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The Essentials of AC/DC Power Supplies

What is an AC/DC power supply?

An AC/DC power supply is an essential component in any electronic device. It acts as the heart of the device, converting alternating current (AC) to direct current (DC), which is the type of electricity that most electronic devices run on.

AC voltage is typically supplied through power outlets in homes and offices, but electronic devices require a steady DC voltage for optimal performance. This means that an AC/DC power supply must convert the higher AC voltage into a lower, safer DC voltage. This conversion is what allows electronic devices to operate properly and efficiently.

AC/DC power supplies are used in a wide range of electronic devices, from computers and cellphones to televisions and industrial applications. These devices require a steady DC voltage for proper functioning, and the AC/DC power supply provides this voltage by converting the AC voltage supplied by the power outlet.

The process of converting AC voltage to DC voltage requires specialized components, such as transformers and rectifiers. These components work together to transform the AC voltage into the DC voltage required by the device. It’s important to note that while AC voltage is the foundation of mains electricity, it can be hazardous and pose a risk to electronic devices if not properly managed.

Therefore, the primary role of an AC/DC power supply is to ensure that the electronic device has a steady supply of safe DC voltage. This helps to protect the device from power fluctuations and surges, which can cause damage and result in costly repairs. AC/DC power supplies are also commonly referred to as AC/DC converters, and play a critical role in maintaining the proper functioning of electronic devices.

How do AC/DC power supplies work?

The basic operating principle of AC/DC power supplies is shown in Figure 1 (Source: Shutterstock). The top left shows the AC voltage as a sine wave, which continuously changes polarity with a typical frequency of 50 or 60 Hz. The first step in the process is to convert the high AC voltage to a lower AC voltage using a transformer, which also provides galvanic isolation for safety.

Next, the AC voltage is rectified to DC using a bridge rectifier composed of four diodes. The result of this process is shown in the top center of Figure 1, with positive polarity. The output capacitor in the power supply then smoothens the heavy peaks in the DC curve, resulting in a stable DC voltage on the output (top right), which is supplied to the load.

Different types of AC/DC power supplies have unique characteristics and operating modes, but the general operating principle is shown in Figure 1.

Figure 1

Types & applications

AC/DC power supplies can be classified into two types: linear power supply and switch-mode power supply (SMPS).

Linear power supplies provide stabilized and constant output DC voltages, unaffected by input voltage fluctuations or load changes. They consist of a transformer, rectifier, filter, and regulator, reducing the amplitude of the input voltage, converting AC to DC, smoothing the output voltage of the rectifier, and stabilizing and regulating the output voltage. They are commonly used in laboratory equipment, medical devices, and audio systems.

SMPS, on the other hand, use a power transistor that switches between on and off states to store and release energy to the load, often using an inductor. Unlike linear power supplies, SMPSes instantly transmit power to the load. The output voltage is controlled by the frequency and duty cycle of a pulse-width modulation (PWM) signal applied to the transistor. SMPS are commonly used in consumer electronics, appliances, and industrial machinery.

Advantages & disadvantages

Advantages of linear AC/DC power supplies include affordability, simple design, and reliability. They have a straightforward structure that reduces the chances of failure, and their resistance to electromagnetic interference makes them suitable for high-frequency applications. However, they tend to be larger and less efficient, with high heat loss, when used in high-power applications.

On the other hand, switch-mode power supplies (SMPS) are more compact, with improved efficiency, due to their alternating on and off states that minimize energy waste. However, they are more complex to design and maintain, and can introduce harmonics into the system, requiring EMI filters for proper operation.