Switching vs Linear Power Supply: What's the difference

Different electronic devices that we use in everyday life, such as smartphones, LED lights, computers, and robots use different electronic circuits and processors for their operation. The utility supplies AC to different households and industries in order to transfer power efficiently over long distances. A stable DC power supply can be obtained using electronic circuits that can be internal or external in nature. Smartphones and laptops power external power supplies, while devices like computers, robots, and servers have power supplies integrated within them. 

Power supplies can be regulated or unregulated, wherein in regulated power supplies, changes in the input voltage profile do not affect the output of the power supply. On the other hand, in an unregulated power supply, the output depends on the input. 

To illustrate the general structure of a DC power supply we can consider it to consist of four basic blocks. Each block represents a particular function that is performed. The basic blocks are described below:

• Transformer: Since the input supply to the transformer is AC, the transformer converts high voltage AC into low voltage AC or converts a low voltage AC to high voltage AC. The transformer also acts as an isolator preventing high-frequency signals from going into the input side.

• Rectification - This is the process of conversion of AC to DC. Switching converters are used to rectify the voltage. The output of the rectifier circuit is DC along with ripples. These ripples need to filter out before connecting them to the load.

• Filter - The rippled DC output of the rectifier is filtered out using filter circuits. These filter circuits are generally capacitors connected in parallel with the load.

• Voltage regulator - These devices are used to regulate the voltage with certain limits as desired by the load that is connected.

DC power supplies are available in switch-mode (also known as switching) or linear configurations. While both types provide DC power, the method by which this power is generated differs. Each type of power supply has advantages over the other depending on the application. Let's take a look at the differences between these two technologies, as well as the benefits and drawbacks of each design.

What is a linear power supply?

Linear power supplies, also known as series power supplies, are low-frequency power supplies that do not use any switching component in their conversion process. Due to the low frequency of operation, the transformers used are bulkier as compared to switching mode power supplies.

Linear Power Supply

How does it work?

In a linear power supply, an alternating current transformer with an iron core and coil is used first to reduce the voltage of the incoming alternating current (AC). The voltage is then rectified by a diode in a rectifier circuit and smoothed to a stable voltage by a capacitor in a smoothing circuit.

The rectifier circuit produces a series of positive peaks of a sine wave, which is not a stable Direct current. As a result, the voltage is converted to a constant level via a smoothing circuit comprised of a capacitor, followed by a stabilizing circuit (control circuit). Control circuits are classified into two types– shunt and series. To maintain a constant output DC voltage, both methods monitor and control it.

Preferred Applications

A linear power supply requires a dedicated AC power transformer depending upon the input/output voltage and power demand. Each device has a fixed power demand and hence each application requires a dedicated transformer.

A linear power supply is resistant to noise and electromagnetic interference and also does not cause RF interference with other devices. They also have a faster response as compared to switching power supply. Hence, they are used in audio frequency, RF applications, and places where excellent regulation and low-ripple output are required. Some of the common applications are mentioned below.

• Low noise amplifiers

• Signal processing

• Data acquisition - including sensors, multiplexers, A/D converters, and sample & hold circuits.

• Automatic test equipment

• Laboratory test equipment

Advantages & Disadvantages

The benefits of linear mode power supplies include simplicity, reliability, low noise levels, and low cost. A linear power supply has simple construction and design making them less complex as compared to a switching power supply. The simple design makes it easier to operate and maintain. The relatively simpler construction also prevents chances of failure making them more reliable. Linear power supply is resistant to EM interference and does not cause RF interference to other devices, making them ideal for high-frequency operations.

Along with the numerous advantages of linear power supply, there are some disadvantages. Because linear regulators are ideal for low-power applications, the disadvantages are understood when higher power is required. When compared to a switch-mode power supply, the disadvantages of a linear power supply include size, high heat loss, and lower efficiency levels. When used in a high-power application, linear power supply units require a large transformer and other large components to handle the power as the frequency of operation is low. Using larger components increases the overall size and weight of the power supply, which can make weight distribution within a given application difficult. It also makes the power supply bulky and less portable. High heat losses occur while regulating a high power demand by the load, this makes the process inefficient.

What is a switching power supply?

A switching power supply is designed using a different technique, developed to address many of the issues associated with linear power supply design, such as transformer size and voltage regulation. The input voltage is not reduced by switching power supply designs; instead, it is rectified and filtered at the input. The voltage is then passed through a chopper, which transforms it into a high-frequency pulse train. Before reaching the output, the voltage is filtered and rectified once more.  The transformation into a high-frequency pulse train helps reduce the size of components, making the device smaller and more compact in nature.

Switching Power Supply

How does it work?

In contrast to linear power supply, switching power supply includes rectification and smoothing before voltage regulation. Using a switching device for the previously rectified current, the low-frequency AC at the input is transformed into a high-frequency pulse wave. The rectified high-frequency current is treated as a pseudo alternating current (AC) which is fed into a high-frequency transformer reduction in voltage as per load requirements. The output is then filtered and the voltage is regulated generally using feedback from the output to the high-frequency transformer.

Preferred Applications

Switching power supplies have a compact design and are more efficient as compared to linear power supplies making them suitable for applications that require higher efficiency and have to be compact in nature. They are widely used as a source of power for mobile phones, computers, servers, and even LED lights. Some devices have them integrated within their body while others have them externally placed. They have also been used in vehicles and security systems.

Advantages & Disadvantages

Unlike linear power supply, the switching transistors in switch mode supply continually switch between full-on and full-off states and spend very little time in the high dissipation state during transitions, which minimizes wasted energy. This reduces the heat generated by wastage of energy and hence increases efficiency. Another major advantage of switching power supply is the compact size because of the smaller transformer size. This makes the device portable and easy to install.

On the other hand, the design and manufacturing of switching power supply are complex which makes debugging and maintenance difficult as compared to linear supply. This power supply also injects harmonics into the system which can adversely affect other connected devices. The effect of noise and electromagnetic interference is quite significant, thus EMI filters are required for them to operate efficiently.

Linear vs switching power supply

Linear and switching power supplies have their own advantages and disadvantages and are used in specific applications depending on various factors as discussed before in the article. The table below shows the differences between linear and switching power supplies.

Key Takeaways

• There are two common power supply designs– Linear and Switching power supply used for a wide range of applications, including RF, computer, audio, smartphones, etc.

• The linear power supply is designed for low noise and there is no high-frequency switching to be used where regulation and low ripple are required along with low electromagnetic emissions and transient response.

• The switching power supply is designed for high efficiency and small size that converts electrical power efficiently. The power supply regulates the output voltage through the process of pulse width modulation with various available topologies, including buck, boost, forward converter, half-bridge rectifier, or flyback depending on the output power requirements.

• In modern-day electronics and electrical appliances, switching power supply is preferred due to their low cost, compact design, and improved efficiency.

References

[1] [Available online: https://electronicscoach.com/difference-between-linear-supply-and-switch-mode-power-supply.html; Accessed. July 13, 2022.

[2] Available online: https://www.seacomp.com/resources/switching-power-supply-advantages-and-disadvantages; Accessed. July 13, 2022.

[3] Available online: https://www.matsusada.com/column/linear_switching_difference.html; Accessed. July 13, 2022.

[4] Available online: https://www.circuitspecialists.com/blog/power-supplies-switch-mode-vs-linear/; Accessed. July 13, 2022.