EMC classification and standards
EMC (Electromagnetic 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 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 two parts: conduction conduction and radiation Radiation.
The Conduction specification can generally be 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 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.
EMI is electromagnetic interference, EMI is part of EMC, EMI (Electronic 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.
Radiated interference (30MHz - 1GHz) is transmitted through space and in the characteristics and laws of electromagnetic waves. But not any device can radiate electromagnetic waves.
Conducted interference (150K--30MHz) is the interference that propagates along the conductor. Therefore, the propagation of conducted interference requires a complete circuit connection between the interferer and the receiver.
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, CLASSA 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, and EMS is A > B > C > D.
EMI circuit:
The role of the X capacitor:
Suppress differential mode noise, the larger the capacitance, the better the low frequency noise suppression effect.
The role of Y capacitor:
Suppress common mode noise, the larger the capacitance, the better the low frequency noise suppression effect. The Y capacitor provides a low-impedance loop from the secondary to the primary ground, causing the current flowing back through the LISN to be directly short-circuited. Since the Y capacitor is not completely ideal, there is also impedance between the secondary sections, so it is impossible to return all of them. There is still a part that flows to the ground. The Y capacitor must be connected directly to the primary and secondary cold ground with as short a straight line as possible. If the dv/dt of the MOS is greater than the dv/dt at turn-off, the Y capacitor is connected to the primary ground; otherwise it is connected to V+.
The role of common mode inductance:
Suppress common mode noise, the greater the inductance, the better the low frequency noise suppression effect. Increase the impedance of the common mode current section to reduce the common mode current.
The role of differential mode inductance:
Suppress differential mode noise, the greater the inductance, the better the low frequency noise suppression effect.
General response strategy before switching power supply design
AC input EMI filter
There are usually two ways in which the interference current is transmitted on the wire: the common mode and the differential mode. Common mode interference is the interference between the carrier fluid and the earth: the interference size and direction are the same, and exist in any relative ground of the power supply, or between the neutral line and the earth, mainly generated by du/dt, and the di/dt also produces a certain total. Mode interference. The differential mode interference is the interference between the carrier fluids: the interference is equal in magnitude and opposite in direction, and exists between the power phase line and the neutral line and between the phase line and the phase line. When the interference current is transmitted on the wire, it can be either common mode or differential mode; however, the common mode interference current can only interfere with the useful signal after it becomes the differential mode interference current.
The above two types of interference exist on the AC power input line, usually low-band differential mode interference and high-band common mode interference. Under normal circumstances, the differential mode interference amplitude is small, the frequency is low, and the interference caused is small; the common mode interference amplitude is large, the frequency is high, and the radiation can be generated by the wire, and the interference caused is large. If an appropriate EMI filter is used at the input end of the AC power source, electromagnetic interference can be effectively suppressed. The basic principle of the power line EMI filter is shown in Figure 1. The differential mode capacitors C1 and C2 are used to short-circuit the differential mode interference current, while the intermediate connection grounding capacitors C3 and C4 are used to short-circuit the common mode interference current. The common mode choke is composed of two coils that are equal in thickness and wound in the same direction on a magnetic core. If the magnetic coupling between the two coils is very tight, the leakage inductance will be small, and the differential mode reactance will become small in the power line frequency range; when the load current flows through the common mode choke, the series The lines of magnetic force generated by the coils on the phase line are opposite to the lines of magnetic force generated by the coils connected in series on the center line, which cancel each other out in the core. Therefore, even in the case of a large load current, the core does not saturate. For the common mode interference current, the magnetic fields generated by the two coils are in the same direction, which will exhibit a large inductance, thereby attenuating the common mode interference signal. Here, the common mode choke coil is made of a ferrite magnetic material having a high magnetic permeability and a good frequency characteristic.
Figure 1 Basic circuit diagram of power line filter
Improve the switching waveform with the absorption loop
During the turn-on and turn-off of the switch or diode, due to the transformer leakage inductance and line inductance, the diode storage capacitor and distributed capacitance, it is easy to generate a spike voltage at the collector, emitter and diode of the switch. The RC/RCD absorption loop is usually used, and the RCD surge voltage absorption loop is shown in Figure 2.
