By Colin Weaving
EMEA Technology Director, Future Electronics
The huge growth in the number of portable devices owned by the average consumer has led to a concomitant rise in the number of battery chargers and mains power adapters. In the absence of a widely accepted and robust standard, any user owning multiple devices has had to keep an array of charging devices and cables to hand.
This is frustrating and irritating for the user. It has also become an issue of public policy, since consumers’ habit of regularly replacing older portable devices with the latest models causes tens of millions of obsolete power supplies to be sent for disposal in landfill sites across Europe each year.
As a result, government agencies have begun pushing for the standardisation of power supplies, with the aim that a single charging unit should be able to charge all devices and never be thrown away. As is usual in the world of mobile technology, however, engineers have moved the goalposts before the regulators have even got started.
First, the technology industry recently introduced yet another new wired power and communications interface: USB Type-CTM, which has many advantages over older USB charging devices.
And second, various wireless charging technologies have begun competing for consumers’ attention, although none has yet emerged as a winner. This too hampers official attempts to impose standardisation by regulation.
So how does this affect small and medium-sized manufacturers of portable and battery-powered devices? What is the best advice for designers who wish to future-proof their next battery power-system design?
A brief history of USB
To understand the full significance of the introduction of USB Type-C, it is important to put it in the context of previous attempts at standardisation through USB. In fact, standardisation was signalled in the technology’s name: the Universal Serial Bus was introduced by Microsoft in 1995 as a replacement for the ageing RS-232 standard, which was itself then thought of as a universal serial communications standard.
The problem with USB is that it quickly branched out, accruing variants both of the communications protocol and of the connector, as shown in Figure 1.
The diversity of connector types even within the USB standard is a cause of considerable frustration to users, since it requires them to use various cables and adapters in order to achieve interoperability between a charger and multiple devices. This is without taking into account the many proprietary modifications of USB technology.
The reason for the emergence of so many connector types was to satisfy the contrasting demands of different devices: in the desktop PC, in which Microsoft first intended USB to be used, power delivery and a robust connection are of prime importance, and space is not at all limited. The industry developed various mini and micro connectors to enable USB technology to be accommodated in smaller devices such as mobile phones, tablets and media players.
USB Type-C offers the potential to replace these many connector types with just one. That is because the USB Type-C technology offers for the first time a combination of small size and very high power capability. The standard provides for up to 100W to be sourced from the host equipment via a 24-pin double-sided connector measuring just 8.4mm x 2.6mm, which is small enough for use in the latest smartphones and wearable devices.
USB Type-C can thus provide a single power supply for a complete PC system: for instance, a monitor could supply a laptop, mouse and keyboard via a self-powered hub. In fact, almost any portable device may be powered by this one, low-profile connector. At a stroke, then, USB Type-C appears to solve the problems both of end-of-life waste reduction and standardisation of charging equipment for all portable devices.
Support for legacy USB equipment
Unfortunately, in reality it is not quite this simple. Within the USB Type-C specification there is provision for multiple power-delivery options. In addition, a USB Type-C cable is capable of supporting the various data rates and power levels laid down in previous specifications from USB 2.0 onwards.
To handle the legacy USB protocols, the host equipment (the Downstream Facing Port, or DFP) registers on connection the specification of the Upstream Facing Port (UFP) by means of various resistor pull-ups. These notify it of the power and voltage rating of the UFP. The power delivery specification in USB Type-C defines two pins called Configuration Channels 1 and 2 (or CC1 and CC2) which have two responsibilities: first, they detect the attachment of a cable to the port, register the type of device attached, and recognise the current limit for that device. Second, they allow the negotiation of non-default power modes. The nominal set-up for the CC circuits using a non-electronically marked cable is shown in Figure 2.
By monitoring the voltage at the cable termination, it is possible to determine the type of device connected to the system. It is of course possible for a piece of USB Type-C equipment to be both a power supplier and a power receiver, and to switch its behaviour during operation.
In default mode the DFP advertises, by means of the value of the resistor, Rp, the current that it is going to supply, and the UFP has to ensure that it conforms to this set-up. The DFP can as standard supply 0.9A, 1.5A or 3A, all at 5V. This default power-delivery set-up can be overwritten using a communication protocol superimposed over the DC levels of the CC pins, and monitored at the pins. This is based on Bi-phase Mark Coding (BMC), which allows adjustment of both the current and the voltage supplied by the DFP, as shown in Figure 3.
|Mode of Operation||Nominal Voltage||Maximum Current||Notes|
|USB2.0||5V||500mA||Default current, based on definitions in the base specifications|
|USB3.1||5V||500mA||Default current, based on definitions in the base specifications|
|USB BC 1.2||5V||Up to 1.5A||Legacy charging|
|USB Type-C, current at 1.5A||5V||1.5A||Supports higher-power devices|
|USB Type-C, current at 3.0A||5V||3A||Supports higher-power devices|
|USB PD||Configurable up to 20V||Configurable up to 5A||Directional control and power-level management|
Fig. 3: The power-supply options specified for a USB Type-C DFP.
Figure 3 shows that, to make the most of the capability of USB Type-C technology, both ends need to support the communications protocol. This has implications for the design of simple chargers: they cannot just provide the maximum current in all situations to all devices. A universal charger will need some intelligence embedded in it to be able to adjust to the needs of various attached systems.
