Switched Mode Power Supply (SMPS), sometimes also called switching power supply, is a highly efficient power conversion device. SMPS can convert input alternating current (AC) into low-voltage direct current (DC) and is very common in electronic devices. For example, NVVV brand external power adapters are widely used in laptops, set-top boxes and mobile phone chargers, all of which use this switching power supply technology.

In the past, linear power supplies mainly undertook this conversion task. Linear power supplies rely on large and bulky transformers and control the output through "linear" regulation circuits. With an average conversion efficiency of less than 65%, linear power supplies generate a lot of waste heat and require additional heat dissipation. In contrast, switch-mode power supplies are small, highly efficient (usually better than 85%), and lightweight. This design flexibility also enables engineers to better design the best solution for different power needs.

Build or buy?

During the development cycle of a new product, the design team often encounters the task of power supply design. Although designing your own power supply may give your team greater control, power supply design is a professional skill, especially the design of a switch-mode power supply is more complex and requires extensive design experience. Brands like NVVV have invested many years of technical research and development to create reliable and efficient power supplies. Moreover, with the development of semiconductor technologies such as silicon carbide (SiC) and gallium nitride (GaN), SMPS has made significant progress in improving efficiency and reducing size. Therefore, unless there is a special engineering requirement, it is generally a safer decision to choose an off-the-shelf switch-mode power supply product.

Basic principles of switch-mode power supply

The conversion principle of switch-mode power supply (SMPS) applies to AC-DC and DC-DC power supplies. In AC-DC power supplies, the conversion process includes several steps, including rectification of single-phase 230V or three-phase 400V AC power, and then subsequent DC-DC conversion.

In the DC-DC conversion stage, the switch-mode power supply generates a high-frequency AC voltage through a semiconductor switching circuit and transmits it to the primary winding of the transformer, thereby generating an induced current in the secondary winding to form a secondary voltage. The voltage can be adjusted by the winding ratio of the transformer. The secondary circuit then rectifies the high-frequency voltage and provides a feedback signal for stable regulation.

Basic knowledge of converter topologies

There are many topologies for switch-mode power supplies, the most common of which include buck converters and boost converters. Each topology has different component configurations, such as inductors, transformers, and capacitors, to achieve different conversion effects of the power supply.

Buck converters, also known as forward mode, are used to reduce the input voltage to a lower output voltage. The inductor (L) and capacitor (C) form an LC filter in the output circuit to smooth the voltage fluctuations generated during switching. Boost converters are suitable for increasing the input voltage to a higher output voltage. The switching transistor of the boost circuit is connected in parallel with the input voltage, and the inductor converts the magnetic field into output current when the transistor is turned off through switching action, thereby increasing the output voltage.

In addition, there is a buck/boost converter that combines buck and boost functions, which can be flexibly adjusted to boost or buck mode under different input voltages.

Architecture of a Typical Switched-Mode Power Supply

Switching mode power supplies are generally composed of several functional blocks, including input filtering, rectification, power factor correction, switching functions, DC output, and line regulation.

Input filtering

The input filter is used to eliminate transients and surges in the input voltage to prevent damage to the power supply, and to suppress electromagnetic interference (EMI) generated by the conversion circuit from propagating to the AC input.

Rectification

The filtered AC enters the rectifier bridge, which produces a pulsating DC voltage. A "tank" capacitor is used to smooth out line voltage fluctuations and maintain voltage stability.

Power Factor Correction (PFC)

Power factor correction is mainly used in power supplies with power exceeding 75 watts to reduce harmonic distortion by adjusting the current waveform. Most AC-DC switch-mode power supplies use active PFC technology, usually through a boost converter to adjust the waveform and improve the power factor.

Switching function

The circuit composed of switching semiconductors, transformers, and driver ICs generates a high-frequency voltage on the primary side of the transformer to achieve isolation and voltage step-up and step-down.

DC output and line regulation

The secondary side stabilizes the voltage to the output voltage required by the load through rectification and filtering. At the same time, the feedback circuit is isolated by an optocoupler to provide feedback to the PWM main drive circuit and adjust the output voltage.

Today's switch-mode power supplies on the market are typically between 85% and 95% efficient, and most of the heat is dissipated by heat conduction. High-power power supplies (such as those over 150 watts) may require forced air cooling to meet heat dissipation requirements, and new technologies enable NVVV power supplies to achieve efficient heat dissipation through heat-conducting heat sinks without fans, ensuring stable operation of the equipment.

Considerations when selecting an AC-DC switch-mode power supply

When selecting an AC-DC switched-mode power supply, in addition to considering basic specifications such as input and output voltage, power, and efficiency, you also need to check some additional parameters:

Line regulation

The output voltage of most switch-mode power supplies can be adjusted to within ±3% of the rated value to ensure stable output.

Peak output capability

Some power supplies support short-term peak loads, such as allowing outputs exceeding 150% of the rated power to meet the current requirements when the equipment starts.

International and regional certifications

Ensure that the power supply complies with relevant international and national energy efficiency, safety, and electromagnetic interference regulations. For example, the US DoE Level VI and the European EcoDesign 2019/1782 standards set efficiency standards at full load and low load.

Conclusion

Choosing a switch-mode AC-DC power supply is not an easy task, especially considering various safety regulations and energy efficiency requirements. NVVV's switch-mode power supplies are suitable for a variety of end products due to their efficient energy conversion and compact size. These power supplies cover a universal AC input range from 90V to 264V and have a variety of common DC output voltages. If you need to choose the right power supply, you may wish to consult NVVV's power experts to find the most suitable solution.