Mobile devices are becoming an integral part of our daily lives. Take smartphones, for example. In addition to simple phone calling, smartphones now have rich features that support social networking, web browsing, messaging, gaming, and large high-definition screens. All of these features have made mobile phones high-power devices. Battery capacity and energy density have been significantly improved to meet higher power demands. Charging for 10 minutes to power a device for a day and charging for an hour to get 80% of the power have become the trend for high-end user experience. If the fast charging requirements and large battery capacity are combined, the charging current of portable devices can reach 4A or even higher. This demand for high power brings many new challenges to battery-powered system design.
USB powered
Portable devices usually use 5V USB power supply. The maximum output current of traditional USB ports is 500mA if using USB2.0 specification, or 900mA if using USB3.0, which cannot meet the fast charging requirements of portable devices. USB adapters (dedicated charging ports, or DCPs) can increase the output current to 1.8A using a micro USB connector. Unfortunately, a typical 5V/2A power adapter can only provide a total power of 10W. If such a power adapter is used as a charger power source, the battery charger can only provide a maximum charging current of 2.5A, which is not enough to quickly charge battery packs of 4,000mAh and higher capacities. In order to increase power, can we continue to increase the output current of the 5V power adapter? In theory, it is possible if we increase the cost and use a dedicated cable. However, this approach will be subject to the following factors:
• Higher adapter currents (e.g. 2A or higher) require thicker cables and specialized USB connectors, which increase system solution costs. In addition, traditional USB cables are not adequate due to power loss and safety issues.
• Typical impedance of adapter cables ranges from 150 to 300mOhm, depending on cable length and thickness. High adapter output currents cause a higher voltage drop across the cable, which reduces the effective input voltage at the charger input. When the charger input voltage is close to the battery charge voltage, the charge current is significantly reduced, which increases the charging time.
For example, using a 5V/3A adapter with a cable resistance of 180mOhm, the voltage drop on the cable is 540mV. This means the input voltage to the charger is 4.46V. We assume that the total resistance from the charger input to the battery pack is 150mOhm, which includes the on-resistance of the charger power MOSFET and the DC resistance of the inductor. Even if the charger can support 3A of current, the maximum charging current for a 4.35V Li-ion battery is only 730mA. A charging current of less than 1A is obviously not high enough to meet the needs of fast charging.
According to the above analysis, the power input voltage must be increased to provide enough voltage to prevent the charger from entering low voltage drop mode. Due to these constraints, if the system requires more than 10W or 15W, it is best to use a high voltage adapter, such as 9V or 12V. At the same power, the high voltage adapter not only requires a lower input current, but also has a larger input voltage margin to provide a fully charged battery voltage. The only limitation of the high voltage adapter is the backward compatibility issue. If the high voltage adapter is plugged into a portable device that supports 5V input, if the system is not shut down (due to overvoltage protection), the device will also be damaged (due to lack of sufficient high voltage protection).
Because of these limiting factors, a number of new hybrid high voltage adapters such as the USD power adapter are entering the market. The common feature of these hybrid voltage adapters is the ability to identify the voltage requirements of the system through handshaking between the adapter and the system controller. The adapter starts with a 5V output as a default value. The voltage is raised to a higher 9V or 12V only when the system confirms that it can support the higher voltage for fast charging. Communication between the system and the adapter can be achieved using VBUS or through the D+ and D- lines with the help of a dedicated handshake algorithm or signal. This new hybrid, adjustable voltage adapter can not only be used as a universal power supply, but also supports the traditional 5V voltage as a normal power supply and a high input voltage system for fast charging.
Fast battery charging
Can we shorten the charging time by using some special battery charging scheme without increasing the input power or charging current? To find the answer, we need to first understand the battery charging cycle.
There are two operating modes in the battery charging cycle: constant current (CC) mode and constant voltage (CV) mode. When the battery voltage is lower than the regulated charging voltage, the charger operates in CC mode. Once the battery pack terminal voltage is sensed to reach the preset regulated voltage, it enters CV mode. When the actual battery current reaches the termination current, the battery charging is terminated. The termination current is usually equivalent to 5% to 10% of the entire fast charging current.
In an ideal charging system, the battery pack itself does not have any resistance, and there is only a constant current mode. It does not have a CV charging mode, and the charging time is the shortest. The reason is that as long as the charging voltage reaches the regulated charging voltage, the charging current will immediately drop to zero and reach the charging termination current.
