How to crack the electromagnetic interference of L

2022-09-29
  • Detail

How to solve the electromagnetic interference of LED power supply is an important part of 3C certification. Friends who are familiar with power circuit design know that electromagnetic interference EMI is a big problem in the design process of LED power supply, so how can we solve this problem

this article will share the treatment of electromagnetic compatibility from this perspective, so that electromagnetic interference is no longer a problem

I. several factors affecting EMC

1 The circuit structure of the driving power supply

the initial LED power supply is a linear power supply, but the linear power supply will lose a lot of energy in the form of heating when it works. The working mode of linear power supply makes it necessary to have a voltage device from high voltage to low voltage. Generally, it is a transformer, and then output DC voltage through rectification. Although it is bulky and has large calorific value, it has the advantage of small external interference and electromagnetic interference, which is also easy to solve

nowadays, many led switching power supplies are used. They are led driving power supplies in the form of PWM, which make the power transistor work in the on and off state. When conducting, the voltage is low and the current is high; When turning off, the voltage is high and the current is small, so the loss on the power semiconductor device is also small. The obvious disadvantage is that electromagnetic interference (EMI) is also more serious

2. Switching frequency

electromagnetic compatibility problems of LED power supply are generally in the power supply of switching circuit. Switching circuit is one of the main interference sources of switching power supply

switching circuit is the core of LED driving power supply. The switching circuit is mainly composed of switching tubes and high-frequency transformers. The du/dt produced by it has large amplitude pulses, wide frequency band and rich harmonics

the main reason for this high-frequency pulse interference is that the switch load is the primary coil of high-frequency transformer, which is an inductive load

Keep the number of experiments

3. At the moment of switching pulse spike

conduction, the primary coil generates a large inrush current, and a high surge spike voltage appears at both ends of the primary coil; At the moment of disconnection, due to the leakage flux of the primary coil, part of the energy is not transmitted from the primary coil to the secondary coil, and attenuation oscillation with spikes is formed in the circuit, which is superimposed on the shutdown voltage to form the shutdown voltage spike

Figure 1 shows the peak of switching pulse

high frequency pulse produces more emissions, and periodic signal produces more emissions. In the LED power supply system, the switching circuit produces a current spike signal, and when the load current changes, it will also produce a current spike. In the formula, n is called the deformation enhancement index, and K is called the deformation enhancement coefficient peak signal. This is one of the sources of electromagnetic interference

4. Grounding

among all EMC problems, the main problem is caused by improper grounding. There are three signal grounding methods: single point, multipoint and hybrid. When the switching circuit frequency is lower than 1MHz, the single point grounding method can be adopted, but it is not suitable for high frequency; In high-frequency applications, it is best to use multi-point grounding. Hybrid grounding is a method of single point grounding for low frequency and multi-point grounding for high frequency. Ground wire layout is the key, and the grounding circuits of high-frequency digital circuits and low-level analog circuits should not be mixed

b design

proper printed circuit board (PCB) wiring is crucial to prevent EMI

reset circuit design of intelligent LED power supply

enhance the anti-interference ability of the disturbed body

in the LED drive power supply system, the input/output is also the transmission line of the interference source and the pickup source of the RF interference signal. We generally need to take effective measures in design:

adopt the necessary common mode/differential mode suppression circuit, and also take certain filtering and anti electromagnetic shielding measures to reduce the progress of interference

if conditions permit, various isolation measures (such as photoelectric isolation or magnetoelectric isolation) shall be taken as far as possible to block the propagation of interference

6. Lightning protection measures

lightning protection of the system should be considered when the LED power supply system used outdoors or the indoor power line and signal line are introduced from outdoors. Common lightning protection devices include: gas discharge tube, etc. When the voltage of the power supply is greater than a certain value, usually tens of volts or hundreds of volts, the gas discharge tube breaks down and discharges, leading the strong impulse pulse on the power line into the earth. TVs can be regarded as two zener diodes in parallel and in opposite directions. When the voltage at both ends is higher than a certain value, it turns on. Its characteristic is that it can transiently pass hundreds or thousands of amperes

second, the hardware processing method of interference measures

to solve the electromagnetic interference problem of LED driving power supply, we can start from the following aspects:

1 Reduce the interference of switching power supply itself

① soft switching technology: add inductance and capacitance components in the original hard switching circuit, use the resonance of inductance and capacitance to reduce du/dt and di/dt in the switching process, so that the voltage drop before the current rise when the switching device is turned on, or the current drop before the voltage rise when the switching device is turned off, so as to eliminate the overlap of voltage and current

② switching frequency modulation technology: by modulating the switching frequency fc, the energy concentrated on FC and its harmonics 2FC, 3fc... Is dispersed to the frequency bands around them, so as to reduce the EMI amplitude at each frequency point

③ selection of components: select components that are not easy to generate noise, conduct and radiate noise. It is usually worth paying special attention to the selection of winding components such as diodes and transformers. The fast recovery diode with small reverse recovery current and short recovery time is an ideal device for the high-frequency rectifier of switching power supply

