Electromagnetic compatibility EMI optimization solution sharing
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Electromagnetic compatibility EMI optimization solution sharing

Posted Date: 2024-01-22

The popularity of modern electronic equipment has brought great convenience to people, but it has also aggravated the deterioration of the electromagnetic environment. Electromagnetic interference (EMI) refers to the interference phenomenon caused by the interaction between electromagnetic waves and electronic devices.

Electronic equipment will generate electromagnetic waves when they are working. Mutual interference of electromagnetic waves will have a negative impact on sensitive circuits. In severe cases, the circuits will not work properly. This is an important reason why reducing EMI can improve system stability. This article discusses how EMI affects consumer electronics and sensitive equipment performance, such as home appliances, alarm systems, and garage door openers.

How to optimize EMI

In the design of switching power supply, circuit design and circuit board layout are two key points in solving EMI problems. In circuit design, switching frequency and ringing at the switch node (Figure 1) can produce electromagnetic interference (EMI).

Figure 1: Typical switching power supply application circuit

Two methods can effectively optimize EMI: switching frequency control method and anti-ringing control method.

Switching frequency control method

EMI is reduced by extending the switching rise and fall times of the switch nodes to reduce the dV/dt rate of change (see Figure 2, Figure 3, Figure 4, and Figure 5).

Anti-ringing control method Ringing on switch nodes can cause EMI problems. The more ringing a device has, the worse its EMI performance will be (Figure 6). Adding a 1k Ω resistor (R) and another switch (S1) between SW and GND mitigates ringing. Under light load conditions, when both the HS and LS switches are off, S1 turns on, allowing part of L1's current to be released to GND through R and S1 (Figure 7).

Figure 6: More ringing at SW (no anti-ringing control)

Figure 7: Less ringing at SW (with anti-ringing control) (Test conditions: VIN = 12V, VOUT = 3.3V, IOUT = 10mA) Figure 8 and Figure 9 show the switching speed control and anti-ringing control. Achieved EMI reduction.

PCB layout that improves EMI The feedback signal of a switching power supply is an analog signal that is very sensitive to electromagnetic interference and is susceptible to interference from its own switching signal. A good layout can reduce this EMI interference, while a poor layout may produce large ripples or even cause the power supply to not work properly. Here are some tips for achieving better EMI performance through component placement and PCB layout: ● Place the input filter capacitor close to the IC (Figure 10).

Figure 10: The input capacitor is placed close to the IC, and the electromagnetic field is smaller ● Use a shielded inductor ● Use a small SW pad layout (Figure 11)

● IC GND and system GND use a single point connection

● Keep the connection between input ground and GND as short and wide as possible

● Connect the ground of the VCC capacitor to the ground of the IC through multiple vias or wide traces

● The connection between the input capacitor and the IN pin should be as wide and short as possible

● Make sure all feedback connections are direct and short

● Feedback resistors and compensation devices are as close to the chip as possible

● Keep SW signals away from sensitive analog signals, such as FB signals. Optimization for consumer electronics and RF-sensitive applications. In a world full of electronic devices, from household appliances to consumer electronics and RF-sensitive devices (such as garage door openers) monitors and alarm systems), EMI phenomena may cause unnecessary interaction and operational problems in the system. The MP2317 series solves this problem by optimizing EMI performance. At the same time, the MP2317 series has a simple package and supports the use of single-layer PCB boards for design, making manufacturing simpler and more economical. The MP2317 series can be used as a secondary side DC/DC converter (Figure 12).

Figure 12: Application in air conditioning

Main features of MP2317 series:

● Wide input voltage range from 7.5V to 26V

● 150uA small quiescent current

● Excellent load line regulation and transient response (Figure 15)

● The efficiency can reach up to 96%, and when converting 12V to 5V/20mA, the efficiency can reach 80% (Figure 13)

● Comprehensive protection (over-temperature protection OTP, low-voltage protection UVLO, over-current protection OCP) to improve reliability and service life

Figure 13: MP2317 efficiency chart

Figure 14: MP2317 (U1) single-layer board layout (test conditions: VIN = 12V, VOUT = 5V)

Figure 15: MP2317 fast load transient response (test conditions: VIN = 12V, VOUT = 5V, L = 10μH) Conclusion: In addition to the EMI optimization issues that are crucial to the reliability of the circuit, the ease of manufacturing the circuit Also very important. MPS's 1A / 2A / 2.5A 26V high-efficiency switching regulators - MP2317, MP2344 and MP2345 series, adopt a small 6-pin SOT23 package and large pin pitch (0.95mm). This packaging method can use a single-layer PCB Layout is performed to simplify the manufacturing process and save manufacturing costs. This series of three switching regulators with different current values ​​use the same package and are Pin-to-Pin compatible. System engineers can flexibly switch to switching regulators with different current values ​​without changing the PCB, thus saving design time and cost.

