Effective management of the power path improves the efficiency of switching chargers

As portable devices such as smartphones, tablets, and camcorders continue to gain popularity, people's requirements for power sources and the ability to use these devices while charging are increasing. Higher power requirements increase the need for batteries with high power density and excellent charging capabilities. Currently, lithium-ion (Li-ion) batteries and lithium polymer (Li-po) batteries are most suitable for the current market requirements for power density, charging capabilities, and price. However, unlike other popular battery technologies such as lead-acid and nickel-metal hydride, lithium battery technology also has the most unstable performance: if the charging and discharging of lithium batteries are not managed properly, it will lead to problems such as long charging time, high power dissipation, low efficiency, and battery life that is lower than the average life. Figure 1 shows the charging curve of a typical lithium-ion battery.

Traditional chargers are relatively simple and perform well in low-power applications. However, they cannot effectively adapt to changes in the charging curve, such as when users switch between different power sources or operate the device during charging. In addition, traditional chargers are usually less efficient and dissipate more power in high-power and high-current applications.

Figure 1: Charging curve of a typical lithium-ion battery

New linear and switching chargers, such as the MP2600 series from MPS, use power path management technology to change the charging curve, thereby more efficiently powering the battery/system with lower power dissipation. At the same time, these chargers also improve system safety and battery life.

There are many different power management topologies, but this article focuses on three: battery feed, automatic selection, and dynamic power path.

Battery-fed topology

The battery-fed topology is one of the simplest and lowest-cost topologies to implement because its circuit consists of the charger, battery, and system, as shown in Figure 2.

Figure 2: Battery-fed topology schematic and signal diagram

This topology has three main characteristics: no matter how the supply voltage changes, the system voltage is always equal to the battery voltage, the power system always takes priority so that IBATT ￡ ICHG, and ICHG ultimately limits the maximum power provided by the input power supply to the system power bus. This topology can also achieve minimal power dissipation when the system is disconnected from the charger. Setting ICHG fundamentally limits the total input current, so that as the system current (ISYS) increases, the charging current (IBATT) will decrease by the same amount, and the operating waveform is shown in Figure 2.

Unfortunately, this topology has the following drawbacks that limit its efficiency and usefulness in a wider range of applications:

When the battery voltage is too low, the system cannot work. When the battery voltage drops below the trickle charge threshold, the charger will limit the total output current to a very low level. The system's additional power requirements will be supplemented by the battery, causing the battery energy to be further depleted. Since the system voltage is always equal to the battery voltage, once the battery voltage drops below the system's minimum operating voltage, the system will stop working.

Although the battery is fully charged, the charger cannot enter the EOC (end of charge) state. If ISYS exceeds the full battery limit (IBF), ICHG cannot drop below IBF, and the charging status always shows charging, even if the battery is fully charged.

The battery cannot be fully charged. Since the system takes priority over the battery, the battery can only be charged at a low current. In addition, the charger can only work within the expected effective charging time, which can avoid charging a bad battery. If the charging time exceeds this time period, the charger will misjudge the battery as a bad battery and stop charging.

Automatic power path topology selection

The automatic power path selection topology adds two switches to the battery direct topology, so that the system power can switch back and forth between the adapter and the battery according to the input voltage changes. The topology structure and working waveform are shown in Figure 3.

Figure 3: Power path automatic selection topology and operating waveform

Compared with the battery-fed topology, this topology is a substantial improvement. It connects the system directly to the AC adapter and is independent of the charger, so it can provide a larger system current, higher efficiency, and allow the system to operate at a low battery voltage. In addition, it is relatively inexpensive. However, when the adapter output voltage changes greatly, the system voltage will also change accordingly, so this topology requires the system to be able to accept a wider input voltage range. In addition, the adapter is also required to have a higher rated power to meet the maximum total power requirements of the system and charger, as well as the power change requirements when the system load changes suddenly.

Figure 4 is a schematic diagram of the automatic power path selection topology using MPS's MP2611. To prevent instability, the MP2611 disconnects the system from the battery when VBATT approaches VIN. In addition, it inserts a blanking period between S1 (M1 and M2) and S2 (M3) to prevent current penetration, which can damage the system and battery.

Figure 4: Automatic power path selection topology using MP2611

Dynamic Power Path Management Topology (DPPM)

[page] Dynamic Power Path Management (DPPM) technology uses an additional detection module to measure the system voltage or input current and monitor the total power demand in real time. Once the power demand exceeds the preset value, the charger reduces the charging current to ensure that the adapter output power is constant without overloading.

For example, the input voltage-based DPPM (Figure 5) determines whether the input current has reached the adapter's output current limit by comparing the input voltage with a preset reference voltage. If the adapter current has reached the limit, the adapter voltage will drop to the preset reference voltage, and then the charger will prevent the system voltage from continuing to drop by dynamically reducing the charging current. As long as the input current remains at or below the limit, there is still current to charge the battery. However, the instability or noise caused by the system voltage drop makes this voltage-based DPPM structure unsuitable for use in some noise-sensitive applications, such as audio equipment.

Figure 5: Dynamic power path management based on input voltage

The input current-based DPPM (Figure 6) uses a sense resistor to evaluate the input current. When the input current reaches the preset current threshold, it dynamically reduces the battery current to prevent the adapter from overloading or the system voltage from dropping. This ensures the stability of the system voltage and reduces the additional power requirements of the adapter. At the same time, this topology also has the ability to reversely supplement the battery power supply.

Figure 6: Dynamic power path management based on input current

Some chargers (such as MPS's MP2607) can optimize and select different dynamic power path management solutions according to different power requirements. MP2607 intelligently selects between the two topologies of DPPM based on input voltage and input current according to different adapter types. If the input is an AC adapter, MP2607 uses the DPPM technology based on input voltage to control the adapter AC voltage, so that the AC adapter can power the system and charge the battery at the same time. The working waveform is shown in Figure 7.

Figure 7: MP2607 dynamic power path management at AC adapter input

In USB input mode, MP2607 uses DPPM based on input current. As shown in Figure 8, considering the limited current capability of USB, the charging current is set below the USB current limit. If the system load current is greater than the USB current limit, the battery will reversely supplement the power supply.

Figure 8: MP2607 dynamic power path management at USB input

In summary, chargers with dynamic power path management, especially those that can switch between different management modes, can provide more sophisticated power solutions for mobile electronic devices, bringing unprecedented convenience, performance and efficiency to users.