④ reasonable use of electromagnetic interference filter: one of the main purposes of EMI filter, electrical noise is a kind of electromagnetic interference, which belongs to radio frequency interference (RFI), and its conducted noise spectrum is roughly 10khz-30mhz, up to 150MHz

in general, the amplitude of differential mode interference is small, the frequency is low, and the interference caused is small; Common mode interference has large amplitude and high frequency, and it can also generate radiation through wires, causing large interference. To weaken conducted interference, the most effective way is to install electromagnetic interference filters in the input and output circuits of switching power supply

led power supply generally adopts simple single-stage EMI filter, mainly including common mode choke and filter capacitor

Figure 2 shows the commonly used LED power filter. L, C1 and C2 are used to filter common mode interference, and C3 and C4 are used to filter serial mode interference. When common mode interference occurs, because the magnetic flux directions of the two coils in L are the same, the total inductance increases rapidly after coupling, so it presents great inductance to the common mode signal, making it difficult to pass, so it is called common mode choke. Its two coils are respectively wound on the ferrite magnetic ring with low loss and high permeability. R is the discharge resistance, which can discharge the charge accumulated on C3 to avoid affecting the filtering characteristics due to charge accumulation. After power failure, it can also make the incoming terminals L and N of the power supply uncharged, so as to ensure the safety of use

Figure 2 commonly used LED power filter

⑤ EMI filter can effectively suppress the electromagnetic interference of switching power adapter

curve a in Figure 3 is the waveform of 0.15mhz-30mhz conducted noise on switching power adapter without EMI filter

curve B is the waveform after adding EMI filter, which can attenuate electromagnetic interference by 50 dB (UV) - 70 dB (UV). Obviously, the effect of inserting EMI filter is better

setting (2) lift out the piston with two M12 eyebolts and remove the inner wall seal ring and ring of the oil cylinder; The noise voltage transmitted to the load before and after the electromagnetic interference filter is added is U1 and U2 respectively, and the calculation formula is 20lgu1/u2

the insertion loss is expressed in dB. The higher the DB value, the stronger the ability to suppress noise interference

the circuit for measuring the added loss is shown in Figure 3. E is the noise signal generator, Zi is the internal impedance of the signal source, ZL is the load impedance, generally 50 ohms. The noise frequency range can be 10khz-30mhz. Firstly, the noise voltage drops U1 and U2 at both ends of the load before and after the EMI filter are measured at different frequencies, and then the insertion loss value of each frequency point is calculated by substituting the formula 20lgu1/u2, and finally the insertion loss curve is summarized

Figure 3: situation before and after adding EMI filter

2 Cut off the propagation path of interference signal

① power line interference can be filtered by power line filter. A reasonable and effective EMI filter of switching power supply should have a strong inhibitory effect on both differential mode and common mode interference on the power line

② improve the electromagnetic compatibility design of PCB

PCB is the support of circuit components and devices in LED power supply system, which provides the electrical connection between circuit components and devices. With the rapid development of electronic technology, the density of PCB is getting higher and higher. The quality of PCB design has a great impact on the electromagnetic compatibility of LED power supply system

practice has proved that even if the circuit schematic is designed correctly and the printed circuit board is designed improperly, it will have an adverse impact on the reliability of the LED power supply system

pcb anti-interference design mainly includes PCB layout, wiring and grounding. Its purpose is to reduce the electromagnetic radiation of PCB and the crosstalk between circuits on PCB

Figure 4: correctly select the main filter capacitor pin as the centralized grounding point

in addition, the AC sound frequency caused by the electromagnetic interference of the transformer is generally about 50Hz, while the AC sound caused by the improper wiring of the ground wire, because the frequency doubling action frequency of the rectifier circuit is about 100Hz, it can be detected by careful differentiation

the correct wiring method is as shown in Figure 4. Select the main filter capacitor pin as the centralized grounding point, strictly distinguish the strong and weak signal grounding wires, and summarize them at the general grounding point

therefore, when designing printed circuit boards, we should pay attention to adopting correct methods, abide by the general principles of PCB design, and meet the anti-interference design requirements

3. Enhance the anti-interference ability of the disturbed body

in the LED power supply system, the input/output is also the transmission line of the interference source and the pick-up source of the RF interference signal. We generally need to take effective measures in design:

① adopt the necessary common mode/differential mode suppression circuit, and also take certain filtering and anti electromagnetic shielding measures to reduce the progress of interference

② if conditions permit, various isolation measures (such as photoelectric isolation or magnetoelectric isolation) shall be taken as far as possible to block the propagation of interference

③ lightning protection measures

the lightning protection of the system should be considered when the LED power system used outdoors or the indoor power line and signal line are introduced from outdoors. Common lightning protection devices that usher in the second spring of the industry include: gas discharge tube, TVs (transient voltage suppression), etc. When the voltage of the power supply is greater than a certain value, usually tens of volts or hundreds of volts, the gas discharge tube breaks down and discharges, leading the strong impulse pulse on the power line into the earth. TVs can be regarded as two zener diodes in parallel and in opposite directions. When the voltage at both ends is higher than a certain value, it turns on. Its characteristic is that it can transiently pass hundreds or thousands of amperes

therefore, the technologies to control the electromagnetic interference of LED power supply mainly include: circuit measures, EMI filtering, component selection, shielding, anti-interference design of printed circuit board, etc

Copyright © 2011 JIN SHI