Mobile devices are increasingly become an indispensable part of our daily lives. Take a smartphone as an example, in addition to simple mobile phone call functions, smartphones now have rich features that support social networks, web browsing, messaging, games, and large HD screens. All these features have made mobile phones into high power devices. Battery capacity and energy density have been significantly improved to meet higher power requirements. The charge can be charged for 10 minutes, and 80% of the power can be obtained for one hour, which has become a trend of high-end user experience. If the fast charging requirements and large battery capacity are counted together, the charging current of the portable device can reach 4A or even higher. This demand for high-power has brought many new challenges to battery power supply system design.
USB power supply
Portable devices typically use 5V USB power. Traditional USB port If you use a USB2.0 specification, the maximum output current is 500mA, or if USB3.0 is used, the maximum output current is 900mA, which cannot meet the quick charging requirements of the portable device. USB adapter (dedicated charging port, ie DCP) uses a micro USB connector to increase the output current to 1.8A. But unfortunately, a typical 5V / 2A power adapter can only provide a total power of 10W. If this power adapter is used as a charger power, the battery charger can only provide a charge current of 2.5A, which is not enough to charge 4,000mAh and higher capacity battery packs. In order to improve power, can we continue to increase the output current of the 5V power adapter? If we increase cost and use a dedicated cable, it is theoretically. However, this law will be subject to the following factors:
● Higher adapter current (such as 2A or higher) requires a thicker cable and a special USB connector, which will cause system solution costs. In addition, traditional USB cables cannot fully meet demand for power loss and security issues.
● According to the length and thickness of the cable, the typical impedance of the adapter cable is not equal to 150 to 300 mohm. The high adapter output current causes the pressure drop in the entire cable to increase the effective input voltage of the charger input. When the charger input voltage is close to the battery charging voltage, the charging current is significantly reduced, thereby extending the charging time.
Take the 5V / 3A adapter using a cable resistance as a 5V / 3A adapter, the pressure drop on the cable is 540mV. This allowed input voltage of the charger is 4.46V. We assume that the charger input to the total resistance of 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 charge current is only 730 mA for the 4.35V lithium-ion battery. The charging current of less than 1A is clearly not high to meet the needs of fast charging.
According to the above analysis, the power supply input voltage must be improved to provide sufficient voltage to avoid the charger into the low pressure drop mode. For these constraints, if the power required by the system is greater than 10W or 15W, it is preferable to use a high voltage adapter, such as 9V or 12V. At the same power, the high voltage adapter requires not only a lower input current, but also has a larger input voltage margin, which provides a voltage filled with electricity. The only limitations of the high voltage adapter are backward compatibility problems. Insert the high voltage adapter into a portable device for supporting 5V inputs, if the system is not turned off (due to overvoltage protection), the device will be damaged (due to lack of high voltage protection).
Due to the presence of these restriction factors, many new mixed high voltage adapters such as USD power supply adapters are pouring into the market. The common characteristics of such hybrid electrical voltage adapters are capable of identifying the voltage demand of the system through the handshake between the adapter and the system controller. The adapter is output as a default in 5V. The voltage is raised to a higher 9V or 12V only when the system confirms it to support higher voltage. Communication between system and adapters can be implemented using VBUS, or by means of a special handshake algorithm or signal to be implemented by D + and D-lines. This new type of mixing, the adjustable voltage adapter can also be used as a universal power source, but also supports traditional 5V voltages as a normal power supply and 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 answers, we need to first understand the battery charging cycle.
There are two working modes in the battery charging cycle: constant current (CC) mode and constant voltage (CV) mode. When the battery voltage is below the regulated charge voltage, the charger operates in CC mode. Once the battery pack voltage reaches a preset regulated voltage, it enters the CV mode. When the actual battery current reaches the termination current, the battery is charged. Termination current is often equivalent to 5% to 10% of the entire fast charging current.
In the ideal charging system, there is no resistance in the battery pack itself, there is only constant current mode. It does not have a CV charging mode with the shortest charging time. The reason is that the charging current will fall into zero and reach the charging termination current immediately as long as the charging voltage reaches a regulated charging voltage.
