The working principle and design requirements of mobile phone fast charging chip

At present, with the increasing capacity of lithium-ion batteries equipped with mobile phones, people hope to charge enough power to their mobile phones in the shortest possible time to meet their daily needs and work needs. For example, the Huawei P9 is equipped with a 3000mAH hour lithium-ion battery. If it is expected to nearly fill the battery within one hour, the charging current needs to be above 3A. In order to achieve such a large current charging, the use of a switch-type charging management chip (hereinafter referred to as a fast charging chip) is a good solution, which is also a common choice in the industry. This article assumes that everyone has 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 fast charging chips applied on mobile phones, and also briefly introduce other emerging charging technologies.

First, the four charging links of the mobile phone

Interpretation of the working principle and design requirements of mobile phone fast charging chip

Figure 1 Four charging sessions of the mobile phone

Figure 1 summarizes the four links we have involved in the actual charging process:

1) The task of the charging adapter is to convert the 220V mains to 5V that the mobile phone can withstand (there are now various charging protocols, such as QC and USB PD (Type C interface), etc., and are also required to be able to send 9V/12V/ 14.5V or even 20V voltage. We have discussed the topic of charging protocol in the previous public number), and have certain power output capabilities, such as 5V/2A, 9V/1A and so on. The charging adapter belongs to the technical category of AC-DC. The commonly mentioned fast charging chip is a general term for the adapter AC-DC chip and the switch-type charging management chip of the mobile phone (using DC-DC technology as the means), but this article is fast. The charging chip refers specifically to the switch-type charging management chip of the mobile phone.

2) The task of the charging line is to transfer the voltage/current from the adapter end to the mobile phone. Since most of the charging lines are actually USB cables. There is a parameter here that needs to be brought to your attention. According to the USB2.0 standard, the cable needs to have a current capacity of up to 1.8A, so if it is a 5V adapter, the maximum power that the USB2.0 cable can transmit is only 9W.

3) The task of the fast charge chip is to convert the adapter's 5V/9V / 12V voltage into the battery voltage, and accurately charge the battery according to the required charging current. From a technical point of view, fast charging chips are the most challenging part of these four links, so the current industry has the ability to provide high-quality and high-reliability fast-charge chips manufacturers are very limited, mainly to Texas Instruments, Fairchild Semiconductor, etc. A few foreign large-scale factories are the main ones. After several years of unremitting independent research and development, Xi'an Microelectronics has launched a series of fast-charge chips, which broke the monopoly of foreign big companies and has been in the major mobile phone programs. Business and brand owners are widely used. The specific working principle of the fast charge chip will be discussed in detail below.

4) The battery is a very important part of this link. The entire charging process is to make the battery fully and safely charged. The main parameters of the battery include: capacity (mAH, 2000mAH, 3000mAH and 4100mAH common in mobile phones), charge cut-off voltage (currently 4.2V, 4.35V and 4.4V specifications, higher charge cut-off voltage, in the same In the case of battery volume, it usually has a higher battery capacity, so the so-called high voltage battery of 4.35V and above is gradually more widely used on mobile phones, and the maximum acceptable charging current and the like. Among them, the maximum acceptable charging current is generally expressed in nC. For example, a 3000mAH battery, 1C charging speed means that the battery can be fully charged within one hour. The maximum charging current that can be accepted is 3A. If the charging speed of 2C is allowed, then the battery can be fully charged in theory for half an hour. The maximum acceptable charging current at this time 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 charge chip.

Second, the classic three-stage charging

In fact, the process of charging a lithium-ion battery is very similar to the process of discharging water into a washbasin in our life:

The first stage: When you start to discharge water to an empty washbasin, the amount of water will be kept small in order not to let the water splash out; the second stage: wait until the bottom of the washbasin is full of a certain water level before the faucet is opened. It is relatively large, and the water already in the basin can buffer the rapid influx of water so that there is no splash of water;

The third stage: When the water level is near the top of the washbasin, we will gradually reduce the water intake to prevent water from rushing out of the washbasin until the entire basin is filled.

The battery is like this washbasin, except that it stores not water but electric charge. The charging of the battery also has three similar phases:

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, the internal lithium ion activity is poor, and the internal resistance is large, so only a small charging current can be accepted (generally around 30 to 50 mA). Otherwise, the battery is prone to heat and aging, which not only damages the battery life, but also has potential safety problems. Therefore, this stage is called trickle charging, and some people call it linear charging or pre-charging.

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

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

Interpretation of the working principle and design requirements of mobile phone fast charging chip

Figure 2 Three-stage charging diagram

A qualified fast-charge chip must be able to automatically control the charging process to seamlessly switch between the above three stages, depending on the battery voltage, without the aid of other hardware or software.