Figure 2 RCD surge voltage absorption loop
When the voltage on the absorption loop exceeds a certain range, the devices are quickly turned on, thereby venting the surge energy and limiting the surge voltage to a certain amplitude. The saturable core coil or the microcrystalline magnetic bead is connected in series on the positive electrode lead of the collector of the switch tube and the output diode, and the material is generally cobalt (Co). When the normal current is passed, the core is saturated, and the inductance is small. Once the current flows in the opposite direction, it will generate a large back EMF, which effectively suppresses the reverse surge current of the diode VD.
Switching frequency modulation
The frequency control technique is based on the energy of the switching interference, which is mainly concentrated on a specific frequency and has a large spectral peak. If these energy can be dispersed over a wider frequency band, the purpose of lowering the peak of the disturbance spectrum can be achieved. There are usually two methods of processing: the random frequency method and the modulation frequency method.
The random frequency method adds a random perturbation component to the circuit switching interval, so that the switching interference energy is dispersed in a certain frequency band. Studies have shown that the switching interference spectrum changes from the original discrete spike interference to continuous distributed interference, and its peak value is greatly reduced.
The modulation frequency method adds a modulated wave (white noise) to the sawtooth wave, forms a sideband around the discrete frequency band in which the interference occurs, and expands the discrete frequency band modulation of the interference into a distributed frequency band. In this way, the interference energy is spread over these distributed frequency bands. This control method can well suppress the interference during turn-on and turn-off without affecting the operating characteristics of the converter.
Soft switching technology
One of the interferences of the switching power supply is the du/dt from the on/off of the power switch. Therefore, reducing the on/off of the power switch on/off is an important measure to suppress the interference of the switching power supply. The soft switching technology can reduce the on/off of the on/off of the switch.
If a small resonant component such as an inductor or a capacitor is added to the switching circuit, an auxiliary network is formed. Leading the resonance process before and after the switching process, the voltage is first reduced to zero before the switch is turned on, so that the phenomenon of voltage and current overlap during the opening process can be eliminated, and the switching loss and interference can be reduced or even eliminated. This circuit is called a soft switching circuit. .
According to the above principle, two methods can be used, that is, the current is zero before the switch is turned off, and no loss or interference occurs when the switch is turned off. This shutdown mode is called zero current shutdown; or the switch is turned on. Before the voltage is zero, no loss or interference will occur when the switch is turned on. This turn-on mode is called zero voltage turn-on. In many cases, turn-on or turn-off is no longer indicated. Only zero-current switches and zero-voltage switches are called. The basic circuit is shown in Figure 3 and Figure 4.
Figure 3 Zero voltage switching resonant circuit
Figure 4 Zero current switching resonant circuit
Soft-switching circuit control technology is usually adopted, combined with reasonable component layout, printed circuit board wiring and grounding technology, it can improve the EMI interference of switching power supply.
Using electromagnetic shielding measures
Generally, electromagnetic shielding measures can effectively suppress electromagnetic radiation interference of the switching power supply. The shielding measures of the switching power supply are mainly for the switching tube and the high frequency transformer. When the switch tube works, it generates a large amount of heat, and it needs to be equipped with a heat sink, so that a large distributed capacitance is generated between the collector of the switch tube and the heat sink. Therefore, an insulating shielding metal layer is placed between the collector of the switching tube and the heat sink, and the heat sink is connected to the casing, and the metal layer is connected to the zero end of the hot end to reduce the coupling capacitance between the collector and the heat sink, thereby reducing Radiation interference from the heat sink. For high-frequency transformers, the magnet-conducting structure should first be selected according to the shielding properties of the magnetizer. For example, the can-type iron core and the El-type iron core have good shielding effect of the magnetizer. When the transformer is shielded, the shielding box should not be placed close to the transformer, and a certain air gap should be left. If a multilayer shield with air gap is used, the resulting shielding effect will be better. In addition, in high-frequency transformers, it is often necessary to eliminate the distributed capacitance between the primary and secondary coils, and an open-loop ring made of copper foil can be placed between the coils along the entire length of the coil to reduce the disaster between them. In combination, this open circuit ring is connected to the iron core of the transformer and to the ground of the power supply to provide electrostatic shielding. If conditions permit, install a shield on the entire switching power supply, which will better suppress radiated interference.