However, in a real battery charging system, there is a series of resistances from the battery voltage sensing point to the battery. These resistances include: 1) PCB trace resistance; 2) the on-resistance of the two battery charge and discharge protection MOSFETs; 3) the current sense resistor used to measure the battery charge and discharge current in the fuel gauge for overcurrent protection; and 4) the internal resistance of the battery as a function of battery aging, temperature, and charge state.
When using a 1C charge rate for new batteries, the charger uses about 30% of the charging time in CC mode to charge about 70% of the battery capacity. Conversely, the charger needs to work in CV mode for 70% of the total charging time to charge the remaining 30% of the battery capacity. The greater the internal resistance of the battery pack, the longer the charging time in CV mode. The battery can only be fully charged when the battery open circuit voltage reaches the maximum charging voltage. If there is a large resistance between the battery charge voltage sensing point and the actual battery, then even after the battery pack senses that the voltage has reached the regulated voltage, the actual battery open circuit voltage is still lower than the required regulated voltage.
For applications such as smartphones and tablets that use 4A or higher charging currents, the challenge is even greater. At such high charging currents, the voltage drop across the PCB traces or internal resistors in the battery pack increases significantly. This causes the charger to enter CV mode prematurely, resulting in a longer charging time. How can the charging time that is prolonged by this high voltage drop be shortened?
By closely monitoring the charge current, the voltage drop in the charge path can be accurately estimated in real time. This resistance compensation technique, called IR compensation, compensates for the additional voltage drop in the charge path by increasing the battery regulation voltage. With this technique, the charger can operate in constant current regulation mode for as long as possible until the actual battery open circuit voltage is very close to the required voltage value. In this way, the charging time in CV mode can be significantly shortened, reducing the total charging time by up to 20%.
System cooling optimization
To achieve fast charging capabilities, higher power adapters such as 9V/1.8A and 12V/2A are required. In addition, in addition to charging the battery, the battery charger can also power the system. This makes it one of the hottest components in a portable power device. In order to provide a more ideal end-user experience, the maximum difference between the temperature of the device case and the ambient temperature should not exceed 15°C. For this reason, the power conversion efficiency and system thermal performance of the battery charger need to meet more stringent requirements. How can the best thermal performance and the most ideal efficiency be achieved at the same time?
Figure 1: This block diagram represents a 4.5A I2C high-efficiency switching charger
Figure 1 is a simplified application circuit diagram of a 4.5A high-efficiency switch-mode charger. This charger can support both USB and AC adapters, and all MOSFETs are internally integrated. MOSFETs Q2 and Q3 and inductor L form a synchronous switching buck-based battery charger. This combination can achieve the highest possible battery charging efficiency and can fully utilize the adapter power to achieve the fastest charging speed. MOSFET Q1 can be used as a battery reverse blocking MOSFET to prevent the battery from leaking to the input through the body diode of MOSFET Q2. In addition, it can also be used as an input current sensing element to monitor the adapter current. MOSFET Q4 can be used to actively monitor the battery charging current. All FETs used in the design should have a low enough on-resistance to achieve high efficiency. To further improve thermal performance, a thermal regulation loop can also be used. When the junction temperature reaches a predefined junction temperature value, it can avoid exceeding the maximum junction temperature limit by reducing the charging current.
Figure 2: Comparison of charging time at different charging currents: 2.5A vs. 4.5A
Experimental test results
Figure 2 shows the relationship between charging current and charging time. It is easy to understand that as long as the battery charging current rate does not exceed the maximum current rate specified by the battery manufacturer, using a large charging current can speed up the charging speed. As shown in Figure 2, the charging time can be shortened by 30%. In other words, when the charging current increases from 2.5A to 4.5A, the charging time is shortened from 269 minutes to 206 minutes.
Figure 3 shows the benefit of using IR compensation technology in a real charger design to reduce charging time. The charging time is reduced by 17%, from 234 minutes to 200 minutes.
Figure 3: Comparison of fast charging using IR compensation. Using the same 4.5A charging current, the charging time can be reduced from 234 minutes to 200 minutes. This result can be achieved when charging a single 8,000mAh battery, only 70mOhm resistance needs to be compensated, without adding additional cost and causing additional heat dissipation.
Summarize
Fast charging is becoming more important than ever for many portable devices. However, this requires new design ideas in the actual charging system, including the use of new high-voltage adapters, optimization of charging current and heat dissipation. In addition, advanced charging modes are required to optimize charging time and extend battery life. The above experimental results verify the effectiveness of the design for fast charging.
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