High-efficiency power supply systems for portable electronic devices

For battery-powered portable devices, in addition to breaking through the limitations of processing power, the performance of portable system power supplies also needs to be continuously improved. This article discusses the considerations for portable embedded system power design and the principles that should be followed in the design. These principles are helpful for any portable embedded system power design that has powerful functions and must be powered by batteries. Based on the building blocks described in this article, readers can choose the appropriate devices and design strategies for specific designs.

Power Management Unit

Specifying the specific functional and architectural blocks for the power circuit is not trivial and directly affects the operating time of the battery-powered system. The power system architecture will vary depending on the embedded product and application area. The figure below shows the power scheme in a typical portable embedded system.

Let's define the requirements for each component in the figure. Assume that the product is powered by a battery pack or an external power supply. The function of the power path controller is to switch to the appropriate power source when multiple power sources are available. In some designs, you may need to consider power supply methods including emerging USB and Ethernet Power (PoE).

Battery protection circuits protect the battery from overvoltage, undervoltage, overheating, overcurrent and other abnormal conditions; dedicated battery charging circuits should charge the battery once other power sources are available; fuel gauge circuits continuously monitor the battery charge status and provide battery status information to the user and power management software.

The system may require multiple DS-DC power converters. For example, switch mode power supplies (SMPS), LDO regulators, charge pumps, etc. These different converters are used for all possible input power sources and different voltages required in the product design.

Digital interface or hardware button controllers are responsible for turning the system on and off - sometimes called soft start. In some recently introduced power converters, digital interfaces can also be used to fine-tune the output voltages produced by the various converters. This fine-tuning is necessary in power-conscious power design.

Criteria for Efficient Power Supplies

In embedded applications, power efficiency is not limited to the traditional definition of the ratio of system output power to system input power. In embedded systems, an efficient power solution should meet the following criteria: 1. Provide long device operation when powered by batteries; 2. Extend battery life (charge and discharge cycles); 3. Limit temperature rise of components and the battery itself; 4. Provide integrated software intelligence to maximize efficiency. In

fact, there is no single guideline to maximize the efficiency of a power solution. However, designers consider the following points when developing power systems: Battery life (charge and discharge cycles) depends on the charging characteristics of the battery; for lithium-ion batteries, manufacturers generally recommend following the optimal charging current (constant current mode) and termination/pre-charge current values. When designing the charger circuit, these specifications must be strictly followed.

Battery Management

For consumer electronics, battery protection must be considered a basic feature because it is closely related to the personal safety of the user. Adequate measures must be taken to detect overvoltage, undervoltage and temperature of the battery; appropriate devices such as temperature-dependent resistors must be selected to ensure that the current can be automatically limited regardless of any abnormal conditions; and a fuel gauge must be used. In addition to the normal fuel detection function, it can also ensure battery safety. Most fuel gauges are installed on the battery and can be used to detect battery temperature, discharge current, etc.

For power path controllers, an often overlooked issue is that when switching from one power source to another, no loop can be formed between the two, no matter how short the time. This may require additional reverse-connected diodes or switches. Similarly, when one of the power sources is powered, the voltage of this power source should not be passed to the input of the other power source.

Since there are many available power converter topologies, it is not easy to choose the right power converter. Generally speaking, linear regulators must be avoided when high efficiency and high output current are required.

When using a switching power supply, the designer should ensure that the appropriate topology (buck, boost, buck-boost, charge pump, SEPIC, etc.) is used to ensure that the power supply can maintain the desired output voltage even when the battery voltage drops to the minimum operating value, which helps to extend the operating time of the device.

For buck converters, synchronous converters usually have higher efficiency than non-synchronous converters. However, this architecture selection depends largely on the required output current and duty cycle of the converter under operating conditions. Therefore, the slight efficiency improvement brought by using a synchronous converter is not enough to offset the increased cost.

Different types of inductors used to filter the output ripple of the switching power supply usually have different effects on the converter efficiency. Among various inductor options, inductors with low DC resistance and low magnetic losses at the operating frequency are preferred.

Thermal design should be inseparable from electrical design. The package of each IC or passive component must be able to handle the heat generated under normal operating conditions. Many chip manufacturers recommend using thermal pads with vias and large pads on the PCB to better dissipate heat. Compact embedded products usually do not have room to add fans, but ventilation channels on the PCB and adequate heat dissipation measures must be considered.

Summary

Power supply design is often considered a pure hardware design. However, to achieve an efficient power solution, designers need to add software intelligence to the power circuit. Some basic functions controlled by software include detecting which power source is selected by the power path switch and reducing the supply current to the circuits that are not needed when the battery is powered.

More sophisticated power management software will also include other parameters, such as: the type of application running on the system, the minimum peripheral requirements, the slowest clock frequency, and the minimum voltage required to run this application, and control the state of the power output, clock generator and interface IC accordingly.

Following the above rules of thumb can significantly improve the power performance of portable devices. For example, a typical 30W multi-output power supply solution can achieve an overall efficiency of 85 to 90%. Several integrated circuit manufacturers now offer a range of highly integrated ICs that have all of the above functions. Depending on the power requirements, some applications may require a single-chip solution, while others may use discrete modules. After all, in the competitive embedded product market, battery life and device operating time are key factors affecting buyers' choices.