Electromagnetic Compatibility (EMC) refers to the ability of a device or system to operate in its electromagnetic environment without causing unacceptable electromagnetic interference to any device in its environment. Therefore, EMC includes two aspects: on the one hand, it means that the electromagnetic interference generated by the equipment in the normal operation of the equipment cannot exceed a certain limit; on the other hand, it means that the equipment has certain electromagnetic interference in the environment. Degree of immunity, ie, electromagnetic susceptibility. Electromagnetic interference is any electromagnetic phenomenon that degrades the performance of a device or system. The so-called electromagnetic interference refers to the degradation of the performance of a device or system caused by electromagnetic interference.
EMC includes EMI (Electromagnetic Interference) and EMS (Electromagnetic Tolerance). The so-called EMI electromagnetic interference is the electromagnetic noise generated by the machine itself that is not conducive to other systems in the course of performing its intended function; and EMS refers to The ability of the machine to perform functions that are not affected by the surrounding electromagnetic environment. This article first introduced EMC's classification and standards, followed by a description of the causes of the EMC of the switching power supply, and finally introduced the main reasons why the switching power supply EMC could not pass.
EMC (Electromagnetic Tlc CompaTIbility) is electromagnetic compatibility, which includes EMI (electromagnetic disturbance) and EMS (electromagnetic anti-harassment). EMC is defined as the ability of a device or system to function properly in its electromagnetic environment and does not constitute unacceptable electromagnetic disturbance to anything in any device in the environment. EMC's entire name is electromagnetic compatibility. EMP refers to electromagnetic pulses.
EMC = EMI + EMS EMI : Electromagnetic Interference EMS : Electromagnetic Compatibility (Immunity)
EMI can be divided into conducting Conduction and radiation RadiaTIon two parts, the Conduction specification can be generally divided into: FCC Part 15J Class B; CISPR 22 (EN55022, EN61000-3-2, EN61000-3-3) Class B; national standard IT class (GB9254 , GB17625) and AV category (GB13837, GB17625). FCC test frequency in the 450K-30MHz, CISPR 22 test frequency in the 150K--30MHz, Conduction can be used spectrum analyzer test, Radiation must go to a special laboratory test.
EN55022 is the Radiation Test & Conduction Test; EN61000-3-2 is the Harmonic Test (Power Harmonic Test); EN61000-3-3 is the Flicker Test.
CISPR22 (Comite Special des Purturbations Radioelectrique) is used in information technology equipment for Europe and Asia; EN55022 for European standards, FCC Part 15 (Federal Communications Commission) for the United States, EN30220 European EMI test standards, power radiation test standards The frequency of EN55013 is in 30MHZ-300MHz.
The EN55011 radiation test standard is that some frequency bands require higher frequencies and some have lower frequency bands. Conduction (150KHZ-30MHZ) LISN is mainly a differential mode current with a common mode impedance of 100 ohms (50 + 50); LISN is mainly a common mode current with a total circuit impedance of 25 ohms (50 // 50).
EMI is electromagnetic interference, EMI is part of EMC, EMI (Electronic Magnetic Interference) electromagnetic interference, EMI includes conduction, radiation, current harmonics, voltage flicker, and so on. Electromagnetic interference is composed of three parts: interference source, coupling channel and receiver. It is usually called the three elements of interference. EMI linear proportional to the current, the current loop area and the square of the frequency: 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.
EMI refers to the product's external electromagnetic interference. Under normal circumstances are divided into Class A & Class B two levels. Class A is an industrial grade and Class B is a civilian grade. Civil use is stricter than that of industry because industrial use permits a slightly larger radiation. The same product in the test of EMI radiation testing, in the 30-230MHz, Class B requirements radiation limits of the product can not exceed 40dBm and Class A requirements can not exceed 50dBm (in the case of three-meter method of anechoic chamber measurement) is relatively loose Many, in general, CLASS A means that under the EMI test conditions, without operator intervention, the equipment can continue to work as expected, and performance degradation or functional loss below the specified performance level is not allowed.
