The input voltage of the DC/DC converter is direct current (DC), and the output voltage is also direct current (DC). It is an on-board power supply that uses a lot of chip parts and is miniaturized so that it can be mounted directly on a printed circuit board like other electronic parts. Taking advantage of its small size and thinness, it is used in equipment in a wide range of industries such as communication equipment. This DC/DC converter and AC/ DC converter are combined to supply the optimum voltage and current to equipment and devices.

The optimum voltage and current of each load electronic component are different. Standard voltages are 12-14V for motors, 5V or 12V for HDDs, ± 12V for op amps, 3.3V or less for HIC, and so on. In addition, the voltage supplied to electronic components is required to be stable against the input voltage of the DC/DC converter and sudden changes in load.

PC board

It is laptop computers that use DC/DC converters around us. Many DC/DC converters are used to convert the voltage of a laptop battery to the input voltage of electronic components such as 3.3V, 5V, and 12V. The voltage of the battery changes greatly depending on the charge and discharge, but the output voltage of the DC/DC converter is controlled to be constant at all times. Thanks to this DC/DC converter, the electronic circuits of the laptop computer are operating normally.

Types of DC/DC converters

DC/DC converters are roughly divided into “insulated type” and “non-insulated type”. Insulation means that the primary side and the secondary side are not electrically connected, and non-insulation means that the primary side and the secondary side are not electrically connected. To put it simply, it is insulated if a transformer is used, and non-insulated if it is not used.

In other words, a non-isolated circuit that does not use a transformer is called a “chopper”. If you use a chopper circuit, you can step up or down the DC voltage, but you cannot step up or down too much. Practically, the limit is about 10 times the step-up pressure for step-up and about 1/10 for step-down. When a higher buck-boost is required, use an isolated DC/DC converter with a transformer inside the circuit.

Insulated DC/DC converter

DC/DC converter that has a transformer is called an isolated DC/DC converter. The transformer has an important function of input/output isolation, and if this function is required, use the transformer even if the buck-boost ratio is small. It is a highly reliable full-scale power supply that can handle large-capacity power supplies. However, since it has a built-in transformer, it is difficult to reduce the size and cost.

The switch element (transistor) of the DC/DC converter operates at high frequencies of several tens of kHz to several hundreds of kHz. Transformers have the property of becoming smaller as the frequency increases, and the oscillation frequency of DC/DC converters is smaller and lighter in units of KHz compared to transformers that operate at 50Hz or 60Hz.

Also, electrical circuits that may come into contact with humans must be “insulated” by this transformer from dangerous voltages. There are various connectors on the side of the laptop computer, which can be easily touched by humans, so the circuit of the laptop computer is insulated from AC100V with a transformer. Therefore, isolated DC/DC converters are always used in the DC/DC converters of laptop chargers. Thanks to the transformer of the isolated DC/DC converter, you can use your laptop with confidence.

Non-isolated DC/DC converter

The chopper circuit is also called a non-isolated DC/DC converter. Since the non-isolated DC/DC converter does not use a transformer, it has the advantage of being small and inexpensive. Electrically, the primary side and the secondary side are electrically connected, but there is no risk of electric shock because the output voltage is low such as 1.8V, 1.5V, 1.3V, 0.8V.

Also, by not using a transformer, there is an advantage that a non-isolated DC/DC converter can be mounted near the load (IC) of the printed circuit board because it is highly efficient, generates less heat, and does not require a heat sink.

A typical non-isolated DC/DC converter is “POL”. POL is an abbreviation for “Point of Load” and means proximity of loads. Placing a POL in front of the load IC reduces voltage drop and makes it less susceptible to noise.

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.