Meeting the challenge of the universal charge standard in mobile phones - Embedded.com

Meeting the challenge of the universal charge standard in mobile phones

Who could have thought that the mobile phone industry could have an impact on the environment? When you think about reducing your ecological footprint, you think about changing your car or using green energy, which has nothing to do with a mobile phone.

However, mobile phone chargers do have an impact on the environment, and mobile phone industry stakeholders have started to address it through the universal charging solution (UCS). UCS will change the way mobile phone makers will design handsets, and we will analyze the trade-offs they need to cope with to implement a safe charging interface.

Pushing for initiatives
A mobile phone user generates around 17kg of carbon dioxide per year, equivalent to 111km by car. It is not much, and it does not seem to be a priority in the fight for the environment.

If you change your point of view, however, note that last year, 1.2 billion handsets have been shipped all over the world, each of them with a dedicated charger. Out of these 1.2 billion mobile phones, we can estimate that 500 million are replacement phones.

It is always exciting to get a new upgraded phone, but people do not have any interest on changing their charger. When you realize that mobile phones average turnover is 18 months and that most of mobile phones have a specific charger (including within the same brand), it is not infrequent to get three or four useless mobile phone chargers at home. Indeed, the probability that your former charger fits your new mobile phone is around 10 percent.

That's why we've been seeing initiatives coming from governments, institutions and key players of the mobile phone industry to reduce this waste. China's Minister of Information and Industry has issued in December 2006 a new regulation (YD/T 1591-2006) aimed at standardizing the wall charger and the connecting cable. Each new mobile phone launched in the China market will have to be compliant with this regulation to be authorized in China.

Another major step has been achieved during the last Mobile World Congress in February 2009 in Barcelona, with the agreement of the GSMA (including the five biggest mobile phone makers in the world) and 17 mobile phone operators to provide in 2012 UCS through USB using a micro-USB connector.

This initiative could help to eliminate up to 51,000 tons of useless mobile phone chargers. Assuming that the production of mobile phone chargers will be cut by two each year, the industry can expect to decrease the greenhouse effect gases from 13 to 22 millions of tons per year.

On top of the clear ecological impact, you will not need to take two or three chargers when you go on a weekend with your family. You will have no problem to find someone to loan a charger that fits your mobile phone.

Standardization
However, to make UCS successful, the first requirement for mobile phone makers is to agree on a standardization of the battery charging through USB.

The Battery Charging Specification Revision 1.0 issued on March 8, 2007 defines limits and detection mechanism for portable device to draw current from USB dedicated chargers. The China Communications Standard YD/1591-2006 had the same scope for mobile phones released in China.

Mobile phones will need to detect if they are plugged to a USB charger or to another terminal for data exchange (e.g. computer, mobile phone etc.). To do so, D+ and D- are internally short-circuited (through a resistor RDCHGR_DAT that must be lower than 200 ohms) in USB chargers.

When a device is connected through the USB port with a mobile phone, the D+ line is driven to a voltage of VDAT_SRC, sourcing a current of IDAT_SRC. If a specified voltage is detected on D- line (determined by RDCHGR_DAT and IDAT_SRC), it means that the mobile phone is connected to a USB charger.

Then, the maximum charger output current must be 1.5A, whereas the maximum charger output voltage must be 5.25V for the battery charging specification. China's YD/T 1591-2006 specifies a maximum charger output current at 1.8A.

Protecting mobiles
Nevertheless, the main challenge is not related to battery charging being standard, which is now well defined but it would rather lie in the quality of USB chargers.

Indeed, a smart phone can be connected to a very poor USB charger with—for instance—no monitoring of the charging, no Users will not accept that their phone can be damaged by an external charger, and that's no doubt that the mobile brand's image can suffer if it is known that their phones are vulnerable when charging.

Thus, as mobile phone makers cannot control the quality of their wall chargers, they will need to include protections directly inside the terminal. Protecting a mobile phone is more challenging than protecting a charger for two reasons:

The PCB space of a mobile phone is reduced because of the integration of hundreds of functions. The thickness of the protections must be compatible with slim phones (7mm) or clamshell phones.

The protection cannot impact the autonomy of the mobile phone, so the leakage current must be very low. To choose the right protection device, mobile phone designers must assess the risks on the internal charging circuitry connected to the USB port.

Risky business
The first well-known risk is ESD events. ESD can occur at any time, whether the mobile phone is charging or not. By only touching the USB connector, the user can induce an ESD surge up to 30kV into the USB circuitry.

It is not a new risk for designers who already know that the lines of their USB port must be protected with devices able to withstand, at least, IEC61000-4-2 level 4 surges (8kV contact discharge). Transient voltage suppressors (TVS) diodes offer is large enough on the market to mitigate this risk.

