of the charger will rise to match it. If the impedance of a component rises higher than the acceptable limits, each component of the charging circuit will switch to protection mode. This ensures that no faults or safety hazards develop, such as overheating at the charging socket.
While the challenge facing 5G smartphone designers relates to managing ever higher power levels efficiently and safely, those posed by wearable devices are somewhat different, and are mainly driven by severe space constraints. As wearable technology is intended to be worn on the body for extended periods, the form factor must be unobtrusive: small, lightweight, and thin rather than bulky.
This means that very little space is available for a battery and its associated electronics – not a problem for simpler wearables, like fitness trackers which can use a small battery for up to a week without needing recharging. However, as newer, higher-capability devices such as smartwatches, with more power-hungry features like full-color touchscreens, become available, battery life becomes much shorter, with some devices possibly not even lasting a day without needing a recharge.
This need for constant recharging calls for an easier and more convenient method than plugging in. This can be met by wireless charging , in which a smartwatch can recharge its battery simply by setting it on a docking device, or for an Apple watch, snapping a small, round magnetized connector to the back of the watch. Although not as fast as wired charging, the time required – typically about an hour for a small device like a smartwatch – is tolerable. Additionally, by avoiding the need for a wired charging port, the device can keep its water resistance certification and remain invulnerable to dust.
Qi charging standard
Wireless charging involves passing an alternating current through a transmitter coil (in the charging dock) to create a magnetic field that induces a voltage in a nearby receiver coil (in the mobile device) which is then used to charge the device battery. An insight into the technology and its parameters can be gained by reference to the Qi charging standard.
This is an open interface standard that defines wireless power transfer using inductive charging over distances of up to 4 cm (1.6 inches), developed by the Wireless Power Consortium. The system uses a charging pad and a compatible device, which is placed on top of the pad, charging via resonant inductive coupling.
Mobile device manufacturers that are working with the standard include Apple, Asus, Google, HTC, Huawai, LG Electronics, Motorola Mobility, Nokia, NuCurrent, Samsung, BlackBerry, Xiaomi, and Sony. First released in 2008, the Qi standard had by 2020 been incorporated into more than 200 smartphones, tablets, and other devices.