Many systems require battery power. Batteries can be used to provide backup power in the event of a power outage, but are primarily used in mobile devices – as large as electric vehicles and as small as hearing aids. In all battery-powered systems, power efficiency is key. The less efficient the power supply, the larger the battery and the higher its cost for the same run time. In addition, the battery provides different voltages depending on the state of charge. This requires a special power converter to regulate the variable voltage provided by the battery to the stable voltage required by the system electronics. Today, most battery-powered systems use rechargeable batteries instead of non-rechargeable primary batteries. This requires the inclusion of a battery charger in the system. In this article, we will look at various battery charging architectures and some innovative new use cases. Of course, power conversion efficiency is a top priority.
Figure 1 shows a system schematic of a battery-powered system. While the exact implementation will vary depending on the use case, in general, all systems will include the main functional blocks shown in the diagram. There is some kind of supply voltage in the system, which supplies power to the system. This connection usually needs to be switchable. If the power supply is a wall AC power converter, unplugging the low-voltage power cord has the same effect as switching the power switch to the off position in Figure 1. This power path management is necessary to avoid the use of valuable battery power by additional circuitry connected to the power supply. In addition, there is a potential second power supply in Figure 1. With the power switch module, it is possible to switch between the power flow of Power 1 or Power 2. For example, Power 2 can be a USB 5V power supply.
Figure 1Simplified system diagram of a battery-powered system.
This power supply is then converted to safely charge the usable battery, and either directly to power the system. If no input power is available, the energy stored in the battery will power the system through a very efficient switched-mode power converter.
Power efficiency of battery-powered systemsBattery charging usually does not require very high power efficiency. In addition to energy harvesting, most battery-powered systems receive enough power to charge the battery. For example, when a phone is connected to a charger, most people usually don't care about the exact efficiency of the charging process.
However, in energy harvesting systems, power efficiency during charging is critical. Ultimately, higher power efficiency during charging directly results in a smaller energy harvester size, which reduces system cost and can reduce system size.
However, all battery-powered systems will value power conversion efficiency when the battery is discharged. The higher the power conversion efficiency in this process, the smaller the battery capacity required, given the same system run time.
The efficiency of this power conversion stage to generate the voltage required to generate a load from the battery needs to be further evaluated. One is full-load conversion efficiency, which provides information on how long a system can run at nominal load, and light-load efficiency, which is important for many systems. This is the power conversion efficiency at very small loads. In the case of a battery-powered smoke detector, it can operate for many years during the smoke detection phase at low load currents until smoke is detected and an alarm sounds. The alarm is initiated by a high current, but the power efficiency at this stage has little to do with the point at which the battery needs to be replaced.
When the load power consumption is very low, the quiescent current IQ is related to efficiency. The lower the quiescent current, the better. This quiescent current, together with the switching scheme, determines the low load efficiency. Figure 2 shows a typical efficiency curve with and without the light-load efficiency mode. The light load efficiency mode is a blue curve, and the fixed switching frequency mode is a black dotted curve. Many power conversion circuits use this mode to improve light-load efficiency. Typically, it works by stopping the use of a constant switching frequency and only generating a few switching pulses when the output voltage drops slightly. In the time between these bursts, the power converter shuts down many functions to save power consumption. These low-power modes may vary slightly depending on the IC in terms of specific architecture, but these special modes consistently deliver very high efficiency at light loads.
Figure 2The power conversion efficiency of the ADP2370 buck regulator is to activate the low-load power-down mode and to use a fixed 600 kHz switching frequency at all loads.
As shown in Figure 2, the difference in efficiency at a 1 mA output load is considerable. When the power-saving mode is activated at a light load of 1 mA, even down to 100 A load, the power conversion efficiency is 50%. At a fixed switching frequency of 600 kHz without the power-saving mode activated, the efficiency is only about 15%.