EMI measures the radiation and conduction of a device when it is in normal operation. During the test, EMI radiation and conduction have two upper limits on the receiver, representing Class A and Class B. If the observed waveform exceeds the B line but is lower than the A line, then the product is Class A. The EMS uses the test equipment to interfere with the product and observe whether the product can work normally under interference. If the work is normal or there is no performance degradation beyond the standard, it is Class A. Automatically restarts and does not exhibit performance degradation beyond the standard after restart. Class B. Can not be restarted automatically restart to C level, hang up to D level. The national standard has Class D regulations, EN only A, B, C. EMI at odd multiples of the operating frequency is the worst.
EMS (Electmmagnetic Suseeptibilkr) Electromagnetic susceptibility is commonly known as “electromagnetic immunityâ€. It is the ability of the device to resist disturbance from the outside, and EMI is external harassment of the device.
The rating in the EMS refers to: Class A. After the test is completed, the device is still working properly. Class B, after the test is completed or the test needs to be restarted, can work normally; Class C, after the manual adjustment, it can be restarted normally and work normally; Class D The device is damaged and it cannot be started no matter how it is adjusted. The strict degree of EMI is B"A, and EMS is A"B"C"D.
Switching power supply EMC interference causes1, the electromagnetic interference generated by the switch circuit
In the EMC design of the switching power supply, engineers must first avoid the electromagnetic interference generated by the switch circuit of the power supply. This is also one of the main interference sources of the switching power supply. The switch circuit is mainly composed of a switch tube and a high-frequency transformer in terms of structure, so the resulting du/dt has a large amplitude pulse, a wide frequency band and a rich harmonic wave. The supply voltage interruption produces the same magnetizing impulse current transient as when the primary coil is connected. This transient is a type of conducted electromagnetic interference that affects both the primary of the transformer and the return of conducted interference to the distribution system. Harmonic electromagnetic interference in the power grid, thus affecting the safety and economical operation of other equipment.
2, electromagnetic interference generated by the rectifier circuit
In the EMC design of switching power supplies, another major source of electromagnetic interference is the rectifier circuit. In some rectification circuits for small and medium-sized power supplies, there is a reverse current when the output rectifier diode is turned off. The time it recovers to zero is related to the junction capacitance and other factors. When the rectifier diode in the high-frequency rectifier circuit is conducting forward, a large forward current flows. When the rectifier diode is turned off due to the reverse bias voltage, there are more carriers accumulated in the PN junction, and thus the carrier current In the period of time before the disappearance of the child, the current will flow in the opposite direction, causing the reverse recovery current in which the carrier disappears to drastically decrease and cause a large current change.
3, electromagnetic interference generated by high-frequency transformers
High-frequency transformers will inevitably produce electromagnetic interference during the operation of the switching power supply. This interference problem is especially common during the testing of large-scale power supplies. The high-frequency switch current loop formed by the primary coil, switch tube and filter capacitor of the high-frequency transformer sometimes generates large space radiation, forming radiated interference, and has a great influence on the EMC design of the power supply. If the capacitance filtering capacity is insufficient or the high-frequency characteristics are not good, the high-frequency impedance on the capacitor will cause the high-frequency current to be conducted into the AC power supply in differential mode to form conductive interference.
4. Disturbance caused by distributed capacitance
Distributed capacitance is a very inconspicuous source of electromagnetic interference in the design of switching power supplies and testing of EMC products. When the switching power supply operates in a high-frequency state, the interference generated by its capacitance is very large. On the one hand, the contact area between the heat sink and the collector of the switch tube is large, and the insulation sheet is thin. The high-frequency current flows through the distributed capacitor to the heat sink and then to the chassis ground. This will cause common-mode interference. On the other hand, there is a distributed capacitance between the primary and the secondary of the pulse transformer, which can couple the primary voltage directly to the secondary side, and generate common-mode interference on the two power lines for the secondary output on the secondary side.