Third, the power path management function

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

Interpretation of the working principle and design requirements of mobile phone fast charging chip

Figure 3 Schematic diagram of fast charge chip operation without power path management function

Figure 3 shows the first generation of fast-charge chips from domestic manufacturers. Please pay attention to the location indicated by the red circle. Here, it is where the positive pole of the battery is located, and this also supplies power to the mobile phone system. Then we can easily think that when the battery voltage is very low, even if the charger is inserted, the fast charge chip is already charging the battery, but since the battery voltage is low, it is not enough to turn on the system, so the mobile phone is Unable to start. The phone will only work if the battery voltage has been charged high enough.

We naturally ask, is there a way, as long as we plug in the charger, even if the battery voltage is very low, can you start the phone? The solution is yes, as long as we make a smart partition of the port that charges the battery and the port that supplies the system, this can be achieved. The industry calls this feature power path management, which is one of the hallmarks of the second generation of fast-charge chips.

Interpretation of the working principle and design requirements of mobile phone fast charging chip

Figure 4 Schematic diagram of fast charge chip operation with power path management function

In Figure 4, the SYS pin that powers the system and the BAT pin that charges the battery are two separate pins with an intelligent switch integrated between them. When the battery voltage is low, the smart switch is in a half-open state, and the voltages of SYS and BAT are naturally pulled apart to ensure that the voltage of SYS is sufficient to support the operation of the latter system; when the battery voltage is high enough, the intelligence The switch is fully turned on, which is equivalent to a wire with a small resistance that shorts SYS and BAT together to minimize power consumption.

Fourth, high voltage work

Today's battery capacity is getting bigger and bigger, so when charging, the power it needs is very large. For example, if a 3000mAH battery is charged with 3A and is calculated with a 3.8V battery voltage, the received power is 11.4W. Assuming that the conversion efficiency of the fast charge chip is 90%, the power to enter the fast charge chip is 12.7W. about. According to the four links of the previous analysis, the cable requirements come at this time: If the fast charge chip is working at 5V, it means that the current on the line is about 2.5A, which is not only beyond the existing one. The transfer capability of the USB 2.0 cable, and such a large current will cause a large power loss on the cable. For example, the parasitic resistance of a 1-meter cable is about 250 milliohms, and its power consumption is almost 1.5W! In order to solve this problem, we can increase the transmitted voltage like the high-voltage power lines commonly used in life, and the current and power consumption on the line will naturally drop. In the previous example, if the transmitted voltage is changed to 9V, the current on the line is only about 1.4A, so the existing USB2.0 cable is enough (cost saving!), and the power consumption on the line is only There is 0.5W. Accordingly, the fast charge chip also needs to work normally at a working voltage of 9V, 12V or even 20V. This is an important requirement for fast charging chips.

Five, conversion efficiency

The mobile phone is an electronic device with user experience first. In terms of charging, individual users not only require fast and safe charging, but also prefer a lower-heating mobile phone. In order to achieve this goal, the requirement for fast charging chips is naturally high conversion efficiency. Of course, in addition, fast charging chips are required to use low thermal resistance packages, such as QFN packages with heat sinks; when designing mobile phones, they should also consider heat dissipation measures. Fortunately, domestic manufacturers are fully aware of the importance of conversion efficiency to users, and the products have surpassed foreign brands in indicators, as shown in Figure 5.

Interpretation of the working principle and design requirements of mobile phone fast charging chip

Figure 5 Conversion efficiency curve of fast charge chip

Sixth, the configurability of charging parameters

As in the previous analysis of the battery, due to the different current capabilities of different batteries, the charge cut-off voltage is also different. This requires that the fast charge chip can be configured with different charging parameters according to different batteries to improve fast charge. The versatility of the chip. Typically, fast-charge chips are implemented through the I2C interface. Other common parameters that need to be configured are:

1) Input voltage limit threshold

2) Input current limit threshold

3) Charging off current

4) Working mode control of the chip

Seven, pressure and surge resistance design

In the mobile phone, the fast charge chip is the first chip that the USB/Type C interface comes in. Repeated insertion and removal of the charging line will bring a high glitch voltage, which requires a fast charging chip with good pressure resistance; in the application of USB2.0, the fast charging chip needs to have a withstand voltage of 20V or more. More threatening is the surge voltage transmitted by the grid through the adapter, especially in areas where the grid is very unstable, such as India, where the surge voltage can be as high as 300V or more, which requires fast charging chips with protection devices (such as TVS). In case of safety, it can work safely without being damaged.

Eight, other requirements and other fast charging technology

It is used in fast charging chips on mobile phones, and its functional expectations include OTG function, battery temperature detection, system reset and transport mode functions. In addition, the low-voltage direct charge technology that is currently emerging is also a very interesting direction. The opposite of high-voltage fast charge is to reduce the output voltage of the adapter and maintain a sufficiently small synchronous voltage with the battery voltage during the constant current charging phase. Differential tracking to increase the charging current while increasing conversion efficiency. The content of these two aspects is not limited. Interested friends are welcome to contact the author of this article, we will further discuss together and work together to make our China fast charge chip do a bigger job!

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