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

SWITCHING  POWER  SUPPLY  KNOWLEDGE

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.

Types of switching power supplies

Constant voltage power supply and constant current power supply

　There are many types of switching power supplies. First, it is divided into a “constant voltage power supply” that keeps the output voltage constant and a “constant current power supply” that keeps the output current constant. Just because it is a switching power supply does not mean that it is a constant voltage power supply.

LEDs and batteries are required to have a constant current, and the demand for them is increasing more and more these days. However, the manufacturers of constant-current power supplies are limited, and there are few types of constant-current power supplies, so there are many cases where you have to use a custom power supply.

Therefore, it may be used as a constant current power supply “simplely” by using the overcurrent protection function of the constant voltage power supply. With overcurrent protection, if more current flows than the rated current, the current value remains the same and only the voltage drops. When the current value drops and the overcurrent state disappears, the voltage value automatically returns to the rated voltage.
It can be used as a constant current power supply by changing the operating point of this overcurrent protection with a volume or the like. However, the operation of overcurrent protection is limited to the ” drooping characteristic” power supply, and it cannot be used as a constant current power supply in the “foldback current limiting characteristic” or “H-shaped method”. Only when the accuracy of the current value is rough and constant current control is sufficient.

Standard power supply and custom power supply

Switching power supplies include “standard power supply”, “catalog power supply”, “semi-standard power supply”, “custom power supply”, and “special order power supply”.

The standard power supply and the catalog power supply have the same meaning, and the power supply manufacturer sells the product specifications, dimensions, and data on the catalog and Homepage to an unspecified number of users.
To make it easy for many users to use, makers have obtained safety standards with average electrical specifications, and the shape and mounting are the standard size in the industry. Users can order from one unit, and can buy from the primary distributor, secondary dealer, local trading company, and online dealer of the power supply manufacturer.

The semi-standard power supply is a model with options added to the standard power supply. There are several options for each model. For example, options such as “ON/OFF control“, “variable output voltage volume”, and “vertical terminal block” are identified by adding alphanumeric characters such as R, V, and T at the end of the model name. As for the price, options are added, and the delivery time is about 1 to 2 months. It is wise to think of it as a made-to-order product.

Custom power supply and special order power supply have the same meaning. A power supply designed and manufactured for a specific user according to the specifications of the final product. Therefore, the electrical specifications and shape are special specifications that standard power supplies do not have. To put it the other way around, there was no suitable power supply for the standard power supply, so it can be said that it was unavoidably customized.
Custom power supplies have a variety of uses, but are mainly used in consumer equipment. Power supplies used in consumer equipment must be designed specifically for them due to strict restrictions on function, shape, size, and cost.
Test data and safety standards are costly, so users only get what they need. Custom power supplies should not be custom-made unless the number of units can be expected, as all design and development costs and standard acquisition costs will be borne by the user.

Generally, it is said that the minimum custom line is 500 or more units a month and 50 million yen a year. Custom power supplies often have trouble with the time of termination and the number of lots ordered, so it is necessary to state them firmly in the contract first.

Normally, “standard product = mass production = cheap” and “custom product = small quantity high variety = expensive”, but in the switching power supply industry, the opposite is true “standard product = small quantity high variety = expensive” “custom product = mass production = low price” It is said to be. It is true from the perspective of one user, but standard products are produced collectively by the power supply manufacturer by collecting orders from a large number of users all over Japan.

Shape of switching power supply

　Switching power supplies are classified by shape. “Unit type”, “Board type”, “DIN rail type”, “Onboard”, “Power module”, “Multi slot”, etc. I will explain each of them.

Unit type

The unit type has a box type (unit type) with a chassis and a cover. You can also choose without a cover. The advantage of not having a cover is that it makes it easier to dissipate heat, extending the load factor (derating rate) and the life of the power supply. Under the same environmental conditions, the temperature of the electrolytic capacitor will drop by about 10 ° C, so the life will be doubled.

The standard way to place the power supply is vertically in the photo above. Other placements are possible, but the load capacity (wattage) can only be obtained by 60 to 70% of the standard placement. Since the power supply manufacturer designs the heat based on this standard placement at the time of design, it has the longest load capacity and the longest life.
Check the manufacturer’s specifications for the load factor (derating factor) depending on how the power supply is placed.

　There is only one way to put the power supply, which should never be done. It is a way to place the chassis on the top. If this is used, heat will be trapped inside the power supply and the temperature of the electrolytic capacitor, transformer, and IC will rise too much, causing a trouble. Even the manufacturer does not guarantee this placement.

