Green challenges faced by mobile phone USB charging

Who would have thought that the mobile phone industry would affect the environment? When you think about how to reduce ecological pollution, you might think of changing car design or using green energy, not mobile phones at all. However, we do see that mobile phone chargers have a great impact on the environment. The mobile phone industry has begun to use the Universal Charging Solution (UCS) to solve this problem. UCS will change the way mobile phone manufacturers design mobile phones. This article will analyze the factors that need to be considered to implement a safe charging interface.

A mobile phone user produces about 17 kilograms of carbon dioxide per year, which is equivalent to a car driving 111 kilometers. This number is not very large and does not seem to be a priority in protecting the environment. However, if you look at it from another perspective, you will find that last year, 1.2 billion mobile phones were shipped worldwide, and each mobile phone has its own dedicated charger. Of these 1.2 billion mobile phones, an estimated 500 million are replacement phones. Getting a new mobile phone is usually exciting, but people obviously have no interest in changing chargers. When you realize that the average mobile phone replacement cycle is 18 months, and most mobile phones have dedicated chargers (including mobile phones of the same brand), it is not surprising to have 3 or 4 unused mobile phone chargers at home. In fact, the probability that the old charger is suitable for the new phone is only about 10%.

That’s why we’re starting to see governments and institutions, as well as big names in the mobile phone industry, setting up organizations dedicated to reducing this waste.

China's Ministry of Information Industry issued a new standard, YD/T 1591-2006, in December 2006 to standardize wall chargers and connecting cables. Every new mobile phone released in the Chinese market must be certified to comply with this standard.

Another major development came at the last Mobile World Congress in Barcelona in February 2009, when the GSMA (which includes the five largest global mobile phone manufacturers) and 17 mobile operators agreed to offer a Universal Charging Solution (UCS) using a micro-USB connector by 2012.

This action is expected to eliminate up to 51,000 tons of unused mobile phone chargers. Assuming that the production of mobile phone chargers is reduced by 20% each year, the industry is expected to reduce greenhouse gas emissions by 13 to 22 million tons each year.

Besides the obvious ecological impact, you no longer have to carry two or three chargers when you go on a weekend getaway with your family; you can simply ask someone to lend you a charger to charge your phone.

standardization

However, in order for UCS to be successful, the first requirement for mobile phone manufacturers is that they agree to the standard for battery charging via USB. The Battery Charging Specification Version 1.0, issued on August 8, 2007, specifies the current limit and detection mechanism for portable devices to draw from USB dedicated chargers. The Chinese communication standard YD/T 1591-2006 also imposes the same requirement on mobile phones released in China.

The phone needs to detect and determine whether it is a USB charger or another terminal for data exchange (such as a computer, another phone, etc.). To do this, D+ and D- are shorted in the USB charger (through a resistor RDCHGR_DAT that must be less than 200Ω). If a device is connected to the phone through the USB port, the D+ line will be driven to the VDAT_SRC voltage and the current will be IDAT_SRC. If a certain voltage is detected on the D- line (depending on RDCHGR_DAT and IDAT_SRC), it means that the phone is connected to a USB charger.

The above battery charging specification stipulates that the maximum output current of the charger must be 1.5A and the maximum output voltage of the charger must be 5.25V. China's YD/T 1591-2006 stipulates that the maximum output current of the charger is 1.8A.

The main challenge is to protect

However, the main challenge is not the battery charging standards that are well defined today, but the quality of the USB chargers. In fact, an expensive smartphone may be connected to a poor quality USB charger, with no charge monitoring, no protection, loud noise, etc. It is unacceptable for users to damage their phones by external chargers, and it is certain that the impression of a brand will be greatly reduced if its phones are easily damaged by charging.

Therefore, since phone manufacturers can no longer control the quality of the wall charger, they need to install the protection circuit directly inside the terminal. Protecting the phone is more challenging than protecting the charger for two reasons:

1. The PCB size of mobile phones has been greatly reduced because hundreds of functions have been integrated. The thickness of the protector must be suitable for ultra-thin mobile phones (only 7mm thick in total) or flip phones.

2. The protection device cannot affect the performance of the phone itself, so the leakage current must be very low.

To select the appropriate protection device, mobile phone designers must perform a risk assessment of the internal charging circuit connected to the USB port.

