Mastering Embedded Power Management for Embedded Systems

Table of Contents:Mastering Embedded Systems Design Series

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The embedded power system is integrated into embedded systems to provide DC power to embedded devices. It is a safe, reliable, and high-performance power supply system. Generally, the input to the embedded power supply is AC mains, and the output is common DC voltages of 12V, 5V, and 3.3V, making it a type of secondary power supply device.

AC power supplyis one of the important energy sources for embedded systems. The power for embedded systems is directly or indirectly provided by this type of power supply. Usually, mains power is used as input, which is transformed through a series of operations to convert high-voltage AC into low-voltage DC.

Batteriesare the primary power source for many embedded systems, such as mobile phones and sensors. Battery-powered devices typically have relatively low power consumption and long operating times. Therefore, embedded systems have strict power consumption requirements, and under different application scenarios, the battery capacity may need to be increased.

Voltage regulatorsare commonly used components in conjunction with AC power supplies and batteries. Since embedded systems often require multiple voltages, voltage regulators are used to reduce the voltage to the required range.

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1. Power Management

A typical hard requirement for embedded systems is to reduce power consumption, as many embedded devices often use battery power and are left unattended for long periods, making power consumption a critical issue. When battery capacity is limited or the number of devices is large, the system’s power consumption becomes crucial.

First, the vast majority of embedded systems will include basic power management functions to reduce power consumption.

(1) System Power-Up Behavior

Components of embedded systems can often only enter low-power mode after the system has started up normally, so they usually operate at a higher power during power-up. Many devices do not need to work during power-up, so effective management of these devices is necessary to reduce power consumption during startup.

(2) Idle Mode

CMOS circuits consume effective power only when the circuit clock is operational, so power consumption can be reduced by turning off unnecessary clocks. Modern embedded systems often use components that provide wake-up functionality through external events, so during periods when certain modules are not in use, the main processor can send a “sleep” command to the relevant components to indicate that they should enter a low-power state. When a specific triggering event is needed to wake the device back up, it can be done through that event.

(3) Power Off

Due to reverse bias leakage, circuit components still consume energy in low-power mode, so for components that consume significant energy in low-power mode or are not used for long periods, power-off treatment can be done to reduce power consumption.

(4) Voltage and Frequency Scaling

Effective power is linearly proportional to switching frequency but proportional to the square of the supply voltage. Running at a lower frequency during full clock frequency and then idling does not save much power. In this case, power can be savedby lowering the voltage.

For example

A certain embedded system’s digital circuit section requires a branch power supply, with an input voltage of 220V AC. The power management module first uses a switch-mode power supply to convert 220V AC into DC voltage, and then uses a low-voltage linear regulator to supply power to each sub-module, as shown in the block diagram in Figure 3-1.

In the power generation circuit, to avoid mutual interference between analog and digital signals, the input 220V AC voltage is converted into two independent DC power supplies, which then separately power the analog and digital circuits. For example, this project design requires different voltages of 12V, 24V, 5V, 8V, -8V, 3.3V, and the corresponding power management system topology is shown in Figure 3-2.

Specific implementation is as follows:

① +12V to +8V

Using LM7808, a three-terminal integrated voltage regulator circuit that can accurately reduce voltage to +8V, ensuring that the input is a 12V DC power supply, with a sufficient voltage drop over the output regulated value of 8V. Care must be taken to avoid overloading the current. In the specific design,the capacitors at both ends of the circuit serve to filter, smoothing the voltage and improving anti-interference capability. The output end can be paralleled with a 220HF/25V electrolytic capacitor, which has a low harmonic frequency and can perform energy storage filtering to eliminate low-frequency interference. However, due to the presence of certain inductance in large electrolytic capacitors, they cannot effectively filter high-frequency signals and pulse interference signals,so in the design, it is common to parallel one or more capacitors with smaller capacitance values to achieve filtering of high-frequency interference signals, as shown in the design in Figure 3-3.

② +12V to -8V

Using the NE555 chip, a chip that combines analog and logic functions well. This chip is an 8-pin integrated circuit, released around 1971 by Signetics, and was the only very fast commercial chip at that time. It has been widely used over the past 40 years, extending many application circuits. The CMOS technology version of the chip (such as Motorola’s MC1455) has been widely used, but the original NE555 specifications continue to be supplied. Although the new IC has some functional improvements, the pin functions have not changed, so it can still be directly replaced in a wide range of applications. A typical power conversion circuit it implements is shown in Figure 3-4.

In its design, when the third pin of NE555 outputs a high level, it charges C1 through D1, and the voltage can reach 11V. When NE555 outputs a low level, D1 is reverse-biased and cut off. C2 transfers charge to C3, and after several repetitions, the voltage of C3 reaches 8V, which is considered -8V relative to the ground.

③ +12V to +5V

Using the switching type integrated voltage regulator chip LM2596, which contains a fixed frequency oscillator and a reference voltage regulator, and has complete protection circuits, thermal shutdown circuits, current limiting, etc. LM2596 is a buck-type power management integrated circuit that can output 3A driving current while having good linear and load regulation characteristics. Fixed output versions are available for 3.3V, 5V, and 12V, while the adjustable version can output various voltages less than 37V. A typical circuit diagram for +12V to +5V using LM2596 is shown in Figure 3-5.

④ +5V to +3.3V

Using LM1117-3.3, which is also alow-dropout linear voltage regulator, as long as the input voltage is within the allowable range, its output voltage can be stabilized at a specific voltage. The circuit for +5V to +3.3V using LM1117-3.3 is shown in Figure 3-6.

⑤ +24V to +5V

Directly using WD5-24S5, the DC-DC power module WD5 series has characteristics such as 5W output power, wide voltage input, input/output isolation, and miniaturized packaging.

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