In addition, the capacity of the unit type power supply is from 15W to 3,000W, and a fan is attached to 300W or more (although 300W or fanless is also available). It has a relatively long life because high-temperature parts such as FETs are brought into close contact with the chassis to dissipate heat. It has a lot of functions and options, and the price is higher than the board type power supply.

Unit-type power supplies are widely used in equipment and devices that require reliability, such as control panels, FA machine tools, semiconductor manufacturing equipment, medical equipment, and communication equipment. In addition, unit-type power supplies account for a large proportion of sales for power supply manufacturers, and the span of discontinued product is more than 20 years.

Board type

The board type is a low-cost power supply for users who do not need a chassis cover (optional chassis and cover can be selected). The best feature is its thinness (height), which is often used side by side with other control boards in the user’s device.

Also, since the board type power supply does not have a fan, heat is dissipated by the heat sink fins on the board, but there is a limit to the size of the heat sink fins, and if the power supply is used vertically, only 60 to 70% of the rated current can be output. The life of the board type power supply is inferior to that of the unit type, but in terms of price, it is 70 to 80% of the unit type power supply, so it is ideal for consumer products and equipment.

DIN rail type

The DIN rail type is a power supply for the control panel, and it can be attached to the DIN rail with one touch by the attachment on the back of the power supply. Since the power supply floats in the air, it is not possible to attach a heavy object, but the structure is such that the wind escapes upward from the air hole on the bottom of the power supply.

The input voltage is generally AC100 / 200V and 3-phase 200V. Depending on the manufacturer, it is also characterized by having three-phase 400V, and the capacity is up to 1500W. Recently, high-performance models with communication that support IoT have also been released, and it is possible to constantly monitor input voltage, output voltage / current value, protection function setting value, cumulative operating time, remaining power supply life time, power supply internal temperature, etc. ..

You can also attach DIN rail brackets to the unit type power supply. Many users use the unit power supply because there are many types of output voltage and the price is cheaper than the DIN rail dedicated power supply.

Onboard

On-board is a type of power supply that is directly mounted on the board, such as a DC / DC converter. The type shown in the photo is used by passing the pin through the hole in the board and soldering it. It is called DIP (Double Inline Package) because the pins are lined up in two rows. A single row of pins is called a SIP (Single Inline Package).

On-board is a type of power supply that is directly mounted on the board, such as a DC / DC converter. The type shown in the photo is used by passing the pin through the hole in the board and soldering it. A pin lined up in two rows is called a DIP (Double Inline Package). A single row of pins is called a SIP (Single Inline Package).

The capacity is about 1.5W to 30W, and recently, a power supply with a wide input voltage is preferred because there are many battery inputs and the voltage fluctuates. It is also characterized by the fact that it does not have an electric capacitor life because it does not use an electrolytic capacitor, which is a life component.

Also, DC / DC converters can be divided into “insulated” and “non-insulated”. Insulation is highly safe because the primary side (input side) and secondary side (output side) are not directly connected electrically. Non-insulated is not insulated, but it is used in circuits where insulation is not required due to its small size and low price.

Power module

The power module is also soldered directly to the pins on the board. For heat dissipation, attach a heat dissipation fin to the aluminum base plate on the opposite side of the pin and blow the wind, or make it adhere to the metal part of the device body to dissipate heat.

The power module cannot be used alone and requires peripheral circuit design. A power module is, so to speak, a package of a part of a power supply circuit. Considering that users design power supplies with discrete parts, using power modules makes circuit design easier and saves design man-hours. It has a capacity of up to 700W and various options are available.

Multi Slot

Source: Cosel ACE Series

The multi-slot has 4 to 6 slots in the output part of the case, and it is used by inserting the output module of the desired voltage and current. It is an epoch-making product that allows the custom-made power supply to be used with the standard power supply, and does not require any development cost, standard acquisition cost, development period, etc. required for the custom-made power supply.

Various output voltages (single output, 2 outputs) are prepared for the modules to be inserted into the slots, and the modules can be connected in series or in parallel. It has acquired various safety standards, has abundant options, and has a capacity of 300w to 3KW. The model name is registered for each combination, and the manufacturer puts the module in and ships it in a completed state.

Switching power supply output

Output power is the value obtained by multiplying the output voltage (V) and the output current (A). The power supply for 5V10A is 50W, and the power supply for 24V4A is 96W.

Output power (W) = Output voltage (V) x Output current (A)

Normally, power supplies with the same rated output power (W) have the same capacity, but in reality, the current value (output power) that can be passed differs depending on the ambient temperature of the power supply. This is because the power (current value) that can be output for each ambient temperature is determined for each power source, more specifically for each output voltage, and for each placement.

The ratio of the power that can be output to the rated output power is called the load factor (derating rate), and the derating table is published in the manufacturer’s catalog or HP as “specification standard” or “instruction manual”. The “ambient temperature” here is not the room temperature but the ambient temperature of the power supply inside the device, and is often specified as room temperature + 15 ° C.

