Working principle and design requirements of mobile phone fast charging chip

Release date：2016-09-05   Views：2464

Working principle and design requirements of mobile phone fast charging chip原创 2016-09-05 Hao Yueguo 

 At present, with the increasing capacity of lithium-ion batteries equipped with mobile phones, people hope to be able to charge their mobile phones with enough power in a short time to meet the needs of their daily life and work. For example, Huawei P9 is equipped with a 3000mAh hour lithium-ion battery. If the battery is expected to be nearly full within one hour, the charging current should be above 3a. In order to achieve such a large current charging, the use of switching charging management chip (hereinafter referred to as fast charging chip) is a good solution, which is also the common choice of the industry at present. This paper assumes that you have a basic understanding of the working principle of DCDC, mainly from the technical point of view to analyze the working principle and design requirements of the fast charging chip applied in mobile phones, and also briefly introduces other emerging charging technologies.

1、 Four charging links of mobile phone

Figure 1 four charging links of mobile phone

Figure 1 summarizes the four steps involved in the actual charging process

1) The task of the charging adapter is to convert the 220 V mains power to the 5V voltage that the mobile phone can withstand (now, according to the requirements of various charging protocols, such as QC and USB PD (type C interface), it is also required to be able to send out 9V / 12V / 14.5V or even 20V voltage. We have discussed the charging protocol in the previous official account, and have some power output capabilities, such as 5V/2A, 9V/1A and so on. Charging adapter belongs to the technical category of AC-DC. The fast charging chip usually refers to the AC-DC chip of the adapter and the switching charging management chip of the mobile phone (using DC-DC technology as the implementation means). However, the fast charging chip in this paper refers to the switching charging management chip on the mobile phone.

2) The task of the charging line is to transfer the voltage / current from the adapter to the mobile phone. At present, most of the charging lines are actually USB cables. Here is a parameter to draw your attention to. According to the USB2.0 standard, the cable needs to be able to transmit a maximum current of 1.8A. Therefore, if it is a 5V adapter, the maximum transmission power of USB2.0 cable is only 9W.

3) The task of the fast charging chip is to convert the 5V / 9V / 12V voltage of the adapter into the battery voltage, and charge the battery accurately and controllably according to the required charging current. From a technical point of view, the fast charging chip is the most challenging part of the four links. Therefore, there are very few manufacturers capable of providing high-quality and high reliability fast charging chips in the industry. The main manufacturers are Texas Instruments and Xiantong semiconductor. After years of unremitting independent research and development, China's Xidi Microelectronics has launched a series of fast charging chips Chip, breaking the monopoly of foreign manufacturers, has been widely used in major mobile phone solution providers and brands. The working principle of fast charging chip will be discussed in detail below.

4) Battery is a very important part of this link, the whole charging process is to make the battery quickly and safely charged. The main parameters of the battery include: capacity (MAH, 2000mAh, 3000mAh and 410mah) and charging cut-off voltage (4.2V, commonly used in mobile phones), 4.35v and 4.4V specifications, higher charging cut-off voltage, and higher battery capacity under the same battery volume. Therefore, the so-called 4.35v and above high-voltage batteries are gradually being widely used in mobile phones), as well as the maximum acceptable charging current. The maximum acceptable charging current is generally expressed as NC. For example, for a 3000mAh battery, the charging rate of 1C means that the battery can be fully charged within one hour, and the maximum acceptable charging current is 3A; if 2C charging rate is allowed, then theoretically the battery can be fully charged in half an hour, then the maximum acceptable charging current is 6A; and so on. As will be seen below, these parameters of the battery will have a direct impact on the selection of a suitable fast charging chip.

2、 Classic three-stage charging

In fact, the process of charging the lithium-ion battery is very similar to the process of discharging water from the faucet to the washbasin in our daily life

The first stage: when starting to discharge water to an empty basin, in order to prevent the water from splashing out, the water quantity will be controlled very small; the second stage: when the bottom of the basin is filled with a certain water level, the faucet can be opened relatively large, and the existing water in the washbasin can buffer the rapid inflow, so that there will be no splash;

The third stage: when the water level is close to the top of the basin, we will gradually reduce the water inflow at this time to prevent water from flushing out of the basin until the whole basin is filled.

The battery is like this washbasin, except that it stores not water, but electric charge. There are three similar stages of battery charging

The first stage: trickle charging. The characteristic of the battery is that when the battery voltage (roughly equivalent to the water level) is very low, its internal lithium ion activity is poor and the internal resistance is large, so it can only accept a small charging current (generally about 30 to 50 MA). Otherwise, the battery is easy to heat up and aging, which not only damages the battery life, but also has potential safety problems. Therefore, this stage is called trickle charging, It is also called linear charging or precharge and so on.

The second stage: constant current charging. When the battery voltage is higher than 2V, the lithium ion activity of the battery is fully activated, and the internal resistance is small, so it can accept high current charging. In this stage, the fast charging chip will provide the battery with an acceptable charging current according to the setting, so the battery can get the largest amount of power at this stage, which can account for more than 70% to 80% of the capacity.

The third stage: constant voltage charging. The battery is a very delicate energy storage element. Its battery voltage is not allowed to exceed ± 50mV of the cut-off voltage, otherwise there will be potential safety hazards. Therefore, when the battery voltage is close to the charging cut-off voltage, the fast charging chip must be able to automatically reduce the charging current and control the "water splash" not to exceed the range until the battery is fully charged.

Figure 2 Schematic diagram of three-stage charging

A qualified fast charging chip must be able to automatically control the charging process according to the level of battery voltage and switch seamlessly between the above three stages without the help of other hardware or software.

