Design for fast charging of portable devices

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Mobile devices are increasingly becoming an integral part of our daily lives. In the case of smartphones, in addition to simple cell phone calling features, smartphones now have a rich set of features that support social networking, web browsing, messaging, gaming, and large HD screens. All of these features have made mobile phones a high-power device. Battery capacity and energy density have been significantly improved to meet higher power requirements. It takes 10 minutes to charge the device to power the device, and one hour to get 80% of the charge, which has become a trend of high-end user experience. If you combine fast charging requirements with large battery capacity, portable devices can charge up to 4A or higher. This demand for high power brings many new challenges to the design of battery-powered systems.

USB powered portable devices typically use a 5V USB power supply. The traditional USB port uses a USB 2.0 specification, the maximum output current is 500mA, or if USB3.0 is used, the maximum output current is 900mA, which does not meet the fast charging requirements of portable devices. The USB adapter (dedicated charging port, DCP) uses a micro USB connector to increase the output current to 1.8A. Unfortunately, a typical 5V/2A power adapter can only provide 10W of total power. If you use this power adapter as a charger power supply, the battery charger can only supply up to 2.5A of charging current, which is not enough to quickly charge a 4,000mAh and higher capacity battery pack. In order to increase the power, can we continue to increase the output current of the 5V power adapter? If we increase the cost and use a dedicated cable, it is theoretically possible. However, this practice is subject to the following factors:
● Higher adapter currents (such as 2A or higher) require thicker cables and specialized USB connectors, which can result in increased system solution costs. In addition, traditional USB cables are not sufficient for power loss and security issues.
● Depending on the length and thickness of the cable, the typical impedance of the adapter cable ranges from 150 to 300 mOhm. The high adapter output current causes an increase in the voltage drop across the cable, which in turn reduces the effective input voltage at the charger input. When the charger input voltage is close to the battery charging voltage, the charging current is significantly reduced, thereby prolonging the charging time.
For example, a 5V/3A adapter with a cable resistance of 180mOhm has a voltage drop of 540mV on the cable. The input voltage of the charger is 4.46V. We assume that the total resistance of 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 current, the maximum charging current is only 730mA for charging 4.35V lithium-ion battery. The charging current of less than 1A is obviously not high enough to meet the needs of fast charging.
Based on the above analysis, the power supply input voltage must be increased to provide sufficient voltage to prevent the charger from entering the low dropout mode. For 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 high voltage adapters is the existence of backward compatibility issues. Plug the high voltage adapter into a portable device that supports 5V input. If the system is not turned off (due to overvoltage protection), the device will also be damaged (due to the lack of adequate high voltage protection).
Due to these limitations, many new hybrid high voltage adapters such as the USD Power Adapter are entering the market. A common feature of this type of hybrid electrical voltage adapter is the ability to identify the voltage requirements of the system through a handshake between the adapter and the system controller. The adapter uses a 5V start output as a default value. The voltage is raised to a higher 9V or 12V only if the system confirms that it can support a higher voltage for fast charging. Communication between the system and the adapter can be done using VBUS or by means of dedicated handshake algorithms or signals via D+ and D- lines. This new hybrid, adjustable voltage adapter can be used not only as a universal power supply, but also as a conventional 5V voltage for normal power supplies and as a high input voltage system for fast charging.