The actual rectification strategy of EMI after switching power supply design--conducting part
Differential mode interference within 1MHZ
1, 150KHZ-1MHz, mainly differential mode, 1-5MHz, differential mode and common mode work together, after 5MHz is basically common mode. The divisional fit and the inductive fit of differential mode interference. Generally, interference above 1 MHz is common mode, and low frequency band is differential interference. Use a resistor to string a capacitor and then to the pin of the Y capacitor. Use an oscilloscope to measure the voltage across the two pins of the resistor to estimate common mode interference.
2. Add the differential mode inductor or resistor after the insurance;
3. The low-power power supply can be processed by a PI filter (it is recommended to use a larger electrolytic capacitor near the transformer).
4, the front-end π-type EMI parts of the differential mode inductance is only responsible for low-frequency EMI, the volume is not too large (DR8 is too large, can use the resistance type or DR6 better) otherwise the radiation is not good, if necessary, string magnetic beads, because The high frequency will fly directly to the front end and will not follow the line.
5, when the conduction cold machine exceeds the standard at 0.15-1MHZ, there is 7DB balance when the heat engine. The main reason is that the primary BULK capacitor DF value is too large, the ESR is relatively large in the cold machine, the ESR is relatively small in the heat engine, the switching current forms the switching voltage on the ESR, it will flow between a current LN line, which is the differential mode. interference. The solution is to use a low ESR electrolytic capacitor or a differential mode inductor between the two electrolytic capacitors.
6, test 150KHZ total over-standard solution: increase the X capacitor to see if it can be down, if it is down, it is differential mode interference. If there is not much effect, then it is common mode interference, or the power line is wound on a large magnetic ring for a few turns, which means that it is common mode interference. If the interference curve is very good, reduce the Y capacitor, look at the board for problems, or just add a magnetic ring in front.
7. It is possible to increase the inductance of the single winding inductor of the PFC input section.
8. The components in the PWM line will adjust the main frequency to about 60KHZ.
9. Use a piece of copper to attach to the transformer core.
10, the common mode inductance of the two sides of the sense of asymmetry, one side of the number of turns less than one can also cause conduction 150KHZ-3MHZ exceeds the standard.
11. There are two main points in the generation of general conduction: 200K and 20M. These points also reflect the performance of the circuit; around 200K is mainly the spike caused by leakage inductance; about 20M is mainly the noise of the circuit switch. Poor handling of the transformer will increase a large amount of radiation, and the shielding is useless, and the radiation cannot pass.
12. Change the input BUCK capacitor to a low internal resistance capacitor.
13. For the non-Y-CAP power supply, when winding the transformer, the primary is wound first, then the auxiliary winding is wound and the auxiliary winding is closely wound to one side, and then the secondary is wound.
14. Connect a common K inductor to a few K to several tens of K resistors.
15. Shield the common mode inductor with copper foil and connect it to the ground of the large capacitor.
16. The common mode inductor and the transformer should be separated from each other during PCB design to avoid mutual interference.
17, condoms magnetic beads.
18. The capacity of the Y capacitor for grounding the two incoming lines from the three-wire input is reduced from 2.2nF to 471.
19. For the two-stage filtering, the 0.22uFX capacitor of the latter stage can be removed (sometimes the X capacitor will cause oscillation before and after).
20. For the π-type filter circuit, there is a BUCK capacitor lying on the PCB and close to the transformer. This capacitor interferes with the L channel of the conduction 150KHZ-2MHZ. The improved method is to shield the capacitor with copper to shield the ground, or Separate this capacitor from the transformer and PCB with a small PCB. Or stand up this capacitor or replace it with a small capacitor.
21. For the π-type filter circuit, there is a BUCK capacitor lying on the PCB and close to the transformer. This capacitor interferes with the L channel of the conduction 150KHZ-2MHZ. The improved method is to use this capacitor with a 1uF/400V or 0.1uF/ Instead of a 400V capacitor, the other capacitor is increased.
22. Add a small few hundred uH differential mode inductor to the common mode inductor.
23. The switch tube and the heat sink are wrapped with a piece of copper foil, and the ends of the copper foil are shorted together, and then connected to the ground with a copper wire.