The first well-known risk is electrostatic discharge (ESD). ESD can occur at any time, whether the phone is charging or not. Users can introduce up to 30kV ESD surge voltage into the USB circuit simply by touching the USB connector. Many designers already know that USB ports must be protected with devices that can withstand at least level 4 surge voltage (8kV contact discharge) in the IEC61000-4-2 standard, so this is not a new risk for them. The large number of transient voltage suppressor (TVS) diodes on the market are sufficient to mitigate this risk.

F1: Short-circuit current waveform at the generator output (8/20μs)

[page]

However, if we assume that we cannot rely on the quality of the USB charger, then a second risk arises: the phone is connected directly to the indoor power lines and is therefore vulnerable to any surge voltages that appear on these power lines. This is a new factor that designers need to consider. The main sources of interference that appear on indoor power lines are:

1. Surge voltage caused by industrial events (nearby workshops, factories, etc.) or actions on external medium and high voltage power lines (switches in the power distribution network, etc.).

2. Surge voltage caused by lightning.

The most dangerous are the lightning surges that occur all the time. Because the surge voltages caused by lightning on the power lines are not only induced in the rare cases when a house is directly struck by lightning. In fact, every time lightning strikes the power line or even the earth near the power line, it will produce a voltage increase in the (buried) ground wire. This wave (voltage and current) will quickly propagate through the power line and pass through the various protection devices in the central power station and even the household distribution box. Then, a residual surge voltage will be transmitted directly to the power socket where the mobile phone is plugged in.

This residual surge voltage is characterized as a peak di/dt associated with the lightning waveform. Extensive experiments and measurements have modeled such a surge current as the following waveform, which is defined as an 8/20μs pulse in IEC61000-4-5.

The current rise time between 10% and 90% of the current peak is specified as 8 μs, and the current must decrease to half of the peak value after 20 μs.

This overcurrent is indeed very dangerous for electronic devices designed to work with low voltage and low current. A good protection device must be able to absorb this overcurrent through the ground (GND) and maintain a low clamping voltage to protect the charging circuit from damage.

This surge voltage is much stronger than an ESD surge and presents a greater challenge to designers.

In France (550,000 km²), 1 million lightning strikes hit the ground every year! So this phenomenon is not surprising. Even if the probability of lightning hitting a house is very small, the probability of hitting a tree or the ground within a few kilometers of the house in a year is close to 100%.

Home appliances are less vulnerable to these surges, but electronic devices are. Computers, televisions and other appliances are designed to be plugged into power lines and are therefore well protected. Other portable devices use dedicated chargers with appropriate protection or are powered by removable batteries. However, if a universal charger is not effectively protected against these surge voltages for cost reasons, then a mobile phone can easily be damaged while charging.

Surge voltages due to industrial events or switching actions on medium and high voltage power lines can also be modeled as 8/20μs waveforms, but the peak current will be lower. Their probability of occurrence is also high.

The second challenge is the power dissipation on the protection device. TVS diodes that can protect devices from surge voltages like 8/20μs are widely available, but their leakage current can be as high as 20μA. A simple calculation shows that for a mobile phone with a 1,000mAh battery and 400 hours of standby time, adding such a device to the bias line will reduce the standby time by about 1%!

Solution suggestions

Designers face difficult trade-offs between miniaturization of protection devices, efficiency under surge voltages, and power consumption.

Figure 2 is an example of a typical topology for a mobile phone, which provides an acceptable compromise between the three parameters mentioned above:

F2: Example of protection topology used for USB VBUS line

The TVS diode is connected between the charger IC and the USB connector. The closer the TVS diode is to the connector, the better the ESD performance. In fact, the parasitic inductance between the ESD event caused by the PCB metal traces that cause the voltage increase (V=Ldi/dt) and the protection circuit can also be minimized. This diode can withstand a contact discharge voltage of 30kV (IEC61000-4-2). For this application, the 8/20μs performance is critical. In this case, to protect the charger chip, the diode will absorb a peak pulse current (IPP) of 27A to GND. This means that the surge current reaching the socket and the USB charger may reach 27A, which will be absorbed by GND without damaging the TVS diode or the charging circuit. The voltage seen by the charger chip during the surge (tens of milliseconds) will not exceed 18.5V, so there is no impact on the integrity of the chip. The total peak pulse power is about 500W.

If for any reason a TVS diode is connected to VBAT or another permanent bias line, the leakage current of the TVS diode must be very low. This solution has a leakage current of 0.5μA which meets the requirements of portable equipment.

Finally, the solution footprint must be very small to be implemented on a PCB that already integrates many functions and many chips. The ESDA8V2-1MX2 is available in a tiny 1.0mmx1.45mm QFN package with a maximum thickness of only 0.6mm.