Load factor (%) = output power (W) ÷ rated output power (W) x 100

For example, suppose you have 100W power supplies A and B. Both power supplies A and B can output 24V 4.2A at the same 100W. The derating of power supply A is shown by the black line graph, and the power supply B is shown by the red line graph.
Both power supplies A and B can be used at 100% from an ambient temperature of 0 to 40 ° C, but power supply A can be used at 100% at an ambient temperature of 50 ° C, while power supply B is 80% and power supply A is 80 at an ambient temperature of 60 ° C. Power supply B can be used up to 60%, and power supply A can be used up to 60% at an ambient temperature of 70 ° C, but power supply B cannot be used.

In other words, when the ambient temperature of the power supply reaches close to 50 ° C, power supply A can be used as it is as a 100W power supply, but power supply B becomes a power supply of 80W. Power supply B will have to use the above capacity of 150W.

Also, since power supply B slides 10 ° C to the left of power supply A, it can be said that the temperature is 10 ° C higher than power supply A. Considering Arrhenius’s law of double speed at 10 ℃, power supply B has half the life of power supply A. Actually, the circuit method and parts used are different, so I can’t say it simply, but it is a guide to some extent.

Life of switching power supply

Strictly speaking, there are four life parts of switching power supplies, “photocoupler”, “relay”, “fuse”, and “solder joint” in addition to “aluminum electrolytic capacitor” and “fan”.

The photocoupler has a life due to deterioration over time due to the ambient temperature and CTR (ratio of output current to input current), and the relay has a life if it is turned on and off 100,000 times.

Since the thermal expansion coefficient of the printed circuit board and the electronic component are different in the solder life, stress is applied to the “solder” at the joint, and circular cracks occur around the reed. There is a difference between a double-sided board and a single-sided board, but it does not occur at the level of several years. The symptom is that there are many temporary output stops, and some of the solder cracks are connected. When vibration is applied, it outputs and stops. After that state continues for a while, the output stops completely. This is a problem that tends to occur in places where the power is turned on and off many times a day or where the temperature fluctuates greatly.

Since the glass tube fuse is also a copper wire, it may be said that it has it, but since it must be blown when a current exceeding the specified value flows, the fuse manufacturer generally recommends 10 years as the recommended replacement period.

The power supply has the above life parts, but in general, the aluminum electrolytic capacitor is the shortest life part of the switching power supply.
Aluminum electrolytic capacitors are made by impregnating electrolytic paper with electrolyte and putting it in a case together with lead wires.

Source: Structure of aluminum electrolytic capacitor / Nippon Chemi-Con Co., Ltd.

The life mechanism of aluminum electrolytic capacitors is as follows.

1. The electrolyte inside the aluminum electrolytic capacitor gradually evaporates to the outside.
2. Decrease in capacitance, increase in ESR (resistance value)
3. Electrolyte expansion due to self-heating, promotion of transpiration speed
4. Electrolyte dry-up
5. Open state

The life of an aluminum electrolytic capacitor is defined as when the electrolyte is reduced by 20% from the beginning. The capacity of the electrolyte is called capacitance and is expressed in μF units. Even if the electrolyte drops by 20%, the ripple of the electrolytic capacitor alone will increase slightly, and there will be almost no effect on the power supply. If you continue to use it, it will eventually dry up and open, and the power output will stop. What percentage does not meet the specifications of the power supply varies depending on the power supply and the location of the electrolytic capacitor. However, the electrolyte will decrease at an accelerating rate after it decreases by 20%, so replace it at this time or replace the power supply with a new one.

The life of an electrolytic capacitor is determined by the load factor (derating rate) and ambient temperature. Several to ten or more electrolytic capacitors are used in the power supply, and the electrolytic capacitor with the shortest life is the life of the power supply. Also, the shortest electric capacitor may change depending on how the power is placed.

In the first place, the life of electrolytic capacitors is listed in the catalog as the L0 value by the electric capacitor manufacturer. Generally, a guarantee of 105 ° C and 5,000H is common. It means that the case temperature of the electrolytic capacitor can be used up to 105 ° C and is guaranteed for 5,000 hours.

5,000 hours has a life of only about half a year in continuous operation, but this is the life when the case temperature of the electrolytic capacitor is 105 ° C. Some products have a long life of L0 = 10,000 hours, but it is difficult to install standard products because there are few models and the price is high and the delivery time is long.

It is not uncommon for the board temperature to rise to 100 ° C if no measures are taken, and it is the know-how of power supply manufacturers how to dissipate this temperature and reduce it to a temperature that does not affect electrolytic capacitors and other electronic components.

