Discussing the design of power supply for embedded systems

Below we 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, it may be necessary to consider power supply methods including emerging USB and Ethernet Power ( PoE ).

The battery protection circuit protects the battery from damage due to overvoltage, undervoltage, overheating, overcurrent and other abnormal conditions; the dedicated battery charging circuit should charge the battery once other power sources are available; the fuel gauge circuit continuously monitors the battery charge status and provides battery status information to the user and power management software.

The system may require multiple DC-DC power converters, such as 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 within the product design.

A digital interface or hardware push-button controller is responsible for turning the system on and off—sometimes called soft-start. In some recent power converters, the digital interface can also be used to fine-tune the output voltages produced by the various converters. This fine-tuning is necessary in power-conscious power supply designs.

The standard 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. When powered by batteries, the device can work for a long time; 2. Extend battery life (charge and discharge times); 3. Limit the temperature rise of components and the battery itself; 4. Provide integrated software intelligence to maximize efficiency.

In reality, there is no single guideline to maximize the efficiency of a power solution. However, designers consider the following when developing a power system: Battery life (number of charge and discharge cycles) depends on the battery's charging characteristics; for lithium-ion batteries, manufacturers generally recommend following the optimal charging current (constant current mode) and termination/pre-charge current values. When designing a 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; suitable devices such as temperature-dependent resistors must be selected to ensure that the current can be automatically limited under any abnormal conditions; and a fuel gauge must be used. In addition to the normal power 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.

An often overlooked issue with power path controllers is that when switching from one supply to another, no loop should be formed between the two, no matter how short the time. This may require additional reverse-connected diodes or switches. Likewise, when powered from one supply, the voltage from that supply should not pass to the input of the other supply.

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

Where a switching power supply is used, designers 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 extend the operating time of the equipment.

For step-down converters, synchronous converters are usually more efficient than asynchronous converters. However, the choice of architecture depends largely on the output current and duty cycle required for the converter to operate. 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 switching power supplies usually have different effects on converter efficiency. Among various inductor options, inductors with low DC resistance and low magnetic loss at the operating frequency are preferred.

Thermal design should be closely tied to electrical design. The packaging of each IC or passive device must be able to handle the heat generated during normal operation. 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.

Figure: Power management solution for a typical portable embedded system.

Conclusion

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 of software control include detecting which power source is selected by the power path switch and reducing the supply current to unnecessary circuits 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 status of power outputs, clock generators, and interface ICs accordingly.

Following the above rules of thumb can significantly improve the power performance of portable devices. For example, the overall efficiency of a typical 30W multi-output power solution can be as high as 85 to 90%. Currently, several integrated circuit manufacturers can provide a series of highly integrated ICs with various functions mentioned above. Depending on different power requirements, some applications may require a single-chip solution, while others may use discrete modules. After all, in the highly competitive embedded product market, battery life and device working time are key factors affecting buyers' choices.