With the rapid development of microelectronics technology, practical applications impose more stringent technical requirements on switching power supplies, not only high efficiency, high power density, but also to ensure electromagnetic interference as small as possible to ensure the reliability of the module and the overall system. So how do you overcome the EMI problem of the power supply during design or application?
Today's switching power supply control mode uses pulse width modulation technology (PWM). When working in high-frequency on-off state, high dynamic power devices such as switching tubes, rectifier diodes, and transformers generate strong harmonics during fast transients. Interference noise and spike interference noise, and through the input and output lines, distributed capacitance conduction, space radiation, crosstalk and other coupling paths affect the normal operation of their own circuits and other electronic system equipment.
I. Overview of electromagnetic interference (EMI)
1. Electromagnetic interference (EMI) hazards
In electronic terminal equipment system applications, the harm of electromagnetic interference (EMI) to the system is obvious, mainly as follows:
Affect the performance of electronic terminal equipment, and even cause it to not work properly;
Due to the interference of electromagnetic noise, the high-voltage breakdown and burning of electrical and electronic equipment may cause fire and explosion of surrounding flammable and explosive materials, causing huge economic losses and personal injury;
Electromagnetic wave energy will cause certain harm to the human body and endanger people's health.
2. The source of electromagnetic interference
There are various circuit topologies for the design of high-frequency switching power supplies, but their core noise sources are generated for the same reason: transients, high currents, and high voltages, resulting in higher harmonic interference and Spike interference. Therefore, high dynamic power devices are the source of electromagnetic interference (EMI) noise.
3. Electromagnetic interference (EMI) elements
The conditions and propagation paths of Electromagnetic Interference (EMI) of switching power supply are shown in Figure 1: disturbance source, disturbance path, and sensitive equipment. The conducted disturbance path mainly passes through the common impedance coupling, capacitive coupling and inductive coupling, and the test frequency band is between 150K and 30MHz. The noise mainly based on the spatial path radiation is radiation disturbance interference, that is, the electromagnetic field energy propagates to the surrounding space in the form of field. The test frequency range is between 30M and 300MHz.
Figure 1 Three major elements of electromagnetic interference
4, the standard for measuring noise
Electromagnetic compatibility (EMC) standards are generally defined by a series of electromagnetic compatibility reliability standards customized by domestic and foreign authorities, governments, and even military departments. It is mandatory to ensure that electronic equipment and various unit circuits comply with electromagnetic compatibility (EMC) standards. At present, the electromagnetic compatibility (EMC) standards at home and abroad mainly include CISPR Pubxxx of the European Radio Interference Committee, VDE xxx of Germany and FCC Part xxx of the United States, and GB9254 xxxx of China's customized EMI standard is completely equivalent to the standard of CISPR Pubxxx. In general, as long as the CISPR Pubxxx Class B power supply and electronic equipment are met, they do not affect each other. Table 1 shows the CISPR Pubxxx Class B standard.
Table 1 CISPR Pubxxx Class B Standard
As shown in Table 1, the noise at frequencies from 015 to 30 MHz is conducted interference noise, which is generally propagated by means of common ground lines, power lines, and distributed capacitances; noise sources of frequencies from 30 MHz to 300 MHz are generally transmitted by external interference in the form of spatial propagation. The kind of noise is called radiated noise.
Second, the solution to electromagnetic interference (EMI)
1. Distribution of noise sources
The switching power supply is a strong source of electromagnetic interference, as shown in Figure 2:
When the switching transistor Q1 and the rectifier diode D2 are in the high-frequency switching state, a transient current and a transient voltage of a large amplitude and a frequency bandwidth are generated;
The primary coil of the high-frequency transformer T1 with leakage inductance is the inductive load of the switch tube Q1, so that the high-speed on-off of the switch tube Q1 generates a large surge current and a peak voltage for attenuating the oscillation;
Especially in high-frequency switching power supplies, the distribution of spurious parameters is the main path of the coupling channel, especially the distributed capacitance existing in the circuit;
In the PCB board, the high-order harmonic interference loop generated due to the unreasonable PCB layout and routing;
If the device with low frequency characteristics works at high frequency, its performance will change, which is one of the causes of noise.
Figure 2 Distribution of noise sources
The frequency of these interference noise sources is from a few hundred KHz to several tens of MHz, or even hundreds of MHz, which are broadband noise signal sources. The principle of high-order harmonic noise sources must be generated when designing the input electromagnetic interference suppression filter. Have full understanding and understanding.
2. Conducted interference suppression measures
The conducted interference shown in Figure 2 is generally divided into: differential mode interference (DM) and common mode interference (CM). Differential mode interference refers to the interference noise present between the input line and its ground return line, the output line and its ground return line. Common mode interference refers to noise with the same amplitude of the interference voltage on the input and output lines and their ground return lines. The reference potential is generally dominated by the earth.
1) Power input EMI filter
Figure 3 Input EMI filter circuit diagram
Figure 3 is a typical input EMI suppression circuit. When the power grid is struck by lightning, when a high voltage is generated and introduced into the switching power supply device through the input line, the lightning protection surge circuit is composed of FS1, ZNR1, and RTH1 for protection.
