Three major considerations on EMC in switching power supply systems (transferred)

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Three major considerations on EMC in switching power supply systems (transferred) [Copy link]

Compared with linear regulated power supplies, switching power supplies have many advantages such as low power consumption, high efficiency, small size, light weight, and wide voltage regulation range. They have been widely used in computers and their peripherals, communications, automatic control, household appliances and other fields. However, the prominent disadvantage of switching power supplies is that they can generate strong electromagnetic interference (EMI). EMI signals have a wide frequency range and a certain amplitude. After conduction and radiation, they will affect the electromagnetic environment and cause interference to communication equipment and electronic products. If not handled properly, the switching power supply itself will become a source of interference. At present, the electromagnetic compatibility (EMC) of electronic products has received increasing attention. Suppressing the EMI of switching power supplies, improving the quality of electronic products, and making them meet EMC standards have become issues that electronic product designers are paying more and more attention to. Let's first look at the external environment's interference with the switching power supply, which mainly comes from grid jitter, lightning strikes, external radiation, etc.; as shown in the following figure:

1. Power supply noise? 2. Power supply reset? 3. Power supply output? 4. Power supply damage? And so on. Transient interference (EMS) will threaten the equipment and cause problems with product functions and performance. Later, we will explain the EMS issues of the switching power supply system through the analysis of PCB; Three major issues to consider in the EMC of switching power supply system products. If you understand and find them, you can solve EMC. 1. Signal (source); 2. Structural design; 3. Ground connection.

Analysis of the current path of the common mode and differential mode signals of conducted interference. Switching power supplies usually rectify industrial frequency AC power into DC power, then change it to high frequency through the control of the switch tube, and then output it through the rectifier filter circuit to obtain a stable DC voltage. The industrial frequency rectifier filter uses large-capacity capacitors to charge and discharge, the switch tube is turned on and off at high frequency, and the reverse recovery of the output rectifier diode generates extremely high di/dt and du/dt during the working process, forming strong surge current and peak voltage, which is the most basic cause of electromagnetic interference in switching power supplies. In addition, the driving waveform of the switch tube, the drain-source waveform of the MOSFET, etc. are all periodic waves close to the rectangular wave shape. Therefore, its frequency is at the MHz level, with very high high-frequency harmonic components! These high-frequency signals interfere with the basic signals of the switching power supply, especially the signals of the control circuit. Simply put: when the MOS tube is turned on in the switching power supply system, the current of the transformer inductance in the L and N loops rises linearly; when the MOS is turned off, the current of the L and N loops is quickly turned off; at this time, the current waveform of the loop is a triangular wave; the harmonic components of the high-frequency triangular wave current form the differential mode interference of the system. The characteristics of the coupling channel affected by stray parameters In the conducted interference frequency band (<30MHz), the coupling channels of most switching power supply interference can be described by circuit networks. However, any actual component in the switching power supply, such as resistors, capacitors, inductors, and even switches and diodes, contains stray parameters, and the wider the frequency band studied, the higher the order of the equivalent circuit. Therefore, the equivalent circuit of the switching power supply, including the stray parameters of each component and the coupling between components, will be much more complicated. At high frequencies, the stray parameters have a great influence on the characteristics of the coupling channel, and the existence of distributed capacitance becomes a channel for electromagnetic interference. In addition, when the power of the switch tube is large, the collector generally needs to be equipped with a heat sink. The distributed capacitance between the heat sink and the switch tube cannot be ignored at high frequencies, and it can form radiation interference facing the space and common mode interference conducted by the power line. Simply put: when the high frequency band is greater than 1MHZ, there is distributed capacitance to the ground in the switching power supply system; the key signals and key traces of the system have distributed capacitance to the ground; the distributed capacitance forms a common mode interference signal from the ground back to L and N. At the same time, the loop of the distributed capacitance forms radiation interference to the space! Magnetic field distribution of common mode and differential mode signals of radiation interference.

1. The magnetic field of the differential mode current is mainly concentrated within the loop area formed by the differential mode current, while the magnetic lines of force outside the loop area will cancel each other out; 2. The magnetic field of the common mode current, outside the loop area, has the same direction as the magnetic field generated by the common mode current, but the magnetic field strength is enhanced. Note: An important basic concept about radiation is that current causes radiation, not voltage. Radiation interferes with the field strength characteristics of common mode and differential mode signals. 1. Smaller common-mode current can generate high-intensity radiation; 2. Many factors can cause common-mode current; For example: A. Common-mode interference voltage is introduced into the power grid (part of our EMS simulation test); B. Radiated interference (such as lightning, equipment arc, nearby radio stations, high-power radiation sources) induces common-mode interference on the signal line; C. Different ground voltages. In other words, the difference in ground potential introduces common-mode interference; D. It also includes the impact of the internal wires of the equipment on the power line. Data results - 40dB test data analysis: Data1:A 20mA differential mode current, at 30MHZ, will generate a radiated electric field with a strength of 100uV/m at a distance of 3m. Data2: A 8uA common mode current, at 30MHZ, will generate a radiated electric field with a strength of 100uV/m at a distance of 3m. A small common mode current and a large differential current generate equal RF energy. Reason: Common mode current cannot cancel magnetic lines of force in the RF return path. Analysis of electromagnetic field loops affected by distributed parameters of radiated interference systems

There is distributed capacitance between the switching power supply system and the ground; the key signals and key traces of the system have distributed capacitance between the ground and the ground; the common mode current flows through the distributed capacitance of the signal lines and cables of the system through the layout and wiring to form a common mode interference signal that returns to L and N from the ground. At the same time, the distributed capacitance loop forms radiated interference to space. Among them, >30MHZ and above are received by the radiated emission antenna; that is, the common mode radiation generates an electromagnetic field in space. At this time, after being received by the radiated interference test receiving antenna, it forms the product's external radiated interference. Three major issues to think about EMC of electronic products. A. Where is the signal (source)? B. Is the structure designed like this? C. Is there a better way to connect the ground?