Fast Battery Charging Can we shorten the charging time without increasing the input power or increasing the charging current through some special battery charging schemes? To find out, we need to first understand the battery charging cycle.
There are two modes of operation during the battery charge 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 terminal voltage is sensed to reach the preset regulated voltage, it enters the CV mode. When the actual battery current reaches the termination current, the battery charge ends. The termination current is typically equivalent to 5% to 10% of the entire fast charge current.
In an ideal charging system, the battery pack itself does not have any resistance and only the constant current mode exists. It has no CV charging mode and has the shortest charging time. The reason is that as long as the charging voltage reaches the regulated charging voltage, the charging current immediately drops to zero and reaches the charging termination current.
However, in an actual battery charging system, there is a series of resistances from the battery voltage sensing point to the battery. These resistors include: 1) PCB trace resistance; 2) on-resistance of two battery charge and discharge protection MOSFETs; 3) current sense resistors for overcurrent protection in the fuel gauge and for measuring battery charge and discharge currents; 4) Internal resistance of the battery as a function of battery aging, temperature and state of charge.
When using a 1C charge rate for a new battery, the charger operates in CC mode with approximately 30% of the charge time, which is sufficient for approximately 70% of the battery capacity. Instead, the charger needs to operate in CV mode with 70% of the total charging time to fill the remaining 30% of the battery capacity. The larger the internal resistance of the battery pack, the longer the charging time in CV mode. The battery can only be fully charged when the open circuit voltage reaches the maximum charging voltage. If there is a large resistance between the battery charging voltage sensing point and the actual battery, the true battery open circuit voltage is still lower than the required regulated voltage even after the battery pack senses that the voltage reaches the regulated voltage.
For applications such as smartphones and tablets that use 4A or more charging currents, the difficulties are even greater. At such large charging currents, the voltage drop across the PCB traces or internal resistors of the battery pack can increase significantly. This causes the charger to enter the CV mode too early, causing a long charging time. How can we shorten the charging time caused by this high voltage drop?
By closely monitoring the charging current, the voltage drop in the charging path can be accurately estimated in real time. This resistance compensation technique, called IR compensation, compensates for the extra voltage drop in the charging path by increasing the battery regulated voltage. With this technology, 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 desired 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, 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 portable power supplies. In order to provide a more ideal end-user experience, the maximum difference between the temperature of the equipment enclosure and the ambient temperature should not exceed 15 °C. For this reason, the battery charger's power conversion efficiency and system heat dissipation performance need to meet more stringent requirements. How can I achieve the best heat dissipation performance and the best efficiency at the same time?

Figure 1: This block diagram represents a 4.5A I2C high efficiency switch charger. Figure 1 is a simplified application circuit diagram of a 4.5A high efficiency switch mode charger. The charger supports both USB and AC adapters, and all MOSFETs are internally integrated. MOSFETs Q2 and Q3 and inductor L form a battery charger based on synchronous switching buck. This combination maximizes battery charging efficiency and maximizes the charging speed of the adapter. MOSFET Q1 can be used as a battery reverse blocking MOSFET to prevent the battery from leaking through the body diode of MOSFET Q2 to the input. In addition, it can be used as an input current sensing component that monitors the adapter current. MOSFET Q4 can be used to actively monitor battery charging current. All FETs used in the design should have low on-resistance to achieve high efficiency. To further improve the thermal performance, a thermal voltage stabilization loop can also be used. When the junction temperature reaches a predefined junction temperature, it can avoid breaking the maximum junction temperature limit by reducing the charge current.

Figure 2: Comparison of charging times at different charging currents: 2.5A and 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, the use of a large charging current can speed up the charging. As shown in Figure 2, the charging time can be reduced by 30%. In other words, when the charging current is increased from 2.5A to 4.5A, the charging time is reduced from 269 minutes to 206 minutes.
Figure 3 shows the advantages of using the IR compensation technique for the actual charging design to achieve a shorter charging time. With a 17% reduction in charging time, it can be reduced from 234 minutes to 200 minutes.

Figure 3: Fast charge comparison using IR compensation. Using a 4.5A charging current, the charging time can be reduced from 234 minutes to 200 minutes. When charging a single 8,000mAh battery, you only need to compensate for the 70mOhm resistor, which can be achieved without additional cost and additional thermal effects.

Summary For many portable devices, fast charging is becoming more important than ever. But this requires a new design concept in the actual charging system, including the use of new high-voltage adapters, optimized charging current and heat dissipation. In addition, an advanced charging mode is required to optimize charging time and extend battery life. The above experimental results verify the effectiveness of the design for fast charging.