Efficient Battery Power Supply Design
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Batteries offer a great power source for electrical devices that need to be mobile or located somewhere where connection to a mains electricity supply or other power source is impossible. Thanks to the foresight of the battery manufacturers, batteries are available worldwide in standard sizes and voltages in various options to suit any application and budget. The biggest problem with battery power is the expectation of users that the device will operate for significant periods with the need for recharging or replacing the batteries. This demand is placing the onus on the designer to improve efficiency and reduce power demand to meet this need.
See my separate article on chasing a minimal power budget for battery-powered projects with a long stand-alone service life requirement.
The simplest method of generating a fixed supply voltage was to connect the appropriate number of batteries in series until the sum of their values produced the required value. This approach was the method of choice with standard single-use 1.5V batteries and no size constraints. Then rechargeable batteries appeared with their standard 1.2V, and the demand from users for smaller, lighter, more portable devices. The solution was to use a boost converter to take the energy from a small number of single-use or rechargeable batteries and generate the required power supply output.
A boost converter is a type of switched-mode power supply that steps up the input voltage to the required output voltage. It achieves this by using an energy store in the form of capacitance or inductance that is switched at sufficient frequency to produce the output needed. The conversion process efficiency will be determined by losses in the L-C circuit and the power requirements of the switching circuit. The good news for designers is that there are plenty of off-the-shelf boost converters from which to choose. They are also known as boost switching regulators and step-up switching converters.
Switching regulators achieve much higher efficiencies than linear regulators because the former use energy storage to perform DC to DC conversion. In contrast, the latter uses resistive losses for conversion that generate heat. Boost switching regulators typically control their output using pulse width modulation to regulate the duty cycle of the switching operation as a closed-loop function. The main losses in this circuit arise due to the diode that provides the feedback element of the control circuit. An efficiency improvement can be obtained by using MOSFET technology to implement the diode function. Additional efficiency savings can come from implementing more complex switching control to skip cycles where switching isn’t required for a period of low loading.
Isolated power supplies are generally less efficient than the equivalent non-isolated counterparts as they require a higher component count and include losses due to the isolating transformer.
Another tool in the designer’s toolkit for extending battery life is the low load efficiency of the device. Low loading is the operating mode where the device is on standby, sleep or is idle, waiting for input or a stimulus to operate. These states typically represent most of the operating life for battery-powered devices, so efficiency savings can significantly impact overall power usage.
Ideally, the power supply should be designed to shut down when there is no load. This approach is essential where the supply has an output L-C filter stage, common with switch-mode designs. In the no-load state, the output voltage will rise to the peak voltage that, at best, is inefficient and, worst-case, results in damage or reduced operating life of the affected components. Regulators can also suffer from unstable feedback conditions where noise present in the circuit becomes the dominant voltage.
Design options for low load conditions include switching to pulse frequency modulation from pulse width modulation, where efficiency improves as load current reduces.
One drawback of switching regulators is the ripple present on the output generated by the switching function. Output filtering and careful PCB layout design can reduce this noise. Still, where the battery-powered device contains sensitive circuitry that requires a noise-free supply, other options need to be explored. Linear regulators do not suffer from this issue, but they are only viable where the battery output voltage is slightly higher than the required device circuit power rails. They cannot step up voltages, and any significant voltage difference in the CD-CD conversion is inefficient.
Hybrid switching regulators are now available to combine elements of a switching regulator with those of a linear regulator to create a low-noise power supply. The linear regulator effectively acts as an output filtering stage. The disadvantages of this solution are the increased complexity of the device, which translates into higher cost, and reduced efficiency. However, for battery-powered devices where a small battery count is necessary, and a switch-mode-based regulator is too noisy, this offers a practical solution.
The limitation of using a linear regulator to clean up the switching regulator’s output is that its performance decreases with increasing noise frequency, adding another trade-off along with physical sizing and efficiency factors when deciding the switching frequency.