Fortunately, an intelligent USB Type-C controller may be implemented with readily available off-the-shelf chipsets which take care of the complexity of legacy USB specifications and the various USB Type-C power options. For instance, the PTN5100 from NXP Semiconductors handles most configuration operations. The LIF-UC110-SG48I, an FPGA running Lattice Semiconductor’s USB Type-C solution for chargers, has a similar capability. In addition, Lattice’s LIF-UC110 is able to manage negotiation and link set-up with legacy USB equipment that uses the D+/D- pins to detect charging capability, as well as supporting communication over the CC1/CC2 pins in USB Type-C devices.
Wireless charging enters the mainstream?
Just as wired chargers appear to have found, in USB Type-C, a universal standard that the industry can rally round, consumer demand has thrown up a new and different form of charging infrastructure for device OEMs to support: the wireless charger.
The concept of wireless charging first appeared 100 years ago, with the ill-fated experiments of Nikola Tesla in the early part of the 20th century, and has been used in electric toothbrushes for many years.
Recently, however, improved wireless charging technologies have come on to the market. Capacitive coupling is little used in chargers, although it is very efficient. The most popular approaches today use one of two methods for inductive charging:
• Inductive coupling
• Resonant coupling
The inductive coupling technique is similar to that used in electric toothbrush chargers. The source end is equipped with a coil, and a matching coil is fitted in the receiver end of the system. Power is magnetically coupled across the air gap using an AC waveform. In effect, the two coils form two windings of a transformer.
The amount of power transferred may be calculated as follows. First, the voltage generated in the secondary coil is given by Maxwell’s law of induction:
Phi b is the magnetic field strength at the secondary side coils, given by the equation:
The equations show that the voltage generated is proportional to the magnetic field strength (and hence the coil area cut by the field), and the number of coils. But the field strength in an electromagnetic system declines with distance as shown by:
that is, by the inverse cube of the separation of the coils.
In short therefore, the equations show that alignment of the coils maximises the field strength through the secondary coil, and the distance between the coils largely determines the system’s efficiency.
This inductive coupling method of charging is used in 5W charging systems conforming to the Wireless Power Consortium’s Qi standard. For high-power transformers providing between 15W and 120W, the Qi standard uses resonant coupling.
The advantage of the system is its efficiency and simplicity, although the efficiency rapidly falls off if the coil alignment is less than perfect. Typically therefore the system is either equipped with multiple coils, or the mechanical design is configured in such a way that alignment is guaranteed. The optimum range for charging is 5mm; 40mm is likely to be the maximum range that can be supported in practice.
In a Qi charging system, the base frequency of the AC waveform is in the range 100 to 200kHz. The Qi specification also provides for communication between the receiver and the base station across
the coupling transformers. The communication system, which uses a standard near-field backscattering technique, allows the device being charged to control the amount of power transferred, to either accelerate, retard or terminate charging. It is also used as part of the Foreign Object Detection (FOD) system in Qi chargers, which ensures metal objects on the charger are not heated to dangerous levels.
Resonant coupling: more efficient at distance
Although efficient at close range, the power transfer in inductive coupling systems drops away quickly with distance and poor alignment. Although these problems can be overcome, another solution exists: to use a resonant coupling circuit, as shown in Figure 4.
The theoretical efficiency of a resonant circuit is given by this equation1:
Where U is proportional to the coupling coefficient k of the circuit (which is a measure of the amount of flux that cuts the secondary field) and the Q of the primary and secondary circuits.
Although the k factor falls away with distance between the two coils – since the amount of flux cutting the secondary coil drops – it does not vary by the cube root, as is the case with closely-coupled inductive charging circuits. This means that power can be transmitted at longer range than in an ordinary inductively coupled circuit, albeit at the cost of a lower overall efficiency than the best that can be achieved from a non- resonant circuit.
Whether a wireless charging system must be optimised for efficiency (for which inductive coupling is appropriate) or range (for which resonant coupling is the right choice), the implementation of a wireless charging controller today is easier than it has ever been.
This is because many semiconductor suppliers now offer integrated chipsets which have built-in support for protocols and standards such as Qi.
For example, Semtech offers the TS80000, which supports both the Qi and Power Matters Alliance (PMA) standards and which is rated for up to 40W of output power. The MWCT1xxx wireless charger parts from NXP also support the Qi and PMA specifications.
A clear view into the future?
This article began with the assertion that standardisation of charging infrastructure would be a good thing for consumers and for the environment. But of course it is ultimately for consumers to decide what is best for them – and this is generally a matter of convenience and ease of use.
This tends to tilt the balance in favour of wireless charging in many environments, such as inside a car or in public spaces such as hotels and restaurants. Here, it will be preferable for many to simply place a mobile phone on a charging mat rather than having to find the right cable and then plugging the device in.
So should manufacturers of portable equipment dispense with the wired connector completely? This seems unwise. First, USB gives the user a high degree of flexibility in terms of communications and interfacing that is not yet matched by wireless technology.
Second, there is still considerable uncertainty about which of the various competing standards for wireless charging will survive, and which will be competed out of existence. Hopes are high for emerging technologies such as Wattup®, but it is still too early to be sure which if any will catch on with users.
And third, wireless charging infrastructure will not be ubiquitous for many years, if ever, and so travellers who depend on their device’s battery will always need to carry their own charger. The wireless charging base unit or mat is itself a bulky piece of equipment; it’s far easier to carry a small USB Type-C cable and charge every device from a laptop or other USB Type-C port.
It is likely, then, that both wired and wireless charging will co-exist, supporting the use cases most appropriate to them. At least the wired charger, however, has a strong prospect of achieving universal adoption with the growing popularity of the high-speed, high-power USB Type-C standard.