Mobile phone USB charging and overvoltage protection solutions for China's latest standards

There are many types of adapters on the market that can charge lithium-ion batteries and provide power for mobile phone systems. At the same time, since China has implemented a unified mobile phone charging interface, any compatible USB interface cable can charge mobile phones. In this way, designers will have no way of knowing what kind of adapter consumers use to charge their phones, and the electrical specifications of these adapters will vary from manufacturer to manufacturer. At the same time, due to the continuous advancement of semiconductor technology, the main frequency and integration of mobile phone platforms are getting higher and higher, and the chip area is getting smaller and smaller, but the voltage resistance of platform chips is also reduced. These have posed severe challenges to designers, requiring designers to design a mobile phone charging system that can meet the safety and reliability requirements for different mobile phone platforms when using different adapters. This article first discusses some of the main problems faced by mobile phone charging systems, and then proposes corresponding measures to help designers cope with these challenges.

Main problems and countermeasures faced by mobile phone charging systems

The main problems faced by mobile phone charging systems include input overvoltage, compatibility with Nokia adapters, compatible design of mobile phone charging systems with different requirements, and layout of peripheral components of mobile phone charging systems and PCB wiring considerations.

Input overvoltage and overvoltage protection

There are many reasons for input overvoltage, such as using an unregulated or incorrect adapter, unstable power grids in some countries causing the adapter's output voltage to change with the mains voltage, transient overvoltage caused by hot plugging of the adapter or transient load changes, etc. Using an unregulated or incorrect adapter and transients during hot plugging of the adapter are the most common causes of input overvoltage.

The common adapters on the market can be divided into two types according to their characteristics: regulated adapters and unregulated adapters. The output voltage of a regulated adapter provides excellent line regulation and load regulation through the internal circuit, while the output voltage provided by an unregulated adapter depends on the load. Figure 1 is a typical curve of the output voltage and load relationship of an unregulated adapter and a regulated adapter.

Figure 1 Load curves of regulated and unregulated adapters

When the adapter is hot-plugged, transient overvoltage will also occur. Due to the parasitic inductance effect of the adapter connection line, a transient output oscillation waveform will be generated during hot plugging, and it will stabilize at the DC value after a period of decay. Figure 2 shows the transient waveform of a 5.5V adapter during hot plugging. Usually, the transient overvoltage peak voltage generated during hot plugging of the adapter is about twice its DC value.

Figure 2 Transient overvoltage waveform when hot plugging an AC adapter with a DC output of 5.5V

With the continuous advancement of semiconductor technology, the integration and main frequency of mobile phone platforms are getting higher and higher, and the chip area is getting smaller and smaller. The resulting problem is that the voltage resistance of the platform chip is also reduced. The voltage resistance of the early platform was relatively high, and the mobile phone platform could withstand the no-load output voltage of the non-regulated adapter or the transient overvoltage when the adapter is hot-plugged. However, due to the high integration and low voltage resistance of mobile phone platforms using advanced process technology, the voltage mentioned above may cause damage to the chip when it is directly added to the mobile phone platform chip. Therefore, mobile phone platforms using advanced process technology require designers to add an input overvoltage protection (OVP) chip between the adapter and the charging module corresponding to the mobile phone platform when applying it to prevent the excessively high voltage output by the adapter from damaging the mobile phone platform chip. For example, the charging pins of MTK's early mobile phone platforms MT6305/5318 and Spreadtrum's SC6600L can withstand a maximum voltage of 15V, and the charging pins of Qualcomm's QSC6240/6270 can withstand a maximum voltage of 18V. None of them require the addition of an OVP chip. However, since the maximum voltage that the charging pins of MTK's MT6223/6235/6238/6253 can withstand is only 9V, an OVP chip is required to prevent the excessive output voltage of the adapter from damaging the mobile phone platform chip.

For the added OVP chip, the highest withstand voltage only needs to be the same as that of several early mobile phone platform chips. This is because the early mobile phone platform chips have been shipped in large quantities, and their long-term application in the market has verified the safety and reliability of their withstand voltage. Therefore, for the added OVP chip, the highest withstand voltage only needs to be above 15V.