1, the mature circuit can be easily carried out PCB design EMI circuit
The above circuit can imagine the impact of EMC, the input filter is here; anti-lightning pressure sensitive; to prevent the impact of the current resistance R102 (cooperate with the relay to reduce the loss); the key to the difference between the X capacitor and Inductor coupled with the filtered Y capacitor; there are also fuses that affect the layout of the safety regulations; each device here is critical, to savor the function and role of each device. The design of the circuit must consider the EMC severity level design, such as setting several levels of filtering, the number of Y capacitors, and the location. The choice of pressure-sensitive size is closely related to our demand for EMC
Second, circuit and EMC
The circuit in the above figure is circled in several parts: it is very important for EMC (note that the green part is not). For example, radiation is known as electromagnetic field radiation is spatial, but the basic principle is the change of magnetic flux. The magnetic flux involves the effective cross-sectional area of ​​the magnetic field. , which is the corresponding loop in the circuit. Electric currents can produce magnetic fields that produce stable magnetic fields that cannot be converted into electric fields; but changing electric currents produce changing magnetic fields, and changing magnetic fields can generate electric fields (in fact, this is the famous Maxwell equation I use in the popular language). The same electric field can produce a magnetic field. Therefore, we must pay attention to those places where there is a switch state, that is, one of the sources of EMC. Here is one of the sources of EMC. (Of course, one of them will be mentioned later.) For example, a dashed-line loop in the circuit is the opening of the switch. And the loop that shuts off, not only when the circuit is designed, the switching speed can be adjusted to influence EMC, but also the area of ​​the routing loop of the layout board has an important influence!
Third, switching devices and EMC
The understanding of the device also has important significance for EMC, such as MOS tube, the main switch MOS is one of the most important sources of EMC, and the opening and closing of the rectifier will also produce high-frequency radiation (the principle is that the current generates a magnetic field, The changing electric current produces electric field); Of course, here mainly introduces the semiconductor switching device, other inductance transformers do not explain;
Which parameters of the switching device have an important impact on EMC, we often say that fast pipe, what is the reference for slow pipe? We all know that the quick opening of the fast pipe is small, and we like to use it for high efficiency. However, for the smooth passage of EMC, we have to abandon the efficiency and reduce the switching speed to weaken the switching radiation.
For MOS transistors, the turn-on speed is determined by the drive resistance and the input junction capacitance; the turn-off speed is determined by the output junction capacitance and the internal resistance of the tube;
Referring to the above two figures, different types of MOS transistors compare the input junction capacitance and the output junction capacitance, 2400PF and 800PF, 780PF and 2200PF. At first glance, the first specification is the fast tube, and the second is the slow tube. The switching speed must be matched with the driving resistance. In the conventional case, the driving resistance is more than 10R-150R. Selecting the driving resistance is related to the junction capacitance, and the driving resistance can be increased appropriately for the fast board. The resistance of the slow-pipe driving can be appropriately reduced.
For diodes, there are Schottky diodes, fast-recovery diodes, common diodes, and a relatively small number of SIC diodes. Switching speed SIC diodes are almost zero, so there is no reverse recovery, switching radiation is minimal, and losses are minimal. The only drawback is that it is expensive, so it is rarely used; followed by Schottky diodes, the forward voltage is reduced, the reverse recovery time is short, followed by fast recovery and common diodes; the trade-off between loss and EMC is needed; It can adopt measures such as changing the absorption and magnetic beads to rectify EMC.
Fourth, EMC filter
The influence of the filter's architecture choice on the filter is very important. In different occasions, the filter is based on the impedance matching to achieve the filtering effect. Everyone can refer to how to filter according to the principles of this figure; for example, the most commonly used output rectifier bridge is π. Filters and LC filters at the output;
The material of the filter is also crucial for designing the filter inductor. Materials with different initial permeability will work in different frequency bands, and the wrong material will completely lose its effect.
V. EMC's Flyback High Frequency Equivalent Model Analysis
First understand EMC from the simplest model:
EMC path, of course, space radiation is related to the loop, the loop is also constructed by the path; analysis of the high-frequency flyback equivalent model, to help understand the mechanism of EMC formation; our test receiving equipment will receive from the L, N side Conduction, in order to reduce the interference received, it must be allowed to flow through the ground loop without flowing from the L, N port to the receiving device; this time our EMI inductors and Y capacitors can be achieved through impedance matching; in addition the primary side of the interference can be Through the original secondary Y capacitor, the transformer stray capacitance and ground coupled to the secondary side, forming more loops; of course, some of the junction capacitance parameters, such as MOS junction capacitance, heat sink junction capacitance can also constitute the flow path;
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