Learn these 4 tips to easily deal with switching power supply EMI
As the basic and mainstream device in the current electric control system, the switching power supply is widely used in many applications such as computers, communications, and electronic equipment. Since there is no alternative equipment, the market size is very large. With the advent of the "low-carbon era", electronic equipment is becoming increasingly miniaturized, thinner, and more energy-efficient, and the market size of switching power supplies has also ushered in further growth.
Global Power Supply Market Size Forecast
According to data released by Markets and Research, the global power supply market size will grow from US$22.5 billion in 2018 to US$34.92 billion in 2023, with a compound annual growth rate of 6.7% from 2018 to 2023.
Figure 1 (Source: China Business Industry Research Institute)
Factors affecting switching power supply
As we all know, a switching power supply uses power semiconductor devices as switching elements and adjusts the output voltage by periodically turning on and off the switch and controlling the duty cycle of the switching elements.
However, due to the poor transient response of switching power supplies, electromagnetic interference (EMI) signals are easily generated, and these EMI signals, through conduction and radiation, will not only pollute the electromagnetic environment, but also interfere with communication equipment and electronic instruments. More importantly, as the size of switching power supplies becomes smaller and the power density increases, EMI control issues are becoming a key factor limiting their use.
Why is EMI important?
EMI stands for Electro Magnetic Interference, which is a performance damage to an electronic system or subsystem caused by unexpected electromagnetic disturbance. Its generation conditions and propagation paths are mainly composed of three basic elements: interference source, coupling path, and sensitive equipment.
What is an interference source? As the name suggests, it is the source of electromagnetic interference. Generally, it is divided into internal interference sources and external interference sources. Internal interference sources include switching circuits, rectifier diodes of rectifier circuits, and stray parameters. External interference sources include power supply interference and lightning interference.
How does the interference source come from? Take the switching circuit as an example. The switching circuit is the core of the switching power supply and is also one of the main interference sources. It is composed of a switching tube and a high-frequency transformer.
Simply put, due to the distributed capacitance between the switch tube and its heat sink and the housing and the leads inside the power supply, the du/dt generated has a large pulse amplitude, a wide frequency band and rich harmonics. When the switch tube load is the primary coil of the high-frequency transformer, it is an inductive load. At this time, the switch tube that was originally turned on is turned off, and the leakage inductance of the high-frequency transformer generates a reverse electromotive force E = -Ldi/dt, whose value is proportional to the rate of change of the collector current and proportional to the leakage inductance. It is superimposed on the turn-off voltage to form a turn-off voltage spike, thereby forming a conducted interference. Of course, not only the switch circuit, but also the rectifier diodes and stray parameters of the rectifier circuit mentioned above are important causes of EMI.
EMI is not a big problem, but if it is not discovered and solved in time, it will take a lot of extra time and money to rectify it later. Especially for some small and medium-sized enterprises, the extremely cumbersome EMI rectification may bring BOM costs and other expensive expenses, and even worse, it will hinder the progress of the later design.
Therefore, we must pay more attention to EMI issues and consider EMI issues at the beginning of the design. The key is to solve it from the source. This article will teach you how to deal with switching power supply EMI.
Four ways to deal with switching power supply EMI
#01
Optimizing Current Paths in Layout
In the design of switching power supplies, PCB design is a key step, which will have a great impact on the performance, EMC requirements, reliability, and manufacturability of the power supply.
Generally speaking, EMI is linearly proportional to the current, the current loop area, and the square of the frequency, that is, EMI=K*I*S*F2. I is the current, S is the loop area, F is the frequency, and K is a constant related to the circuit board material and other factors. This relationship shows that reducing the path area is the key to reducing radiated interference. In other words, the components of the switching power supply should be arranged closely to each other.
Therefore, during the PCB design process, if short and wide PCB traces are used, the voltage drop can be reduced and the inductance can be minimized; at the same time, the component layout can be optimized by using high-frequency switching. A good way to do this for power lines is to place the power line and the return path on adjacent layers of the PCB overlapping each other.
#02
Control device switching speed
In order to improve the power density in the design of switching power supplies, MOSFETs with higher switching frequencies are usually selected. By increasing the switching speed, the volume of the output filter can be significantly reduced, thereby achieving a higher power level per unit volume.
However, as the switching speed increases, the du/dt of the power switch tube when it is turned on/off will also increase, which is precisely one of the main causes of EMI. In addition, high du/dt will also have an adverse effect on the insulation of the motor windings, accelerate the aging of insulating parts such as enameled wires and insulating rings, and bring new challenges to the insulation design of the motor. Therefore, controlling the switching speed of the device and thus reducing the du/dt of the power switch tube when it is turned on/off has also become an important measure to suppress the interference of the switching power supply.
Figure 2: MOSFET equivalent circuit
(Image source: Mouser)
From this point of view, how to choose MOSFET is also a key step. The SuperFET ® V MOSFET from ON Semiconductor sold by Mouser Electronics is a good choice.
SuperFET is a technology developed by Fairchild Semiconductor (acquired by ON Semiconductor in 2015) that adds additional manufacturing steps to reduce R DS(ON) . SuperFET devices can achieve ideal low R DS(ON) in small chip sizes, even when the breakdown voltage reaches 600V . In other words, the device package size using SupeRFET technology can be greatly reduced.
