[Explanation] What is a switching power supply? You can easily understand the switching power supply!




Although home appliances have a built-in power supply, we do not usually see the power supply. It is a power supply that can be seen and touched by an AC adapter such as a laptop computer.

The circuit method is different between the AC adapter and the “switching power supply”. The circuit of the AC adapter is simple because of cost priority, but the switching power supply is built into the equipment and devices used in factories, hospitals, stations, etc., so stability, reliability, and long life are required.


What is a switching power supply?


 First, I will explain the “power supply” using the laptop computer that we usually use as an example. You can tell that a laptop computer also uses a power supply and can be said to be a black “AC adapter”.

On the other hand, a personal computer has a printed circuit board on which many electronic components such as semiconductor integrated circuits (ICs) are mounted. Switching power supplies play an important role in supplying energy to the electronic components built into these electronic devices.

Many electronic components operate on low voltage direct current (DC). For example, a microprocessor operates at DC 3.3V or less, an LED operates at 12V, a motor or sensor operates at DC 12V, an operational amplifier operates at ± 12V, a driver IC operates at DC 5V, and a DC / DC converter operates at DC 48V.


If AC voltage (AC) is applied to electronic components as it is, the electronic components will be damaged, so it will be converted to low-voltage DC voltage with an AC adapter or switching power supply.

Originally, it would be nice if there was a battery that could be used semi-permanently, but there is no such battery in the first place, and even if it can be used, it takes about half a day. What power supply can be input to a computer anywhere? There is no choice but to take power from the outlet. Therefore, a power supply that converts the alternating current voltage (AC) of the outlet to direct current (DC) is required.

The equipment and devices used in factories and laboratories are the same as those of personal computers, and there are motors, relays, fans, electronic parts, etc. inside, all of which operate at DC voltage. In order for a device or device to operate stably, the switching power supply must supply a stable DC voltage to the device side even if the AC voltage fluctuates. Therefore, the switching power supply is also called “DC stabilized power supply”. At the same time, high reliability (long life), safety (no smoke and ignition), and environmental resistance (low noise) are also required.


Switching power supply and dropper power supply

Heavy and large dropper power supply

The regulated DC power supply is roughly divided into a dropper power supply (linear power supply) and a switching power supply according to the circuit method. Dropper power supplies were a common circuit system 50 years ago, but their size, weight, and cost are 5 to 10 times heavier than switching power supplies. For example, a 300W power supply for a personal computer is not practical because it weighs only a few kilograms with a transformer alone.

The circuit system of the dropper power supply is simple and the voltage is converted by the transformer. Since switching noise is not generated because switching control is not performed, it is currently used only for limited applications of measurement equipment that dislikes noise.

History of switching power supplies

On the other hand, the switching power supply is a compact and lightweight power supply developed by NASA when launching a rocket on the moon about 50 years ago. The dropper power supply was too big and heavy to power the Apollo. A switching power supply creates high-frequency pulses by high-speed switching (ON / OFF) of semiconductors such as transistors and converts the output voltage.

Roughly speaking, turning 100V on 1/2 turns it to 50V, and turning it 1/4 on turns it to 25V. Since current flows only while the transistor is on, it consumes less power and is more efficient. The efficiency of the dropper power supply is 40 to 50%, while the efficiency of the current switching power supply is from a little less than 80% to 95% if it is good. Since it switches at high speed, even a large transformer can be made smaller.

However, since switching power supplies switch at high speed, switching noise is added to the output voltage according to the switching frequency. You can cut the tip of the whiskers of noise by attaching a capacitor to the output terminal, but not all can be cut. Power supply noise is subject to various safety standards and laws and regulations because it can affect not only the power supply itself and the equipment on which the power supply is installed, but also other equipment and the human body. Most countries have established EMC standards based on the international standards IEC (International Electrotechnical Commission) and CISPR (International Special Committee on Radio Interference). For example, there are EN standards in Europe, FCC standards in the United States, JIS standards in Japan, and VDE standards in Germany.

The noise of the switching power supply is about 10 times as large as that of the dropper power supply (series power supply). The noise generated from the power supply is blocked and attenuated by surrounding the power supply with a metal plate or inserting a noise filter. Dropper power supplies are still used in some medical and measuring equipment, but due to the improved performance of noise filters, they are generally used in combination with switching power supplies and noise filters.