For safety reasons during charging, mobile phone platforms generally limit the charging voltage to below 7V. If the adapter output voltage is higher than 7V and is directly connected to the mobile phone charging module, charging is not allowed. In addition, due to the implementation of the unified domestic charging interface standard, the DC output voltage of the adapter is mostly concentrated in the range of 5 to 6V. In view of the characteristics of domestic adapters, the OVP chip is mainly used to avoid the cumulative damage to the mobile phone platform chip caused by transient overshoot during hot plugging of the adapter.

Figure 3 Single-chip mobile phone charging system solution suitable for domestic adapters

The AW3206 from Shanghai Awin is an OVP chip that can meet the requirements of domestic mobile phone charging systems. The OVP protection voltage of AW3206 is 6.8V, which is suitable for domestic mobile phone charging systems with adapter output voltage of 5-6V. For transient overvoltages caused by hot plugging, the 100ns overvoltage protection response time of AW3206 can ensure the safety of the mobile phone charging system. The ESD protection of AW3206 up to ±8KV (HBM) and the latch-up protection of ±450mA are both a strong foundation for increasing the safety and reliability of mobile phone charging systems.

To increase the safety and reliability of the mobile phone charging system, AW3206 has the following features:

1. 6.8V input protection voltage, suitable for domestic mobile phone charging system with adapter output voltage of 5-6V;

2. The input current limiting protection with integrated K-Charge technology can not only ensure a relatively large charging current when the chip temperature is low, but also intelligently adjust the output current to limit the junction temperature when the chip junction temperature is too high, taking into account both performance and safety;

3. Integrated charging P-MOSFET with anti-reverse flow function, which not only saves costs but also prevents battery current reverse flow during standby;

4. Lithium-ion battery overvoltage protection and overtemperature protection.

Problems and solutions for charging systems of mobile phones compatible with Nokia adapters

According to the survey data of market research organization Gartner this year, Nokia has a global market share of 34.2%, still the largest giant in the mobile phone industry, and Nokia's market share is even higher in some emerging market countries. For example, IDC's survey data shows that Nokia's market share in India was as high as 54% at the end of 2009. Due to the popularity of Nokia mobile phones, Nokia adapters are also readily available, so designers need to consider charging systems that are compatible with Nokia adapters.

Figure 4 Output characteristic curve of Nokia adapter AC-3C

However, among the standard Nokia adapters, a large part of them have an output voltage higher than 7V. Figure 3 is the output characteristic curve of Nokia adapter AC-3C. It can be seen from the figure that the output voltage of AC-3C is 7.5V when no-load, while the output voltage of some Nokia chargers can be as high as 8-9V. In order to adapt to Nokia adapters, there was a mobile phone charging system solution designed with high-voltage LDO as shown in Figure 5:

Figure 5: Mobile phone charging system solution for Nokia adapter

However, this solution has some problems. First, due to the large process size of the high-voltage LDO (in order to withstand high input voltage), the on-resistance RDS(ON) will be relatively large. The output voltage of the Nokia adapter will gradually decrease as the output current increases. The larger the charging current, the lower the output voltage. The excessive LDO on-resistance will further reduce the voltage, and the charging module behind the LDO also has a certain on-voltage drop, so the voltage added to the battery may be too low and the battery may not be fully charged. In addition, LDOs are mostly packaged in the form of SOT23-5L. When charging with high input voltage, the power consumption inside the LDO is relatively large, and there will be problems with heat dissipation. The lack of OVP protection function, the large board area of ​​the entire solution, and the high cost are also the disadvantages of this solution. Therefore, a single-chip mobile phone charging system solution suitable for Nokia adapters is urgently needed by designers.

Figure 6 Single-chip mobile phone charging system solution for Nokia adapter

The step-down OVP chip AW3208 launched by Shanghai Awin is an OVP chip specially designed for Nokia adapters. The OVP voltage of AW3208 is as high as 10.5V. For Nokia adapters with an output voltage of 8-9V, AW3208 works in the step-down LDO mode, and the voltage output to the mobile phone platform charging module is 5.25V (CHRIN voltage), ensuring that the mobile phone platform charging module can charge normally. For adapters with an output voltage of 5-6V, the output mode of AW3208 is the pass-through mode, which minimizes the conduction voltage drop as much as possible, ensuring that the battery can be fully charged even when using an adapter with a relatively low output voltage.