3、 Power path management function

The purpose of the battery is to power electronic devices such as mobile phones. If the battery is out of power, the mobile phone will not work, so you must plug in a charger to charge. Let's take a look at the experience of several different fast charging chips.

Fig. 3 working diagram of fast charging chip without power path management function

Figure 3 shows the first generation of fast charging chips introduced by domestic manufacturers. Please pay attention to the position of the red circle. This is where the positive pole of the battery is located, and it also supplies power to the mobile phone system. Then it is easy to think: when the battery voltage is very low, even if the charger is inserted, that is, the fast charging chip is already charging the battery, but the battery voltage is very low at this time, which is not enough to turn on the system, so the mobile phone cannot be started. Only when the battery voltage has been charged high enough, the phone can work normally.

We naturally ask, is there a way to start the phone even if the battery voltage is very low, as long as we plug in the charger? There are ways. As long as we make an intelligent partition between the battery charging port and the system power supply port, we can achieve this goal. The industry calls this function power path management, which is also one of the hallmarks of the second generation of fast charging chips.

Fig. 4 working diagram of fast charging chip with power path management function

In Figure 4, the sys pin that supplies power to the system and bat pin that charges the battery are two independent pins, and an intelligent switch is integrated between them. When the battery voltage is low, the intelligent switch is in the half open state, and the voltage of sys and bat is naturally opened to ensure that the voltage of sys is enough to support the work of the later stage system; when the battery voltage is high enough, the intelligent switch is fully opened, which is equivalent to a wire with small resistance to short circuit sys and bat together to minimize the power consumption between them.

4、 High voltage operation

Now the battery capacity is increasing, so when charging, it needs a lot of power. For example, if a 3000mAh battery is charged with 3a and calculated by 3.8V battery voltage, the accepted power is 11.4w. Assuming that the conversion efficiency of the fast charging chip is 90%, then the power entering the fast charging chip is about 12.7W. According to the above analysis of the four links of charging, the requirements for the cable come at this time: if the fast charging chip is working at 5V, it means that the current on the line is about 2.5A, which not only exceeds the transmission capacity of the existing USB2.0 cable, but also causes large power loss on the cable. For example, the parasitic resistance of 1 meter cable is about 250 milliohm, and its power consumption is about 1.5 w! In order to solve this problem, we can increase the transmission voltage like the common high-voltage transmission line in our life, and the current and power consumption on the line will naturally drop. In the previous example, if the transmission voltage is changed to 9V, the current on the line will be only about 1.4A, so the existing USB2.0 cable will be enough (cost saving!) The power consumption on the line is only 0.5W. Accordingly, the chip needs to work normally at 9V, 12V or even 20V. This is an important requirement for fast charging chip.

5、 Conversion efficiency

Mobile phone is an electronic device with the highest user experience. In terms of charging, individual users not only require fast and safe charging, but also prefer low heating mobile phones. In order to achieve this goal, the requirement of fast charging chip is naturally high conversion efficiency. Of course, in addition, we also need to choose low thermal resistance package for fast charging chip, such as QFN package with heat sink; in the design of mobile phone, more consideration should be given to heat dissipation measures, etc. Fortunately, domestic manufacturers are fully aware of the importance of conversion efficiency to users, and their products have overtaken foreign brands in terms of indicators, as shown in Figure 5.

Fig. 5 conversion efficiency curve of fast charging chip

6、 Configuration of charging parameters

As the previous analysis of the battery, due to the different current capacity that different batteries can accept, the charging cut-off voltage is also different, which requires that the fast charging chip can configure different charging parameters through software according to different batteries, so as to improve the versatility of the fast charging chip. Generally, the configuration of fast charging chip is completed through I2C interface. Other common parameters to be configured include:

L input voltage limiting threshold

L input current limiting threshold

L charging cut-off current

Working mode control of L chip

7、 Design of withstand voltage and anti surge capability

In the mobile phone, the fast charging chip is the first chip seen in the USB / type C interface. The repeated plugging and plugging of the charging line will bring high burr voltage, which requires the fast charging chip to have a good withstand voltage capability; in the application of USB2.0, the fast charging chip needs to have the withstand voltage capability of more than 20V. What is more threatening is that the surge voltage transmitted from the power grid through the adapter, especially in the area where the power grid is very unstable, such as India, the surge voltage will be as high as 300V, which requires that the fast charging chip can work safely without being damaged with protective devices (such as TVs).

8、 Other requirements and other fast charging technologies

The function expectation of fast charging chip used in mobile phone also includes OTG function, battery temperature detection, system reset and transport mode function and so on. In addition, the emerging low-voltage direct charging technology is also a very noteworthy direction, which is the opposite of the high-voltage fast charging. It is to reduce the output voltage of the adapter and keep the synchronous voltage difference tracking with the battery voltage at the constant current charging stage, so as to increase the charging current and improve the conversion efficiency at the same time. Limited to space, these two aspects will not be expanded one by one. Welcome interested friends and the author of this article to contact, we do further discussion, and work together to make our China's fast charging chip better, stronger and bigger!

Introduction to Xidi Microelectronics

Xidi microelectronics is an original chip manufacturer specializing in the design and sale of fast charging chips and high-performance DCDC. The company has successfully produced 1.5A, 2.5A, 3a, 4.5a and 5V / 9V / 12V are all series of fast charging chips. Their high-quality and high reliability fast charging products are widely used in mobile phone design manufacturers and brands, breaking the long-term monopoly of foreign brands in this market situation.

Contact information of the author:

Hao Yueguo, 180, 1783, 1819 (same as wechat), Owen.Hao @ halomicro.com ;

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