Normally, electrolytic capacitors (case temperature) are used at about 70 ° C, so if the temperature drops from 105 ° C to 30 ° C, the life will be extended by 2 to the 3rd power = 8 times. In other words, 5,000 hours will extend the life to 40,000 hours (4.5 years).
Some manufacturers publish the calculated lifespan of this electrolytic capacitor on HP, while others do not.

Electronic components mounted on switching power supplies

Many electronic components are used in switching power supplies. Even if you cannot design a circuit, you can understand the power supply circuit block if you understand the part names and roles.

The explanation is based on the electronic components mounted on the 100W board-type power supply.

• Bridge diode (rectifier diode): A module that packages four diodes into one to convert AC waveform to DC full-wave rectification.
• Varistor: Used to protect elements such as ICs from static electricity and lightning surges by utilizing the characteristic that the resistance value changes depending on the voltage.
• Ceramic capacitor (capacitor): An electronic component that stores and discharges electricity (electric charge).
• X-capacitor (capacitor): It is a role to reduce the normal mode noise of the noise terminal voltage by connecting between the phases (between lines) of the power supply line and short-circuiting the lines at high frequency.
• Choke coil (inductor): Selects and removes only the common mode component that is the source of radiation noise.
• MOSFET: Abbreviation for Metal-Oxide-Semiconductor Field Effect Transistor, which translates as “metal oxide semiconductor field effect transistor”. The voltage is controlled by switching with this MOSFET.

• Photocoupler: An IC package containing an LED and a light receiving element (photodiode). When a current is passed through the LED, the light receiving element turns on. The LED and the light receiving element are electrically insulated.

Example of using a switching power supply

VPN router

　It is a VPN router installed for corporate security measures. It is used in an air-conditioned server room with little dust. A single output board type power supply of about 75W is used as the power supply. No L sheet metal and cover are required to fit in the device case. Previously, it was divided into overseas and domestic, but recently the price difference between the AC100V dedicated input power supply and the AC100 / 200V wide input has become smaller, so the overseas and domestic are common.

control panel

　An electric control panel used to control machines and equipment in factories, buildings, and condominiums. The power supply has a 24V output and is often used by fitting it on a DIN rail. Recently, those with a communication function have also been released.

Platform door

The Ministry of Land, Infrastructure, Transport and Tourism has been actively promoting the introduction to each railway company for about 10 years, and it has become commonplace. It is very effective in preventing accidents where people with poor eyesight accidentally fall onto the railroad tracks. The platform door is divided into a control unit and a sensor unit. The control unit uses a 5V / 24V output that operates the control board and motor, and the sensor unit uses a power supply of about 50 to 150W with a 12V output. There is a lot of dust such as vibration and iron powder, and the environment is not good.

FA machine tool

It is in operation at many factories such as CNC lathes (computer control), machining centers (automatic tool change function), and laser machining machines. Since it may be operated for 24 hours and the line stops when the equipment stops, the performance and reliability of the power supply are more important than the price. The power supply used here is often a unit type 5V or 24V single output power supply.

CT scan

It is installed in the laboratory of a hospital, and the operating time is short, and the environmental conditions of the power supply are very good. The power supply uses a 24V output power supply for driving the motor. The installation space for the power supply is small, and we have devised ways to distribute the power supply of about 600W.

FA robot

Abenomics deregulation a few years ago made it possible to operate a robot next to a worker (Cobot) if regulations such as motor capacity were met. The movement of the human arm in the board assembly process of an electric / electronic manufacturer can be substituted for the horizontal movement of the “double-arm robot”. The robot’s arm is made of soft vinyl chloride, and when a person touches it, the sensor automatically stops it. Due to labor shortages, various types of products are being sold by each robot manufacturer.

Dialysis machine

The environmental conditions are very good as it is used in an air-conditioned hospital room. The power supply uses a multi-output power supply that has acquired the IEC medical standard. The capacity is about 600W, and insulation and low leakage current are required. In addition, the power supply cannot be changed because the entire device has acquired medical standards.

LED sign

LED signboards are sometimes used outdoors, so waterproof and dustproof properties are essential for the power supply. A power supply that has acquired the IP standard and is PSE certified is used. In addition, the environmental conditions are extremely harsh because the signboard is exposed to direct sunlight and the temperature inside the signboard rises. Depending on the type of LED module, both constant current power supply and constant voltage power supply are used as the power supply.

Stadium large-scale vision

These days, it is a large-scale vision installed in most baseball stadiums and stadiums. Since thousands of LED modules are arranged side by side, LED color matching is the most important factor. Domestic LED manufacturers with little variation in LED Vf values are often used, and one power supply is installed in each LED module.

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