What is an AC/DC power supply?
The heart of any electronic device is its power supply. This article provides a basic explanation of what AC/DC power supplies are and how they operate.
An AC/DC power supply is a device that converts alternating current (AC) to direct current (DC). AC is typically supplied by a power outlet in a home or office, while DC is used by most electronic devices.
AC/DC power supplies are used in a variety of electronic devices, including computers, cellphones (i.e., wall chargers) and TVs. They are also used in many industrial applications.
Today, a steady DC voltage (e.g., 3.3 VDC, 5 VDC, 12 VDC) is required by most electronic systems and devices. However, AC voltage (100–120 VAC or 200–240 VAC, depending on the country) is the foundation of mains electricity. The supplied AC mains on the input is converted to the necessary DC voltage on the output side by an AC/DC power supply.
Therefore, the primary job of an AC/DC power supply (also known as an AC/DC converter) is to convert the higher, and to some extent hazardous, AC voltage into the lower and safe DC voltage that the device needs.
How do AC/DC power supplies work?
Although there are different types of AC/DC power supplies, each with their own characteristics and operating modes, we can trace the general operating principle to the block diagram in Figure 1.
If we look at the AC diagram in Figure 1 (top left), we see a sine wave. This means that the AC voltage continuously changes its polarity. Typical frequency values are 50 or 60 Hz, depending on the country.
In the first step, the high AC voltage is converted into a lower AC voltage by a transformer. By separating the primary circuit (higher voltage) from the secondary circuit (lower voltage), the transformer also ensures galvanic isolation, thus increasing safety.
Because most devices require a DC voltage on the input side, the AC voltage must be rectified in the next step. The result of this process is shown in the DC diagram in Figure 1 (top center). After the rectification process, we see positive polarity. This is realized by a bridge rectifier, composed of four identical diodes.
For a stable DC supply, the process of rectification isn’t enough. The heavy peaks in the DC curve need to be smoothened. This is done by the output capacitor in the power supply. The capacitor can store energy very fast, which it supplies between two peaks. This process fills the drops to a certain amount and smoothes the curve. The result is a stable DC voltage on the output in Figure 1(top right), which is supplied to the load.
Types & applications
AC/DC power supplies can be grouped into two main classes:
- Linear power supply: This provides one or more stabilized and constant output DC voltages, regardless of the fluctuations of the input voltage or the load. Following the same principle shown in Figure 1, a linear power supply consists of a transformer, a rectifier, a filter and a regulator. The transformer reduces the amplitude of the power grid voltage, the rectifier converts the AC voltage into a DC voltage, the filter stores the energy to smooth the output voltage of the rectifier and the regulator stabilizes and regulates the output voltage. This type of power supply is used in laboratory equipment, medical devices and audio systems.
- Switch-mode power supply (SMPS): A power transistor, alternating between the on and off states, allows energy to be stored and then released to the load, typically using an inductor as an energy-storage element. Unlike linear power supplies, SMPSes instantaneously transmit the power to the load. To switch the transistor, a pulse-width–modulation (PWM) signal is applied. The frequency and duty cycle of the PWM signal modify the value of the output voltage. This type of power supply is widely used in consumer electronics, appliances and industrial machinery.
Advantages & disadvantages
The advantages of linear AC/DC power supplies include their affordability, simplicity and reliability. Compared with SMPSes, linear power supplies are less complicated because of their straightforward design and construction. They are more reliable because of their very simple structure, which reduces the likelihood of failure. Due to their resistance to electromagnetic interference (EMI) and lack of RF interference with other devices, linear power supplies are perfect for high-frequency activities.
However, there are also some drawbacks compared with SMPSes, such as size, excessive heat loss and poorer efficiency levels. Because the frequency of operation is low, linear power supply units need a large transformer and other substantial components to handle the power when employed in a high-power application. The process is inefficient because substantial heat losses happen while regulating a high-power demand by the load.
In contrast to linear power supplies, SMPSes continuously alternate between the on and off states with very little time spent in the high dissipation state between transitions, minimizing wasted energy. This improves efficiency by lowering the heat produced by energy waste. Due to the decreased size of the transformer, another significant benefit of SMPSes is their compact size.
Unlike linear power supplies, SMPSes are more difficult to design and manufacture, which makes debugging and maintenance more challenging. Additionally, this power source introduces harmonics into the system, which may have a negative impact on other attached devices. Due to the enormous impact of EMI and noise, EMI filters are necessary for these devices to function properly.
How can AC/DC power supplies be used in different environments?
AC/DC power supplies are widely used in different contexts and environments, with consumer electronics being one of the main fields of application. In such applications, it is now possible to achieve very high levels of efficiency, as required by the most recent international regulations to reduce electrical-power waste. Furthermore, the use of wide-bandgap materials (especially gallium nitride) obtains efficient, compact and low-heat–dissipation devices.
Other relevant applications include:
- Industrial: Both low-power and high-power AC/DC power supplies are used, providing highly reliable and safe operation of industrial machines.
- Semiconductor-fabrication equipment: Applications like ion implantation, wire bonders, hardware emulators and more require high-power AC/DC power supplies.
- Medical devices: AC/DC power supplies designed for medical applications, such as diagnostic equipment, personal health devices or other medical machinery, must be compliant to strict safety standards.
- Building and factory automation: Compact and robust AC/DC power supplies are required in these application sectors. Widely used for these applications are DIN rail devices (see Figure 2), which can be easily mounted on DIN rail TS-35/7.5 or TS-35/15 rails.