Tech Tips

Difference between Linear and Switching Power Supplies

Mechanism of linear power supply and switching power supply

Both linear and switching power supplies are power supplies that provide a stable Direct current (also abbreviated as "direct current" or "DC").
Direct current flows through circuits of common electronic devices, such as smartphones, computers, air conditioners, and factory robots. Since each type of device and each internal circuit of such electronic equipment requires different voltages, a Direct current source appropriate for each voltage is necessary.
Of the linear and switching power supplies, the linear power supply appeared first. Linear power supplies are also called series power supplies. In a linear power supply, an AC transformer consisting of an iron core and coil is first used to reduce the voltage to the incoming alternating current (AC). The voltage is then rectified by a diode in a rectifier circuit and smoothed by a capacitor in a smoothing circuit to a stable voltage.
The voltage output from the rectifier circuit is a series of positive peaks of a sine wave, which is not a stable Direct current. Therefore, the voltage is converted to a constant level through a smoothing circuit consisting of a capacitor and then through a stabilizing circuit (control circuit). There are two types of control circuits: shunt and series. Both methods monitor and control the output DC voltage to keep it constant. The difference between the input voltage and output voltage is heat, so a large heat sink is required.
A linear power supply requires a dedicated AC transformer according to the input voltage, output voltage, and power. Therefore, each device has a fixed capacity and requires a power supply with a dedicated transformer for each application.

Difference between Linear Power Supply and Switching Power Supply - How Linear Power Supply (Series Power Supply) Works

Switching power supplies, on the other hand, feature rectification and smoothing prior to voltage regulation. A pulse wave is converted to a pulse wave using a switching device for the previously rectified current. By turning the switch on and off at high speed, the rectified current is treated as a pseudo alternating current (AC) with a pulse wave, and the voltage is regulated using a high-frequency transformer.

Difference between Linear Power Supplies and Switching Power Supplies - How Switching Power Supplies Work

Comparison of linear power supply and switching power supply

The typical difference between a linear power supply and a switching power supply is the amount of noise and the size of the power supply itself. Let us first explain the difference in noise.
As mentioned above, the switching power supply repeatedly turns the switch on and off at high speed. This causes noise due to switching. In terms of noise, linear power supplies produce less noise than switching power supplies.
Comparing a linear power supply and a switching power supply that can produce the same output, the noise of the linear power supply (R4G18-2) is 0.5 mVrms and 1 mArms in the datasheet specifications, while the switching power supply (R4K18-2) is about 5 mVrms and 5 mArms.
In other words, in this case, the voltage noise of a switching power supply is ten times greater than that of a linear power supply, and the current noise is five times greater than that of a linear power supply.
Next is the size of the power supply. Linear and switching power supplies are smaller, with switching power supplies being smaller. The reason for this stems from the size of the transformer. If the voltage is varied by the same amount, the transformer will be smaller at higher frequencies.
In a linear power supply, the frequency input from an outlet or other source is input directly to the transformer, whereas in a switching power supply, the current after rectification is sent to the transformer as a high-frequency pulse. Therefore, the transformer can be small.
For example, when compared with a power supply that can output a DC voltage of 18V, the linear power supply (R4G18-2) measures 124 x 84 x 325 mm (HxWxD) and weighs about 3 kg. On the other hand, the switching power supply (R4K18-2) measures 124 x 35 x 128 mm (HxWxD) and weighs about 500 g.
Both are the same height, but the linear power supply is 2.4 times wider and 2.5 times deeper than the switching power supply and weighs six times as much. This is because transformers use an iron core, and the need for a larger transformer affects the weight of the transformer.
Switching power supplies came into general use around 1990, later than linear power supplies. The old AC adapters were very large and heavy because they used linear power supplies.
In recent years, GaN (gallium nitride) power supplies with low energy loss have also been commercialized.
GaN power supplies are semiconductors made of gallium nitride, just like conventional silicon-based semiconductors. Transistors using GaN have lower power loss than conventional transistors.
The development of these new technologies has led to the creation of smaller, more powerful switching power supplies.

Other differences

There are many differences between linear and switching power supplies, in addition to noise and magnitude. Other differences are explained here.

Switching power supplies have less energy conversion loss.

Switching power supplies were originally developed by NASA for use in space. Therefore, they are developed for high energy efficiency. Switching power supplies are more efficient because linear power supplies release more energy as heat.

Linear power supplies respond more quickly to load fluctuations

The output of the switching power supply is controlled by the control circuit. Linear power supplies, on the other hand, are controlled by the response of the regulator circuit. Therefore, the linear power supply has a faster response to load fluctuations.

If the output power is small, the linear power supply can be made cheaper, and if it is large, the switching power supply can be made cheaper.

Linear power supplies are cheaper to make because of their simpler structure. However, as mentioned above, linear power supplies have lower efficiency and generate more heat than switching power supplies, so when power consumption increases, measures must be taken for the power consumed by the linear power supply itself and the heat emitted by it.
Therefore, as the power consumption increases, the total cost of the switching power supply will be lower. As a rough guide, linear power supplies can be operated at a lower cost if the power is lower than 400W.

The differences between linear and switching power supplies described so far are summarized in the table below.

Linear power supply Switching power supply
Noise (electric) Small Large
Circuit (electric) Simple Complex
Power supply unit size Big and heavy Small and light
Conversion loss (heat generation) Large Small
Response Fast A little slow
Low cost Advantageous in low-power power supply High-power (high-power) power supply