The π-type filter circuit composed of R1, R2, C2, C4, LF1, and LF2 is an input filter circuit, which mainly suppresses electromagnetic noise that is connected in the power grid, prevents interference to the switching power supply, and also suppresses high internal generation of the switching power supply. Frequency noise interferes with the power grid and greens the electromagnetic pollution of the power grid.
Capacitors C1, C3, and C5 are Y capacitors. The choice is mainly to consider the leakage current. The larger the capacitance, the better the EMI effect, but the larger the leakage current, the risk of non-compliance with safety regulations.
Inductors LF1, LF2 are common mode chokes, mainly for common mode noise, generally use high magnetic permeability, take mH level sense value; differential mode interference is mainly filtered by X capacitors C2, C4, generally do not add differential mode inductance, Because it is easy to be saturated.
2) Noise suppression measures for switching tubes and rectifier diodes
During the working process of the switching power supply, due to the junction capacitance of the switching tube and the rectifier diode, a sharp peak is generated during the fast switching, and is transmitted or transmitted through the coupling channel. In addition, the junction capacitance of the switching transistor and the leakage inductance of the primary winding of the transformer may resonate to generate an interference signal. Therefore, the countermeasures that can be used are:
A magnetic bead ring is connected between the D pole and the G pole of the switch tube to reduce the current change rate of the switch tube, thereby achieving the purpose of reducing the peak voltage;
Adding an RC buffer absorption circuit between the gate and the source of the switch tube, thereby reducing the peak voltage generated when the switch tube is turned on and off quickly;
By reducing the voltage difference between the switch tube and the peripheral components, the degree of charging of the junction capacitor of the switch tube is reduced to some extent;
Increase the G-pole drive resistance of the switch. As shown in Figure 4.
Figure 4 Several circuits for spike voltage absorption
3) High frequency transformer noise suppression measures
The high-frequency transformer is an energy storage component of the switching power supply, which generates noise interference during the charging and discharging process of the energy. In particular, the leakage inductance formed by the high-frequency transformer and the distributed capacitance form an oscillating circuit that generates high-frequency oscillations and radiates electromagnetic wave energy outward, causing electromagnetic interference. The countermeasures that can be used for this are:
The transformer is shielded between the primary and secondary, and the copper foil is grounded to isolate the primary and secondary interference noise. The distributed capacitor and the grounded copper foil form a common mode interference noise loop, so that it cannot be transmitted to the secondary side and acts as an electromagnetic shield. ;
Reduce the operating frequency of the switching power supply and slow down the rapid charging and discharging process of energy;
In the transformer, the sandwich winding method is adopted to reduce the leakage inductance and the inter-turn capacitance, reduce the voltage spike, reduce the parasitic oscillation, and reduce the interference.
3. Radiation interference suppression measures
When the switching power supply is working, it will radiate interference to the space:
The amplitude of the radiated noise is inversely proportional to the distance of the radiation source. If the spatial structure is patched and the distance cannot be extended, the shielding technique is adopted;
Since the power input line easily introduces noise through the power grid, and the input line is connected with the high-frequency transformer, a high-frequency dynamic current is generated to generate an electromagnetic field, which is coupled to the output line and generates electromagnetic interference to the power supply target device, so the input line and the output line must be Try to stay away from it;
Wires with dynamic high current flow are as short as possible, as thick as possible, away from low frequency signal lines;
High-power, high-frequency on-off devices should be connected to the ground with the shortest and thickest leads and capacitors when grounding;
The dynamic high current loop is one of the inductive coupling paths of radiation, and the loop area should be as small as possible.
Third, the conclusion
Whether the switching power supply is stable and reliable is the technical core requirement of the entire electrical equipment system. In particular, whether the electromagnetic interference filtering circuit design meets the requirements determines the stability and reliability of the electrical equipment system. When designing the EMI filter circuit of the switching power supply, it should comprehensively consider the suppression of high-power device noise, the layout of the PCB structure, the design of the high-frequency transformer, the grounding, etc., as far as possible to meet the measurement standards of electromagnetic interference noise at home and abroad. The designed switching power supply products can be more widely used in the market and generate value.
For the self-built power module, not only the development cycle is long, the production cost is high, and the consistency and reliability of the product are difficult to guarantee. At this time, a high-quality power module can be selected for product design.
The isolated power supply module independently developed and produced by Zhiyuan Electronics has a wide input voltage range and is isolated from 1000VDC, 1500VDC, 3000VDC and 6000VDC series. The package is available in various forms and is compatible with international standard SIP and DIP packages. At the same time, Zhiyuan Electronics has built a first-class testing laboratory in the industry to ensure the performance of power products. It is equipped with the most advanced and complete testing equipment. The full range of isolated DC-DC power supplies pass the complete EMC test, and the electrostatic immunity is up to 4KV, surge resistance. With a frequency of up to 2KV, it can be applied to most complex industrial sites, providing users with a stable and reliable power isolation solution.
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