EMC message: With the development of the times, more and more electronic and electrical equipment or system products need to be inspected and tested, among which EMC testing is one of the necessary inspection and testing indicators. However, EMC testing projects are relatively expensive, EMC laboratories are expensive, and most measurement equipment requires imported equipment. As a result, few inspection and testing institutions have the ability to build EMC laboratories. The EMC performance of the product is given during the design stage. If EMC factors are not considered during the design of general electronic products, it will easily lead to failure of the EMC test and fail to pass the testing or certification of relevant EMC regulations. For example, product design and R&D engineers design effective filter circuits according to needs and place them in the preamplifier of the product's I/O (input/output) interface, which can eliminate the interference noise that enters the system due to conduction at the entrance of the circuit system. Department; design isolation circuits (such as transformer isolation and photoelectric isolation, etc.) to solve the conduction interference entering the circuit through power lines, signal lines and ground wires, while preventing interference caused by public impedance and long-term transmission; design energy absorption loops, Thereby reducing the noise energy absorbed by circuits and devices; by selecting components and rationally arranging circuit systems, the impact of interference can be reduced.

EMC skills: rectification tips

1. 150kHz-1MHz, mainly differential mode, 1MHz-5MHz, differential mode and common mode work together, and after 5MHz, it is basically common mode. Differential mode interference is divided into capacitive coupling and perceptual coupling. Generally, the interference above 1MHz is common mode, and the low frequency band is differential mode interference. Use a resistor in series with a capacitor and then connect it to the pin of the Y capacitor. Use an oscilloscope to measure the voltage at the two pins of the resistor to estimate common mode interference.

2. Add differential mode inductor or resistor after insurance.

3. The low-power power supply can be processed by a PI filter (it is recommended that the electrolytic capacitor close to the transformer be larger).

4. The differential mode inductor in the front-end π-type EMI parts is only responsible for low-frequency EMI. Don’t choose a size that is too large (DR8 is too large, it is better to use a resistor type or DR6), otherwise the radiation will be difficult to pass. You can string magnetic beads if necessary, because The high frequency will fly directly to the front end and will not follow the line.

5. The conduction exceeds the standard at 0.15MHz-1MHz when the engine is cold, and there is a 7dB margin when the engine is warm. The main reason is that the primary BULk capacitor DF value is too large. The ESR is relatively large when the engine is cold and the ESR is relatively small when the engine is hot. The switching current forms a switching voltage on the ESR, which will press a current flowing between the LN lines. This is the differential mode. interference. The solution is to use electrolytic capacitors with low ESR or add a differential mode inductor between two electrolytic capacitors.

6. The solution to the total exceedance of the standard when testing 150kHz: Increase the X capacitance and see if it can be reduced. If it is reduced, it means differential mode interference. If it doesn't have much effect, then it's common mode interference, or you can wind the power cord around a large magnetic ring a few times. If it comes down, it means it's common mode interference. If the interference curve is very good at the back, reduce the Y capacitance and see if there is any problem with the layout, or add a magnetic ring in front.

7. The inductance of the single winding inductor in the PFC input part can be increased.

8. The components in the PWM circuit adjust the main frequency to about 60kHz.

9. Use a piece of copper sheet to stick to the transformer core.

10. The inductance on both sides of the common mode inductor is asymmetrical. If one side has one less turn, it may cause the conduction 150kHz-3MHz to exceed the standard.

11. Generally, there are two main points for the generation of conduction: around 200kHz and 20MHz. These points also reflect the performance of the circuit; around 200kHz is mainly the spike caused by leakage inductance; around 20MHz is mainly the noise of the circuit switch. If the transformer is not properly handled, a large amount of radiation will be added. Even shielding is useless because the radiation cannot pass through.

12. Change the input BUCk capacitor to a low internal resistance capacitor.

13. For the power supply without Y-CAP, when winding the transformer, first wind the primary winding, then wind the auxiliary winding and close the auxiliary winding to one side, and then wind the secondary winding.