However, in the actual battery charging system, there is a series of resistors from the battery voltage sensing point to the battery. These resistors include: 1) PCB cable resistance; 2) Two battery charge and discharge protection MOSFET on-resistance; 3) Excessive excessive protection effects in power monitoring and used to measure battery charge current; and 4) The internal resistance of the battery in the relationship between the aging conditions, temperature, and charging state.
When using a 1C charging rate for the new battery, the charger uses about 30% of the charging time in CC mode, and can sufficiently 70% battery capacity. Instead, the charger needs to operate 70% of the total charging time in CV mode to full 30% battery capacity. The larger the internal resistance of the battery pack, the longer the charging time in CV mode. The battery can be fully full when the battery open voltage reaches the maximum charging voltage. If there is a larger resistance between the battery charging voltage sensing point and the actual battery, even if the battery pack senses the voltage reaches the regulator voltage, the real battery open voltage is still below the desired voltage regulator.
For smartphones and tablets, the use of 4A or larger charging currents, is more difficult. Under such a large charging current, the pressure drop on the PCB track or the internal resistors in the battery pack will increase significantly. This will cause the charger to enter the CV mode too early, resulting in dragging the charging time. How can I shorten the charging time due to this high pressure drop?
The pressure drop in the charging path can be estimated to estimate the pressure drop in the charging path by closely monitoring the charging current. This resistance compensation technique called IR compensation can compensate for additional pressure drop in the charging path by increasing the battery regulator voltage. With this technique, the charger can operate in constant current regulating mode as long as possible until the actual battery open voltage is extremely close to the required voltage value. Thus, the charging time in CV mode can be significantly shortened, so that the total charge time is reduced by 20%.
System cooling optimization
To achieve fast charging, you need to use a higher power adapter such as 9V / 1.8A and 12V / 2A. In addition, in addition to charging the battery, the battery charger can be powered by the system. This makes it possible to have one of the highest temperatures in the portable power supply. In order to provide a more desirable end user experience, the maximum difference between the temperature and the ambient temperature of the equipment housing should not exceed 15 ° C. For this reason, the battery charger's power conversion efficiency and system heat dissipation need to meet more stringent requirements. How can we achieve the best cooling performance and the ideal efficiency?
Figure 1: This block diagram represents 4.5A I2C high efficiency switch charger
Figure 1 is a simplified application circuit diagram of a 4.5a high efficiency switch mode charger. This charger can simultaneously support USB and AC adapters, and all MOSFETs are integrated within. MOSFET Q2 and Q3 and inductor L constitute a battery charger based on synchronous switch buck. This combination can achieve the highest battery charging efficiency as much as possible, and the fastest charging speed can be achieved with adapter power. MOSFET Q1 can be used as a battery reverse blocking MOSFET to prevent battery from leaking to the input from the body diode of MOSFET Q2. In addition, it can also be used as an input current sensing element that can monitor the adapter current. MOSFET Q4 can be used to actively monitor battery charging current. All FETs used in the design should have sufficiently low on-resistance to achieve high efficiency. To further improve heat dissipation performance, a heat dissipation regulator loop can also be used. When the tuned temperature reaches a predefined junction temperature, it can avoid breakthrough of maximum junction temperature by reducing the charging current.
Figure 2: Charging time under different charging current comparison: 2.5A and 4.5A
Experimental test results
Figure 2 shows the relationship between the charging current and the charging time. It is easy to understand that the charging speed can be accelerated as long as the speed of the battery charging current does not exceed the maximum current rate specified by the battery manufacturer. 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 will be shortened from 269 minutes to 206 minutes.
Figure 3 shows the advantages of shortening the charging time obtained by IR compensation techniques for the actual charger design. The charging time is reduced by 17%, which can be shortened from 234 minutes to 200 minutes.
Figure 3: Comparative comparison using the IR compensation method. Also use 4.5a charging current, the charging time can be shortened from 234 minutes to 200 minutes. This results can be achieved if the 70mohm resistance is charged for a single 8,000mAh battery. This result can be achieved without increasing additional cost and additional heat dissipation.
Summarize
For numerous portable devices, fast charging is becoming unprecedented. However, this requires a new design idea in the actual charging system, including the use of new high voltage adapters, optimizing charging current and heat dissipation. In addition, advanced charging mode is required to optimize charging time, extend battery life. The above experimental results verify the efficacy of the design for fast charging.
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Source: Wiku Electronic Market Network
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