EMC includes EMI (electromagnetic interference) and EMS (electromagnetic tolerance). The so-called EMI electromagnetic interference is electromagnetic noise generated by the machine itself in the process of performing its functions, which is not conducive to other systems; and EMS refers to The ability of the machine to be unaffected by the surrounding electromagnetic environment while performing its intended function. This paper first introduces the classification and standards of EMC, and then explains the causes of EMC interference of switching power supply. Finally, the main reason why the switching power supply EMC can't pass is introduced.
EMC classification and standards
EMC (Electromagne TIc CompaTIbility) is electromagnetic compatible and 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 without posing unacceptable electromagnetic disturbances to anything in any device in the environment. EMC's overall name is electromagnetic compatibility. EMP refers to electromagnetic pulses.
EMC = EMI + EMS EMI : Electromagnetic Interference EMS : Electromagnetic Compatibility (Immunity)
EMI can be divided into conduction ConducTIon and radiation RadiaTIon. Conduction specifications can be generally divided into: FCC Part 15J Class B; CISPR 22 (EN55022, EN61000-3-2, EN61000-3-3) Class B; GB IT class (GB9254) , GB17625) and AV class (GB13837, GB17625). The FCC test frequency is 450K-30MHz, the CISPR 22 test frequency is 150K-30MHz, the Conduction can be tested with the spectrum analyzer, and the Radiation must be tested in a special laboratory.
EN55022 is the Radiation Test & Co nduction Test; EN61000-3-2 is the Harmo nic Test; and the EN61000-3-3 is the Flicker Test.
CISPR22 (Comite Special des Purturbations Radioelectrique) is applied to information technology devices for Europe and Asia; EN55022 is the European standard, FCC Part 15 (Federal Communications Commission) is applicable to the United States, EN30220 European EMI test standard, power radiation test standard is The EN55013 frequency is between 30 MHz and 300 MHz.
The EN55011 radiation test standard is that some frequency segments have higher requirements and some have lower frequency requirements. 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 (Electro nic Magnetic Interference) electromagnetic interference, EMI includes conduction, radiation, current harmonics, voltage flashing and so on. Electromagnetic interference is composed of three sources: interference source, coupling channel and receiver. It is usually called the three elements of interference. The EMI linearity is 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 board material and other factors.
EMI refers to the external electromagnetic interference of the product. In general, it is divided into Class A & Class B. Class A is an industrial grade and Class B is a civil grade. Civilian use is stricter than industry, because industrial use allows radiation to be slightly larger. For the radiation test of the same product in the test EMI, at 30-230MHz, the radiation limit of the Class B required product should not exceed 40dBm and the Class A requirement should not exceed 50dBm (for example, the measurement of the three-meter anechoic chamber) is relatively loose. In general, CLASS A means that under EMI test conditions, without operator intervention, the equipment can continue to work as expected, and performance degradation or loss of function below the specified performance level is not allowed.
EMI is the measurement of radiation and conduction when the device is in normal operation. At the time of testing, 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 below the A line, then the product is Class A. EMS is to interfere with the product with the test equipment, and observe whether the product can work normally under the interference. If it works normally or does not exceed the performance degradation specified by the standard, it is Class A. It can be automatically restarted and does not appear to exceed the performance degradation specified in the standard after the restart, which is Class B. Can not automatically restart the need to manually restart to level C, hang up to level D. The national standard has a D-level regulation, and EN has only A, B, and C. The odd multiple of EMI at the operating frequency is the worst.
EMS (Electmmagnetic Suseeptibilkr) Electromagnetic sensitivity is commonly known as "electromagnetic immunity", which is the ability of equipment to resist external disturbances. EMI is the external disturbance of equipment.
The level in EMS refers to: Class A, the device is still working normally after the test is completed; Class B, it can work normally after the test is completed or tested, and Class C needs to be manually adjusted to restart and work normally; Class D The device is damaged and cannot be started anyway. Strict EMI is B"A, EMS is A"B"C"D.
Causes of switching power supply EMC interference
1. Electromagnetic interference generated by the switching circuit
In the EMC design of the switching power supply, the first thing the engineer needs to avoid the rabbit is the electromagnetic interference problem generated from the switching circuit of the power supply, which is 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 that the du/dt generated by it has a large amplitude pulse, and the frequency band is wide and the harmonics are rich. The interruption of the power supply voltage produces the same magnetizing impulse current transient as when the primary coil is turned on. This transient is a conducted electromagnetic interference that affects the primary of the transformer and also causes conducted interference back to the distribution system, resulting in Harmonic electromagnetic interference from the power grid, which affects the safety and economic operation of other equipment.
2. Electromagnetic interference generated by the rectifier circuit
In the EMC design of switching power supplies, another large source of electromagnetic interference is the rectifier circuit. In some rectifier circuits of small and medium-sized power supplies, there is a reverse current when the output rectifier diode is turned off. The time it returns to zero is related to the junction capacitance and other factors. When the rectifier diode in the high-frequency rectification circuit is conducting in the forward direction, a large forward current flows, and when it is turned off by the reverse bias voltage, since there are more carriers accumulated in the PN junction, the current is carried. During a period of time before the sub-disappearance, the current will flow in the opposite direction, causing the reverse recovery current of the disappearance of the carrier to drastically decrease and a large current change.