24. Wrap the common mode inductor in a piece of copper and connect it to the ground.
25. Connect the switch tube to the ground with a metal sleeve.
26, increase X2 capacitor can only solve the frequency band of about 150K, can not solve the frequency band above 20M, only the first input of nickel-zinc ferrite black magnetic ring in the power input, the inductance is about 50uH-1mH.
27. Increase the X capacitor at the input.
28. Increase the input common-mode inductor.
29. Reverse the auxiliary winding supply diode to ground.
30. Change the auxiliary winding power supply filter capacitor to a thin electrolytic capacitor or increase the capacity.
31. Increase the input filter capacitor.
32, 150KHZ-300KHZ and 20MHZ-30MHZ are both conduction, but a differential mode circuit can be added in front of the common mode circuit. You can also check if there is any problem with the grounding. The grounding place must be strengthened. The ground wire on the main board must be straightened out. The wires between the different grounding lines must be smooth and not interlaced.
33. On the rectifier bridge and capacitor, when considering the common mode component, it should be adjacent and capacitor. When considering the differential mode component, it should be diagonal and capacitor.
34. Increase the differential mode inductance at the input end.
1MHZ---5MHZ differential mode common mode mixing
A series of X capacitors are connected in parallel with the input terminal to filter out the differential interference and analyze which interference is exceeded and solve it.
1. For differential mode interference exceeding the standard, the X capacitance can be adjusted, the differential mode inductor is added, and the differential mode inductance is adjusted.
2. For common mode interference exceeding the standard, a common mode inductor can be added, and a reasonable inductance is used to suppress;
3. The rectifier diode characteristics can also be changed to handle a pair of fast diodes such as FR107 pair of common rectifier diodes 1N4007.
4. For power supplies with Y capacitors, the interference is dominated by differential mode before 1M, and 2-5M is differential mode and common mode interference. For NO-Y, the situation is different, and the common mode before 1M is also very powerful. Add a lot of X capacitors in front, filter the differential mode, and change the transformer to have no effect on the differential mode. If there is still a change, it is common mode. The method of differential common mode separation: add a lot of X capacitors at the AC input, from small to large, so that the differential mode can be filtered out, and the rest is common mode, and then compared with the total noise, the differential mode can be seen. size.
5. When winding the transformer, put all the end of the same name on one side, which can reduce the conduction interference of 1.0MHZ-5.0MHZ.
6. For two low-mode inductors with low power, reducing the differential mode inductance turns can reduce the conduction 1.2MHZ interference.
7, increase the Y capacitor, can reduce the conduction of the middle section 1MHZ-5MHZ interference.
8. For the switching power supply EMI without Y capacitor, the 1MHZ-6MHZ exceeds the standard. If the EM is reduced after adding the Y capacitor, you can add more layers of adhesive tape between the primary and secondary of the transformer.
9. Connect the MOS tube heat sink to the MOS tube S pole.
10. Parallel small-capacity high-voltage ceramic or high-voltage chip capacitors on the input filter capacitor.
5M---20MHZ is mainly based on common touch interference, and adopts a method of suppressing common touch.
1. For the grounding of the outer casing, 2-3 turns of a magnetic ring on the ground wire will have a greater attenuation effect on the interference above 10MHZ;
2, you can choose the core of the transformer to adhere to the copper foil, the copper foil should be closed.
3. The size of the absorption circuit of the back-end output rectifier and the parallel capacitance of the primary large circuit.
4. Use a very thin triple insulated wire around the primary winding of the transformer and wrap around a shield winding. One end of the shield winding is connected to the other end of the power supply terminal and connected to ground through a capacitor.
5, the common mode inductance can be changed to one side, the number of turns is one more than the other side, and the other has the role of differential mode.
6. Add a small heat sink to the D of the switch tube and connect the negative pole of the high voltage end. The primary start end of the transformer is connected to the D pole of the MOS tube.
7. Connect the secondary heat sink to a primary L/N line with a 102 Y capacitor to reduce conducted interference.
8. If the interference of the Y capacitor is increased, the transformer winding method can be modified to improve the number of layers of tape between the primary and secondary. If the interference of the Y capacitor is not improved, the circuit can be changed. Change the transformer winding method.
9. Increase the inductance of the transformer appropriately, which can reduce the conduction interference of the RCC switching power supply during half load.