For adapters with relatively high output voltage, the internal power consumption of AW3208 working in LDO mode will be relatively large during charging. In addition to the over-temperature protection and over-current limiting functions, AW3208 also integrates the innovative K-Charge technology, which continuously monitors the junction temperature of the chip during charging. If the junction temperature of the chip continues to rise after reaching a certain value, the chip will reduce the output current to limit the internal power consumption of the chip, and try to avoid the junction temperature of the chip from continuing to rise to the point of entering repeated overheating protection state, thereby solving the problem of not being able to charge or charging time being too long.

In addition, based on safety and reliability considerations, AW3208 has all other functions and features of AW3206.

Compatibility design considerations for mobile phone charging systems for different applications

For different applications, the requirements of mobile phone charging systems are different, and sometimes they may conflict with each other. For example, in order to adapt to Nokia adapters, the OVP voltage of the OVP chip needs to be higher than 9V. However, in China, if the output voltage of the adapter is too high, the certification of domestic mobile phone certification laboratories requires that the mobile phone charging system cannot charge and is in a protection state. When facing these two conflicting requirements, designers can only design two different solutions. If there is a solution that can be compatible with these two conflicting requirements at the same time, this solution is undoubtedly the best solution for designers.

Since the pin distribution of AW3206 and AW3208 is exactly the same, and from the application point of view, the two chips only have different OVP voltages, and the peripheral devices and schematics are exactly the same (see Figure 3 and Figure 6). In addition, for the mobile phone platform, the software control is also exactly the same, so AW3206 and AW3208 just meet the above two conflicting requirements through a compatible design.

For designers, when designing a mobile phone charging system, they can first design the schematic diagram and PCB layout according to Figure 3 or Figure 6. After the design is completed, they only need to change the BOM instead of the PCB to meet different requirements.

Selection of peripheral components of OVP chip in mobile phone charging system and some considerations on PCB layout and wiring

1. Selection of input capacitor and output capacitor

The input pin ACIN of AW3206 and AW3208 needs an input capacitor of not less than 1uF to ground. In addition to decoupling, this input capacitor can also effectively reduce the transient overshoot voltage generated by the parasitic inductance effect of the connecting line when the adapter is hot-plugged. In addition, this capacitor is recommended to be an X7R or X5R ceramic capacitor with a withstand voltage of not less than 15V.

AW3206 and AW3208 also need an output decoupling capacitor from the output pin CHRIN to ground. This capacitor is especially important for AW3208, because the output capacitor plays a vital role in the output stability of AW3208 working in LDO mode. Without this capacitor, the output voltage of the CHRIN pin may oscillate. It is recommended to select an X7R or X5R ceramic capacitor with a withstand voltage of 6.3V and a capacitance value of not less than 1uF.

2. Some considerations and suggestions for PCB layout and wiring

When laying out the PCB, it is necessary to consider that the input capacitor and output capacitor from the input pin ACIN and the output pin CHRIN to the ground should be as close to the ACIN and CHRIN pins as possible, and a single layer of routing should be used directly between the pads and pins of the capacitor to avoid using two layers of routing through through holes.

PCB wiring needs to consider that the routing from the ACIN pin to the charging interface, the routing from the OUT pin to the sampling current resistor, and the routing from the sampling resistor to the battery should be as wide as possible while meeting the charging current density, and the parasitic resistance of the routing should be reduced as much as possible.

In order to obtain better heat dissipation performance, the heat sink of AW3206/AW3208 should be directly connected to the large-area ground layer of PCB together with the GND pin. At the same time, as many through holes as possible should be made in the ground layer under the heat sink, and all the ground layers should be connected together by through holes. Through through holes and large-area ground layers, thermal resistance can be reduced and heat dissipation performance can be improved.

Summarize

This article discusses some of the problems faced by mobile phone charging systems and proposes corresponding countermeasures to these problems in order to help designers design mobile phone charging systems that can meet more stable and reliable requirements, so that their products can stand out from the crowd instead of being "indistinguishable from the crowd."