However, if we assume that we cannot rely on the quality of USB chargers, the second risk lies in the fact that the mobile phone is directly connected to the power mains of the house and exposed to any surges appearing on these power mains.

That is the new element designers need to take into account. The main perturbations appearing on home power mains are:

Surges due to industrial events (plants, factories nearby) or actions on external mid/high voltage power lines (switch on and off in the power distribution network);

Surges due to lightning. The most critical are very often lightning surges. A surge on power mains due to lightning is not only induced when a house is blasted, which is, fortunately, extremely rare.

Each lightning strike blasting a power line or merely the earth near a power line will generate a voltage increase in ground (buried) lines.

This wave (voltage and current) will propagate through the power lines going through various protection devices in power centrals and even in the electrical dashboard of the house.

However, a residual surge will be transmitted directly to the receptacle to which the mobile phone is plugged. This residual surge is characterized by a sharp di/dt related to the waveform of a lightning strike. Experiments and measurements have led to modelize the current of such a surge by the following waveform called 8/20 us defined in the IEC61000-4-5 (Figure 1, below).

Figure 1: Shown is a waveform of short-circuit current (8/20ms) at the output of the generator.

The rising time of the current between 10 percent and 90 percent of its peak value is set to 8s. The current must be half of its peak value after 20 microseconds. This overcurrent is really critical for electronic devices, which are designed to work with low voltages and low currents.

A good protection must sink the overcurrent through the GND while keeping a low clamping voltage to avoid destroying the charging circuitry. These surges are more powerful than ESD surges and represent a tougher challenge for designers.

In France (550,000km), one million lightning strikes blast the ground each year. So the phenomenon is not exceptional. Even if the probability of a lightning blasting your home is very low, the probability that a lightning blasts a tree or the ground a few kilometers around once a year is close to 100 percent.

Home appliances are not vulnerable to these surges, but electronic devices are. Computers and TV sets are designed to be plugged to power mains and are protected in consequence. Other portable devices have dedicated chargers properly protected or are powered through removable batteries.

But, if a universal charger is not protected against such surges for reasons of cost, the mobile phone is dramatically exposed. Surges due to industrial events or actions on high/mid voltages power lines can also be modelized by an 8/20 microsecond waveform but with lower peak current value. Their probability is also very high.

The second challenge is the current consumption of the protection device. TVS diodes able to protect against 8/20s-like surges are many, but their current leakage can go up to 20 microAmperes. A very simple calculation shows that for a 1,000mAh battery capacity and 400hrs of autonomy in standby mode, adding such a device on a biased line will decrease the autonomy of around 1 percent.

Proposed solution
Designers face a difficult tradeoff between miniaturization of protection devices, efficiency in case of surges and power consumption.

Figure 2 below shows an example of a typical topology for mobile phones, which can provide an acceptable compromise between the three parameters. The TVS diode is connected between the charger IC and the USB connector. The closer to the connector, the better are the ESD performances.

Figure 2: Shown is an example of protection topology for VBUS line of USB.

Indeed, the parasitic inductances between the ESD event and the protection due to PCB metal tracks responsible for voltage increases (V = Ldi/dt) are minimized. This kind of diode can withstand 30kV contact discharge (IEC61000-4-2).

The performance in 8/20 microseconds is key for this application. In this case, the diode will sink a peak pulse current (IPP) of 27A to the GND to protect the charger IC.

It means that the current surge going through the receptacle and the USB charger can reach 27A, it will be sunk to the GND and not damage neither the TVS diode nor the charging circuitry. The voltage seen by the charger IC will not go beyond 18.5V during the surge (tens of microseconds) and will have no impact on the integrity of this IC.

The total peak pulse power will be around 500W. If for any reasons, the TVS diode is also connected to VBAT or another permanently biased line, the leakage current of the TVS diode must be very low. The leakage current of this solution at 0.5 microAmperes can fit portable devices' requirements.

Finally, the solution must have a very small footprint to be implemented on a PCB with already many functions and many ICs.

Conclusion
The UCS is a real achievement for the mobile phone industry, and it is a clear step forward to protect the environment. The key challenge for mobile phone makers is now to transfer the protections against surges and ESD from the charger to the mobile phone.

The protection devices dedicated to this application will have to be small to present a low leakage current and withstand high level of current when submitted to 8/20 microsecond waveforms. Protection devices gathering the totality of these performances are not easy to find, but the TVS market is now starting to propose solutions compatible with all these requirements.

Jean Garcia is a Product Marketing Engineer at

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