Feature: Batteries
Power efficiency in battery-powered systems The charging of a battery usually does not have to be very power efficient. In most battery-powered systems, except energy harvesting, there is enough power available to recharge a battery. For example, when a mobile phone is connected to a phone charger, the exact efficiency of the charging process is typically not relevant to most people. In energy harvesting systems, however, the power
efficiency during the charging process is key. Ultimately, higher power efficiency during charging directly translates to smaller energy harvesters, which reduce system cost and may decrease system size. All battery-powered systems, however, value power
conversion efficiency while the battery is discharged. Higher power conversion efficiency during this process directly translates to smaller battery capacity for the same system operating time. The efficiency of such a power conversion stage from the
battery to generate the voltage required for the load needs to be evaluated further. There is a full load conversion efficiency, which provides information about how long a system may run at nominal load, and there is also the light load efficiency, which matters in many systems. This is the efficiency of power conversion during very light load conditions. For example, when we look at a battery-powered smoke detector, it runs at a low load current smoke detecting stage for up to many years until the moment when smoke is detected and an alarm sounds. The alarm is enabled by a high current, but the power efficiency during this phase is not so relevant for the point in time when batteries need to be replaced. For a very low power load efficiency, the quiescent current
IQ is relevant. The lower, the better. This quiescent current together with the switching scheme determines the low load efficiency. In Figure 2, there is a typical efficiency plot shown with and without light load efficiency mode. Light load efficiency mode is the blue curve and fixed switching frequency mode is the black dashed curve. Many power conversion circuits have such a mode to increase the light load efficiency. Typically, the way it works is that the constant switching frequency is stopped and a few switching pulses are generated only when the output voltage dips slightly. In between those bursts, the power converter shuts down many functions to save power. These low power modes may differ slightly in regard to the exact architecture from IC to IC, but the result of such special modes is always a very high efficiency at light loads. The efficiency difference at 1mA output load is quite high in Figure 2. With power save mode activated at a light load of 1mA (even down to 100μA load), we see 50 per cent power conversion efficiency. At 600kHz fixed switching frequency, without power save mode activated, we only get roughly 15 per cent efficiency.
Power conversion challenges As mentioned, power conversion efficiency is very important
34 April 2025
www.electronicsworld.co.uk
in battery-powered systems. All existing types of topologies may be chosen for a battery-powered system. However, one topology that is used often is the four-switch buck-boost converter. Many systems require 3.3V of supply voltage and are powered by a single lithium-ion battery cell. Such cells provide a nominal voltage of 3.6V, but toward their discharged state, they only provide between 2.8V and 3.0V. For the longest run time of the system, we need to utilise as much energy from the battery as possible. In 3.3V systems, this forces us to step down from 3.6V to 3.3V when the lithium-ion battery is fully charged. However, when the battery is toward the end of its discharge, 2.8V need to be boosted to 3.3V. This requirement calls for a buck-boost circuit. Many different types of buck-boost circuits exist. Just to name a few, suitable topologies include the transformer-based flyback, two-inductor single-ended primary-inductor converters (SEPICs), and the four-switch buck-boost. The four-switch buck-boost is usually chosen since it typically offers the highest power conversion efficiency compared to the other two topologies. Figure 3 shows the concept of a four-switch buck-boost
topology. It is possible to avoid a buck-boost topology altogether
by using two lithium-ion batteries in series, rather than just one. Then only a simple step-down stage power converter is needed. However, this requires additional effort and cost for the second battery cell. Also, the charging of two battery cells is a bit more challenging than just charging a single cell. When two cells are used in series, the max voltage of the two cells in series is 7.2V. This requires a power converter with a higher voltage semiconductor process than the typical 5.5V max processes derivates. This is not a problem but may make the semiconductor of the DC-to-DC power converter slightly higher cost.
Selecting the right battery charger There are many battery charger ICs available on the market. A battery charger is a device that provides voltage and current in a manner that safely recharges batteries. When choosing an integrated circuit, the first decision that needs to be made is whether to use a linear charger or a switching charger. Linear chargers are like linear regulators. They can only step down an available voltage. The input current roughly equals the output current. For example, if a depleted battery has a voltage of 0.8V,
and the available system voltage is 3.3V, the linear charger must step down to 2.5V. If the charge current is 1A, 2.5W is dissipated into heat by the linear charger. While this may be possible, imagine a system voltage of 12V. Here the dissipated power amounts to 11.2W. Linear chargers are a reasonable choice for applications with low charging currents and a system voltage close to the battery voltage. For all other applications, switching chargers are
recommended. Most battery charger ICs available are switch-
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