What Is Switched-Mode Power Supply?
A switched-mode power supply (SMPS) is a power supply that incorporates a highly efficient power conversion circuit called a switching regulator.
Electronic devices with ICs and microcontrollers require stable direct current with minimal voltage fluctuation. There are two types of regulated power supplies: linear power supplies and switching power supplies, but linear power supplies have been the mainstream.
In the case of linear power supplies, the circuit is simple. It converts a 100 V AC voltage into a constant AC voltage and rectifies the AC using the diode’s property of forward current flow but not reverse current flow. It further uses a capacitor to smooth it out. However, this had the disadvantage that it did not allow power supplies to be made smaller or more efficient.
Switch-mode power supplies have solved this problem. While linear power supplies convert commercial alternating current into voltage and then rectify it, switched-mode power supplies rectify commercial alternating current into direct current and then convert it into voltage, which is exactly the opposite method.
Then, by switching on and off, it is converted into pulse wave AC and fed into a high-frequency transformer, which is the mechanism of a switched-mode power supply. The characteristic of switching power supplies is that they are smaller and lighter, but their circuits are more complex.
Applications of Switched-Mode Power Supplies
Compared to conventional linear power supplies, switched-mode power supplies have much more complex circuits, but they have the advantage of being extremely compact due to the use of IC stabilization circuits. Since a large and heavy power transformer (a device that uses electromagnetic induction to convert the height of AC power voltage) is not installed, as is the case with simple power supplies, the size and weight of the power supply can be reduced.
Because of this miniaturization and light weight, applications for switching power supplies include AC adapters for cell phones. They are also highly compatible with small electronic devices such as PCs and tablets, which are often carried around.
Recently, in order to further promote miniaturization, high-power and high-efficiency GaN devices have been incorporated into switching power supplies for AC adapters to realize AC adapters that are even smaller than conventional Si devices.
Principle of Switched-Mode Power Supplies
The principle of switched-mode power supplies is the opposite of that of conventional linear power supplies. Linear power supplies use a transformer to convert commercial current to voltage and then rectify it, whereas switched-mode power supplies rectify commercial alternating current to DC first and then convert it to voltage. However, once rectified, voltage conversion cannot be performed using a transformer.
Therefore, in a switched-mode power supply, the rectified current is converted into pulse-oriented AC by high-speed switching of semiconductor elements such as transistors and MOSFETs, and fed into a high-frequency transformer. This increases the number of components and circuits required and makes them more complex, but this complexity is the key to switching power supplies.
There are several types of control methods for switched-mode power supplies, but a typical one is the pulse width modulation (PWM) method. This method stabilizes the voltage by adjusting the on-time of the switching on/off cycle, or the width of the pulse wave, so that the area of each pulse is the same. It can be said that switching power supplies allow the output to be adjusted by switching on and off, which is another feature that increases efficiency.
In addition, the pulses of a switched-mode power supply have a high frequency of tens to hundreds of kHz, so a small and light transformer is all that is required. However, high frequencies increase the loss of the iron core, so a ferrite core is used. This increases the efficiency of the power supply and saves energy.
The ferrite core is attached to a core rod of a material called ferrite that wraps the cable, which absorbs the magnetic field generated by the high-frequency noise current flowing through the cable and converts it into heat, thereby reducing noise.
The weak point of switched-mode power supplies is the generation of noise due to high-speed switching, which is expected to be reduced by ferrite technology.
Other Information on Switched-Mode Power Supplies
Frequency of Switched-Mode Power Supplies
Switching power supplies convert output voltage to a specified voltage value by adjusting the ON/OFF time for switching operation by semiconductor devices. The frequency of the signal that controls this ON/OFF switching is called switching frequency.
Reason Why 24V Voltage Is Often Used
In electrical products that use switched-mode power supplies, the output voltage required for the power supply is often 24 VDC. The reason for this is that control circuits require that voltage, although there are various theories.
One theory is that since DC was once often powered by batteries, it was determined by an integral multiple of the 1.5V of a dry cell battery cell. In small devices, 6V, 9V, 12V, etc. are also used, but these are also integer multiples of 1.5.
In the days before PLCs (programmable controllers) replaced control circuits used in factory automation, circuits consisted of electromagnetic relays, and the voltage was used to turn on the relays. As a remnant of this, 24V is still often used today. There are also other reasons such as the fact that 24VDC is more resistant to noise environments.
Noise of Switched-Mode Power Supplies
Switched-mode power supplies use switching elements to turn current on and off at high speed, so it is inevitable that they generate high-frequency noise. In the history of the development of switched-mode power supplies, noise suppression has been an essential issue as well as increasing efficiency. Nowadays they are equipped with various noise countermeasures.
Switched-mode power supplies are themselves noise sources. Noise is not only added to the output power line, but also becomes electromagnetic waves that affect electronic equipment.
Noise suppression measures include:
- Reflection
- Filtering inductors and capacitors to prevent the transmission of noise components
- Absorption
Absorb noise with ferrite cores, etc., and convert it into heat or other energy - Bypass
Dropping noise to ground with capacitors, etc. - Shield
Radiated noise components are dropped to the ground with a metal case or absorbed with a ferrite material or other radio wave absorbing material
Detailed Examples of Noise Suppression
Common mode noise and differential noise are two of the most common types of noise.
Common mode noise:
This is noise that leaks through parasitic capacitance generated between the switching power supply circuit board and the equipment chassis, and returns to the power supply side through the GND (ground) as a loop path. It is called differential noise because the direction of the noise current at the positive and negative poles of the power supply is the same for each.
Differential noise:
In this case, the current noise from a noise source in series with a switching power supply circuit returns to the power supply side via the power supply line. As the name implies, the direction of each noise current at the positive and negative poles of the power supply is opposite, and is also called normal mode noise.
Generally speaking, common mode noise is the more radiated of the two types of noise, but countermeasures must be taken for both if the allowable noise level is exceeded. The first method is to shorten the cable length of the path or to use stranded cables.
For more serious countermeasures, a noise filter must be added. Choke coils are effective against common-mode noise. A bypass capacitor to ground, called a pass capacitor, is also used. Differential noise suppression is achieved by connecting capacitors between power lines flowing in opposite directions.