In 2016, Fairchild Semiconductor launched the SuperFET III MOSFET series. The SuperFET V recommended this time is a new generation of high-voltage MOSFET exclusive to ON Semiconductor, which adopts an advanced charge balancing mechanism to achieve excellent low on-resistance and lower gate charge performance. As the fifth generation of high-voltage super junction (SJ) MOSFET, this MOSFET from ON Semiconductor has an excellent quality factor (FOM), which not only improves the heavy load efficiency, but also improves the light load efficiency.
Figure 3: Onsemi SuperFET V MOSFET
(Image source: Mouser)
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It is understood that this series of devices has three product groups, namely FAST, Easy Drive and FRFET, which can provide better performance than similar products in a variety of different applications and topologies, including:
Take Easy Drive for example, it can use charge balancing technology to achieve low on-resistance and excellent performance in terms of lower gate charge. This technology is designed to greatly reduce conduction losses, provide excellent switching performance, and withstand extreme dv/dt rates, which helps manage EMI issues and achieve easier power supply design.
In addition to the SuperFET V series, Mouser Electronics also sells another model of ON Semiconductor called M3S 1200V SiC MOSFET. This series of MOSFETs uses silicon carbide as a material and is optimized for fast switching applications. At the same time, M3S has low switching losses and uses a TO247-4LD package to achieve low common source inductance.
Figure 4: M3S 1200V SiC MOSFET
(Image source: Mouser)
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This series has excellent performance when driven by an 18V gate driver, but is also suitable for 15V gate drivers. It can reduce EON losses while increasing power density, and can be applied to AC-DC conversion, DC-AC conversion, DC-DC conversion and many other aspects.
#03
Reduce the impact of parasitic parameters
In the frequency range of EMI, commonly used passive components are no longer considered ideal, and their parasitic parameters seriously affect their high-frequency characteristics.
Theoretically, the accuracy of parasitic parameter extraction is the key to effectively predicting EMI levels through simulation. Although this is easy to calculate for components with simple structures, it is not easy to obtain for some complex components, such as multilayer boards and DC busbars.
Therefore, when selecting components, it is necessary to select components with small parasitic parameter effects, such as the ESR and ESL of capacitors, and the parasitic capacitance of inductors. In addition, when designing filters, the impact of PCB parasitic parameters on filter impedance must also be considered. After all, its essence is to increase the impedance to interference, so that interference cannot pass through the propagation path.
#04
Protect sensitive circuits
The main circuit of the switching power supply is composed of input electromagnetic interference filter (EMI), rectifier filter circuit, power conversion circuit, PWM controller circuit, output rectifier filter circuit, while the auxiliary circuit includes input over-voltage and under-voltage protection circuit, output over-voltage and under-voltage protection circuit, output overcurrent protection circuit, output short-circuit protection circuit, etc.
Common power supply protection methods include surge protection soft start circuit; overvoltage, undervoltage and overheat protection circuit; phase loss protection circuit; short circuit protection. The figure below is a typical input EMI suppression circuit. When the power grid is struck by lightning, the high voltage is generated and introduced into the switching power supply equipment through the input line. The lightning protection surge circuit composed of FS1, ZNR1 and RTH1 is used for protection.
Figure 5: Input EMI filter circuit diagram
(Image source: Mouser)
The π-type filter circuit composed of R1, R2, C2, C4, LF1 and LF2 is an input filter circuit, which is mainly used to suppress the electromagnetic noise in series with the power grid to prevent interference with the switching power supply. At the same time, it also suppresses the high-frequency noise generated inside the switching power supply from interfering with the power grid, thereby weakening the electromagnetic pollution of the power grid.
It can be seen that the protection of sensitive circuits is also the only choice to solve the EMI problem, which puts forward requirements for the protection function of the device. Take the MP44019 CrM/DCM multi-mode PFC controller of Monolithic Power Systems (MPS) as an example. This controller uses very few external components to provide simplified high-performance active power factor correction and provides very low power supply current, which can achieve low standby power consumption of less than 50mW.
Figure 6: MP44019 CrM/DCM multi-mode PFC controller
(Photo source: MPS official website)
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More importantly, MP44019 integrates multiple protection functions in one, including overvoltage protection (OVP), secondary overvoltage protection (OVP2), overcurrent limiting (OCL), overcurrent protection (OCP), undervoltage protection (UVP), power-on (BI) and power-off (BO), V CC undervoltage lockout (UVLO) and overtemperature protection (OTP).
The device can be generally used in AC-DC conversion, DC-AC conversion and DC-DC conversion, and can use dead zone extension technology to reduce switching frequency under light load. In addition, in discontinuous conduction mode (DCM), variable on-time control is used to reduce total harmonic distortion (THD) compared with traditional constant on-time control (COT).
Figure 7: Typical application circuit diagram of MP44019 controller
(Image source: Mouser)
EMI issues should not be underestimated
Technology is changing with each passing day. With the rapid development of new industries such as artificial intelligence, medical treatment, new energy, and automotive electronics, the demand for switching power supplies will also show a rapid growth momentum. Behind the rapid growth is the more stringent technical requirements put forward by downstream companies for switching power supplies. High efficiency, high power density, and the reliability and stability of the module and the overall system will become the key elements of the switching power supply. In this context, it is imperative to solve the EMI control problem, and the above four tricks are the "magic weapon for winning."
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