Power Conversion ChallengesAs mentioned above, power conversion efficiency is very important in battery-powered systems. Battery-powered systems can choose from all existing types of topologies. One of the commonly used topologies is a four-switch buck-boost converter. Many systems require 33 V supply voltage, powered by a single lithium-ion battery. This battery provides 36 V nominal voltage, but later in the state of discharge, they only provide 28 V to 3Voltage between 0 V. To extend the uptime of the system, we need to use as much energy as possible from the battery. In 3In a 3 V system, when the lithium-ion battery is fully charged, we need to change its voltage from 36 V down to 33 v。However, when the battery discharge is nearing its end, we need to place 28 V boost to 33 v。This requires a buck-boost circuit. There are many different types of buck-boost circuits. To name a few, applicable topologies include transformer-based flyback, dual-inductor single-ended primary inductor converters (SEPICs), and four-switch buck-boost topologies. The four-switch buck-boost topology is typically chosen because it has the highest power conversion efficiency compared to the other two topologies.
Figure 3 shows the concept of a four-switch buck-boost topology.
Figure 3An example of a quad switching buck-boost power converter, such as the LT3154 buck-boost DC-DC converter.
Using two lithium-ion batteries in series instead of one eliminates the buck-boost topology altogether. In this case, only a simple step-down power converter is required. However, we need to put extra effort and cost into the second battery. In addition, charging two batteries is more challenging than charging only one. When two batteries are used in series, the maximum voltage is 72 v。Power converters require higher voltage semiconductor processes instead of the typical maximum 55 V process. This is not a problem, but the semiconductor cost of DC-DC power converters can be slightly higher.
Choose the right battery chargerThere are many battery charger ICs on the market. A battery charger is a device that provides voltage and current in a safe way to charge a battery. When choosing an integrated circuit, you first need to decide whether to use a linear charger or a switching charger. A linear charger is like a linear regulator that can only reduce the available voltage. The input current is roughly equal to the output current.
For example, if the depleted battery voltage is 08 V, the available system voltage is 33 V, then the linear charger must be stepped down to 25 v。If the charging current is 1 A, the linear charger consumes 25 W power. This is possible, but if the system voltage is 12 V, the power consumption will be 112 w。Therefore, for applications where the charging current is low and the system voltage is close to the battery voltage, a linear charger is a reasonable choice.
For all other applications, a switching charger is recommended. Most battery charger ICs on the market are switch-mode battery chargers. These belong to the classic switched-mode power supply (SMPS) devices with special features to support battery charging. It can be charged with constant voltage or constant current, and sometimes both, and special features are also available to ensure safe charging. This can be a timer to detect defects in the connected battery, or it can include a temperature sensor to limit the battery temperature during charging to avoid thermal runaway in different situations. Another popular feature is the safety check between the battery pack and the battery charger, which monitors whether the batteries connected to the system are licensed.
Figure 4 shows a stand-alone SMPS battery charger solution. The MAX77985 is used to implement step-down SMPS battery charger and power path switching. Power path switches are an essential feature for most applications. Once the battery is fully charged, it disconnects the input rail from the battery to prevent battery power from being drained through circuits that may be connected to the input power cord. In addition, the solution has a digital I2C interface that changes some settings of the charger IC as well as for telemetry purposes. To make the battery charger as flexible as possible, the digital interface allows different battery types and battery sizes to be set.
Figure 4The MAX77985 stand-alone battery charger simplifies the circuit diagram.
Among the many different characteristics, there is one item that is particularly noteworthy. The integrated power switch in the MAX77985 not only charges the battery in buck mode, but can also be used to boost the battery voltage to a higher system voltage. In a way, this battery charger is a combination of a system power converter and a pure battery charger.
Battery-powered devices require many different electrical functions. Some products offer only basic functionality, while others highly integrate most of the functionality in a single integrated circuit. This product is called a system power management integrated circuit (PMIC) and is particularly popular in battery-powered applications. There are a variety of reasons for this. One reason is that many battery-powered systems are quite small, requiring a compact system solution. The second reason is that each individual IC has a certain quiescent current, and the IC always consumes some power when it is turned on or off, which eventually drains the battery. In most cases, combining many different integrated circuits into a single PMIC device can reduce the quiescent current of the system.
Over the past 20 years, the advent of high-capacity lithium-ion batteries has changed the face of battery-powered systems. A number of integrated circuits can be used to efficiently charge and discharge these batteries. Today, in order to increase the capacity per unit weight and volume, speed up the charging speed of the battery, and ensure the safety of the battery, the industry is conducting a lot of research into the future battery structure. With the continuous development of battery technology, the innovation of battery charging and discharging integrated circuits will also be endless.
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