10. Shielding the primary main winding with the secondary auxiliary winding of the transformer is much better than shielding the primary main winding with the primary auxiliary winding of the transformer.
11. The overall conduction exceeds the standard. With the oscilloscope, the G and D pole waveforms of the switch tube overlap. The optical power supply resistor is connected from the output filter common mode inductor to the output positive pole. It is not connected to the high current. .
12. Connect a 681/250V Y capacitor to the L line and the N line at the input end, and connect the other end of the Y capacitor to the secondary ground.
13. Use the secondary auxiliary winding to shield the primary main winding to reduce conducted 3-15 MHz interference. It is much better to shield the primary main winding with the secondary auxiliary winding than to shield the primary main winding with the primary auxiliary winding.
14. Place a layer of copper on the bottom of the PCB to connect the primary large capacitor negative.
15. Wrap the entire power supply with a piece of copper, and connect the copper piece to the primary large capacitor negative.
16. Reduce the Y capacitor capacity.
For 20--30MHZ
1. For a class of products, it is possible to adjust the Y2 capacitance to the ground or change the Y2 capacitor position;
2. Adjust the position and parameter value of the Y1 capacitor between the secondary and the secondary side;
3. On the outer copper foil of the transformer, the shield is layered in the innermost layer of the transformer, and the arrangement of the windings of the transformer is adjusted.
4, change the PCB LAYOUT;
5. The output line is connected to a small common-mode inductor surrounded by a double line;
6. Connect the RC filter at both ends of the output rectifier and adjust the reasonable parameters;
7. Add magnetic beads between the transformer and the MOSFET;
8. Add a small capacitor to the input voltage of the transformer.
9, you can use to increase the MOS drive resistance.
10, may be caused by electronic load, you can use a resistive load.
11. Connect the D terminal of the MOS tube to the ground with a capacitor of 101.
12. The output rectifier diode can be replaced by a smaller capacitor.
13. The RC loop of the output rectifier diode can be removed.
14. Add two Y capacitors to the input to ground to reduce the conduction of 25MHZ-30MHZ interference.
15. Add a copper skin to the core of the transformer and connect the copper to the ground.
16. After the conduction of 25MHZ exceeds the standard, a common mode inductor can be added at the output end, and a suitable magnetic bead with a long magnetic force can be set on the source detecting resistor of the switch tube.
The actual rectification strategy of EMI after switching power supply design--radiation part
30---50MHZ is generally caused by high-speed turn-off of MOS tube
1. You can increase the MOS drive resistance;
2. The RCD snubber circuit uses a 1N4007 slow tube;
3. The VCC supply voltage is solved with a 1N4007 slow tube;
4. Or a small common mode inductor connected in parallel with the front end of the output line;
5. Connect a small snubber circuit in parallel with the D-S pin of the MOSFET;
6. Add BEAD CORE between the transformer and the MOSFET;
7. Add a small capacitor to the input voltage of the transformer;
8. When cardiac PCB LAYOUT large electrolytic capacitors, transformers, circuit composed of MOS ring small as possible;
9. The circuit loop formed by the transformer, the output diode, and the output smoothing electrolytic capacitor is as small as possible.
50---100MHZ is generally caused by the output rectifier reverse recovery current
1. Magnetic beads can be placed on the rectifier tube;
2. Adjusting the absorption circuit parameters of the output rectifier;
3. The impedance of the Y-capacitor branch can be changed on one secondary side, such as adding BEAD CORE at the PIN pin or connecting a suitable resistor in series;
4. It is also possible to change the MOSFET, the output of the rectifier diode body to the space radiation (such as iron clip card MOSFET; iron clip card DIODE, change the ground point of the heat sink).
5. Increase the shielding copper foil to suppress radiation to the space.
The switching power supply above 200MHZ has a basic amount of radiation and generally can pass the EMI standard.
Summary of countermeasures for switching power supply EMI
1. Shielding treatment of external structures;
2. Cable processing outside the product;
3. Cable processing inside the product;
4, PCB wiring processing;
5. Selection of the oscillation frequency of the switching power supply;
6, the choice of IC model;
7. Selection of frequency and bandwidth of magnetic materials;
8. Selection, winding and design of transformers;
9. Handling of the grounding method of the radiator.
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