14. Connect a resistor of several k to tens of k in parallel to the common mode inductor.

15. Shield the common mode inductor with copper foil and connect it to the ground of the large capacitor.

16. When designing the PCB, the common mode inductor and transformer should be separated to avoid mutual interference.

17. Condom magnetic beads.

18. The Y capacitor capacity of the three-wire input that connects the two incoming wires to ground is reduced from 2.2nF to 471.

19. For those with two-stage filtering, the 0.22uFX capacitor in the rear stage can be removed (sometimes the front and rear X capacitors will cause oscillation).

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 that conducts 150kHz-2MHz. The improvement method is to wrap the capacitor with copper to shield it and connect it to the ground, or Separate this capacitor from the transformer and PCB with a small PCB. Or stand this capacitor up, 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 conducting 150kHz-2MHz. The improvement method is to use a 1uF/400V or 0.1uF/400V capacitor for this capacitor. Instead of a capacitor, increase the other capacitor. 22. Add a small differential mode inductor of several hundred uH before the common mode inductor. 23. Wrap the switch tube and the radiator with a piece of copper foil, short-circuit both ends of the copper foil together, and then connect it to the ground with a copper wire.

24. Wrap the common mode inductor with a piece of copper and connect it to ground.

25. Cover the switch tube with metal and connect it to the ground.

26. Increasing the X2 capacitor can only solve the frequency band around 150kHz, but not the frequency band above 20MHz. Only a first-level nickel-zinc ferrite black magnetic ring can be added to the power input, with an inductance of about 50uH-1mH.

27. Increase the X capacitor at the input end.

28. Increase the common mode inductance at the input end.

29. Reverse the auxiliary winding power supply diode to ground.

30. Change the auxiliary winding power supply filter capacitor to a slim electrolytic capacitor or increase its capacity.

31. Increase the input filter capacitor.

32. The conduction at 150kHz-300kHz and 20MHz-30MHz is insufficient. A differential mode circuit can be added before the common mode circuit. You can also check to see if there are any problems with the grounding. The grounding place must be tightened and firmly connected. The ground wires on the motherboard must be straightened out. Different ground wires must be routed smoothly and not cross each other.

33. The capacitors on the rectifier bridge should be connected diagonally when considering the common mode components. When considering the differential mode components, the capacitors should be connected diagonally.

34. Increase the differential mode inductance at the input end.

2. The sources of product electromagnetic compatibility disturbance include:

1. Switching circuit of equipment switching power supply: the main frequency of the disturbance source is from tens of kHz to more than a hundred kHz, and the high-order harmonics can extend to tens of MHz.

2. Rectification circuit of equipment DC power supply: The frequency limit of power frequency rectification noise of power frequency linear power supply can extend to hundreds of kHz; the frequency limit of high-frequency rectification noise of switching power supply can extend to tens of MHz.

3. Brush noise of DC motors in electric equipment: the upper limit of the noise frequency can extend to hundreds of MHz.

4. Operating noise of AC motors in electric equipment: high-order harmonics can extend to tens of MHz.

5. Harassment emission from frequency conversion speed regulation circuit: The frequency of the disturbance source of the switching speed regulation circuit ranges from tens of kHz to tens of MHz.

6. Switching noise caused by equipment operating status switching: The upper limit of the frequency of noise generated by mechanical or electronic switching actions can extend to hundreds of MHz.

7. Electromagnetic disturbance of crystal oscillators and digital circuits of intelligent control equipment: the main frequency of the disturbance source is tens of kHz to tens of MHz, and the high-order harmonics can extend to hundreds of MHz.

8. Microwave leakage from microwave equipment: the main frequency of the disturbance source is several GHz.

9. Electromagnetic harassment emission from electromagnetic induction heating equipment: The main frequency of the harassment source is tens of kHz, and the high-order harmonics can extend to tens of MHz.

10. The local oscillator and its harmonics of the high-frequency tuning circuit of the TV electroacoustic receiving equipment: the main frequency of the disturbance source is tens of MHz to hundreds of MHz, and the high-order harmonics can extend to several GHz.

11. Digital processing circuits of information technology equipment and various automatic control equipment: the main frequency of the disturbance source is tens of MHz to hundreds of MHz (the main frequency can reach several GHz after internal frequency multiplication), and the high-order harmonics can extend to more than ten GHz .

Review Editor: Huang Fei


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