Without a widely accepted and robust standard, the huge growth in the number of portable devices has led to a correlating rise in the number of mains power adapters.
Without a standard this trait has also become an issue of public policy, with millions of obsolete power supplies sent for disposal in landfill sites globally annually.
Government agencies are pushed for the standardization of main power adapters of power supplies to control this waste. But engineers have moved the goalposts before the regulators have even gotten started.
First, the technology industry recently introduced yet another new wired power and communications interface: USB Type-C™, 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 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 standardization through USB. The Universal Serial Bus was introduced by Microsoft in 1995 as a replacement for the aging RS-232 standard.
However this “universal” serial bus quickly branched out, accruing variants both of the communications protocol and of the connector, as shown in Figure 1.
Figure 1: While the older versions of USB have generated nine different connector designs, the new USB Type-C has just one. (Image credit: Microchip)
The reason for the emergence of so many connector types was to satisfy the contrasting demands of different devices, growing ever smaller over time.
USB Type-C offers the potential to replace these many connector types with just one. 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, including a monitor and peripherals (typically). Almost any portable device may be powered by this one, low-profile connector. USB Type-C appears to solve the problems both of end-of-life waste reduction and of charging equipment for all portable devices.
Support for Legacy USB Equipment
Unfortunately, reality bites with inherent flexibility. 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 recognize 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.
Figure 2: Power connection over USB Type-C.
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 behavior 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 setup. 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||900mA||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|
Figure 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 needs intelligence embedded in it to be able to adjust and match various systems.
Fortunately, an intelligent USB Type-C controller may be implemented with 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 a universal standard, consumer demand has introduced a new form of charging for device OEMs to support the wireless charger.
The concept of wireless charging first appeared 100 years ago and more recently has been used in electric toothbrushes for many years.
Improved wireless charging technologies have now come into the market. 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:
E = emf, or voltage induced
N = number of turns in the coil
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 alignment of the coils maximizes 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 this system is its efficiency and simplicity, although the efficiency rapidly falls off if the coil alignment is poor. Typically 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 switching 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 allows the device being charged to control the amount of power transferred. 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
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, 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.
Whether a wireless charging system must be optimized 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 standardization 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 favor of wireless charging in many environments, where it will be preferable for many to simply place a mobile phone on a charging mat.
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. 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. The wireless charging base unit or mat is itself a bulky piece of equipment; it’s far easier to carry a cable.
It is likely, then, that both wired and wireless charging will co-exist. 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. Fingers are crossed.
Reference 1. WiTricity white paper, Highly resonant wireless power transfer: safe, efficient and over distance, by Dr Morris Kesler.