3. Electromagnetic interference generated by high frequency transformer
High-frequency transformers will also inevitably generate electromagnetic interference during the operation of the switching power supply. This interference problem is especially common during product testing of large power supplies. The high-frequency switching current loop formed by the primary coil, the switching tube and the filter capacitor of the high-frequency transformer sometimes generates large space radiation, which forms radiation interference, and has a great influence on the EMC design of the power source. If the capacitance of the capacitor is insufficient or the high frequency characteristics are not good, the high frequency impedance on the capacitor causes the high frequency current to be conducted in differential mode to the AC power source to form conducted interference.
4, the interference caused by distributed capacitance
Distributed capacitors are a very inconspicuous source of electromagnetic interference during the design of switching power supplies and EMC product testing. When the switching power supply operates in a high frequency state, the interference caused by the distributed capacitance is very large. On the one hand, the contact area between the heat sink and the collector of the switching tube is large, and the insulating sheet is thin. The high-frequency current will flow through the distributed capacitor to the heat sink and then to the chassis ground, which will cause common mode interference. On the other hand, there is a distributed capacitance between the primary and secondary of the pulse transformer, which can directly couple the primary voltage to the secondary side and generate common mode interference on the two power lines with the DC output on the secondary side.
The main reason why switching power supply EMC can't pass
1, familiar with the circuit can be easily carried out PCB design EMI circuit
The influence of the above circuit on EMC can be imagined, the filter at the input is here; the pressure sensitivity of lightning protection; the resistance R102 to prevent the inrush current (to reduce the loss with the relay); the key difference mode X capacitance and The inductor is matched with the filtered Y capacitor; there is also a fuse that affects the layout of the layout; each of the devices here is critical, and the function and function of each device should be carefully evaluated. The EMC severity level to be considered when designing the circuit is designed with ease, such as setting several levels of filtering, the number of Y capacitors, and the position. The choice of pressure sensitive size is closely related to our demand for EMC.
Second, the circuit and EMC
The circuit in the above figure is looped in several parts: it is very important for EMC (note that the green part is not). For example, radiation everyone knows that electromagnetic field radiation is spatial, but the basic principle is the change of magnetic flux, and the magnetic flux relates to the effective cross-sectional area of ​​the magnetic field. That is, the corresponding loop in the circuit. The current can generate a magnetic field, producing a stable magnetic field that cannot be converted to an electric field; but the changing current produces a changing magnetic field, and the changing magnetic field can generate an electric field (in fact, this is the famous Maxwell equation I use in plain language), change The electric field is similar to generate a magnetic field. Therefore, we must pay attention to those places where there is a switch state, that is, one of the EMC sources, here is one of the EMC sources (one of the following will of course talk about other aspects); for example, the dotted loop in the circuit, the switch is opened And the loop that is turned off, not only the switching speed can be adjusted to the EMC when designing the circuit, but also the layout area of ​​the layout routing has an important impact!
Third, switching devices and EMC
The understanding of the device also has important significance for EMC. For example, MOS tube, main switch MOS is one of the most important EMC sources, and the opening and closing of the rectifier also generates high-frequency radiation (the principle is that the current generates a magnetic field, The changing current generates an electric field); of course, the semiconductor switching device is mainly introduced here, and other inductive transformers are not described;
Which parameters of the switching device have an important impact on EMC, we often say that the fast tube, the slow tube is what is the reference? We all know that the fast-opening loss is small, and we like to use it for high efficiency. However, in order for EMC to pass smoothly, we must abandon the efficiency and reduce the switching speed to reduce 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 tubes, compared to the input junction capacitance and output junction capacitance, 2400PF and 800PF; 780PF and 2200PF; at first glance, the first specification is the fast tube, the second is the slow tube, this When the switch speed is determined to match the drive resistance; in general, the drive resistance is more in the 10R-150R. The drive resistance is related to the junction capacitance. The drive resistance of the Allegro can be increased appropriately, and the drive resistance of the slow tube can be appropriately reduced.
For diodes, there are Schottky diodes, fast recovery diodes, common diodes, and a relatively small number of SIC diodes. The switching speed SIC diode is almost zero, which is equivalent to no reverse recovery, minimum switching radiation, and minimal loss. The only drawback is that it is expensive, so it is rarely used; the second is Schottky diode, the forward voltage is reduced, the reverse recovery time is short, followed by fast recovery and ordinary diode; need to compromise between loss and EMC; The EMC can be rectified by measures such as changing absorption and magnetic beads;
Fourth, EMC filter
The choice of filter architecture is very important to the filter. In different occasions, the filter is filtered according to the impedance matching. You can choose how to filter according to the principle of this figure; for example, the most commonly used output rectifier bridge adopts π Type filtering and LC filter at the output;
The material of the filter is also very important for designing the filter inductor. Materials with different initial permeability will work in different frequency segments, and the wrong material will completely lose its proper effect.
V. Analysis of EMC's flyback high frequency equivalent model
First understand EMC from the simplest model:
The path of EMC, of ​​course, the space radiation is related to the loop, and the loop is also constructed by the path; the flyback high frequency equivalent model is analyzed to help understand the mechanism of EMC formation; our test receiving equipment will receive from the L and N terminals. Conduction, in order to reduce the received interference, it is necessary to let the interference flow through the ground loop without flowing from the L, N port to the receiving device; at this time, our EMI inductance and Y capacitor can be achieved by impedance matching; Through the primary secondary side Y capacitor, the transformer stray capacitance and the ground coupling to the secondary side, more loops are formed; of course, some junction capacitance parameters, such as MOS tube junction capacitance, and heat sink junction capacitance can also constitute a circulation path;
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