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How to solve the LED power supply design EMC / EMI problems​

Release date:2015-10-24

Electromagnetic compatibility (EMC) is the study of accident electromagnetic energy in electricity generation, transmission and reception, as well as the harmful effects caused by this energy. EMC's goal is in the same environment, electromagnetic phenomena involving different devices are able to operate normally, but not in this environment, the ability to produce any equipment intolerable electromagnetic interference. Traditionally said, EMC contain EMI (electromagnetic interference) and EMS (electromagnetic susceptibility) two aspects.

Electromagnetic interference (EMI) is any conduction or electromagnetic effects associated with voltage and current generated will reduce the performance of a device, equipment or system, or adversely affect the electromagnetic phenomena.

LED power of electromagnetic interference, the main aspects of engineers to consider are: circuit measures, EMI filtering, component selection, interference shielding and printed circuit board design.

For the design of LED power engineers, electromagnetic interference is a key issue has been present in the design. How can I solve this problem? Let's look at several factors influence electromagnetic compatibility.

  First number of factors influence EMC

  (1) drive power

Initially the LED power is linear power, linear power at work but will loss a lot of energy in the form of heat. Linear power work, so he must have changed from high to low pressure pressure device, generally are transformer, then rectified output DC voltage. Though bulky, heat, advantage is that the external interference small, electromagnetic interference, but also easy to solve.

And now more and more use of LED switching power supply, are in the form of PWM LED driver power supply is to work in a power transistor is turned on and off states. In conduction, low voltage, large current; shutdown, high voltage, current, and therefore the loss arising from the power semiconductor device is small. The disadvantage is that more obvious, electromagnetic interference (EMI) and more serious.

    (2) Switching Frequency

LED power electromagnetic compatibility problem is usually the power switch circuit. The switching circuit is one of the main sources of interference switching power supply.

LED drive power switching circuit is the core of the switching circuit mainly by the switch and the high frequency transformer. It produces du / dt has a relatively large pulse, wide-band and rich harmonics.

The main reason is the high-frequency pulse interference: load switch high-frequency transformer primary coil is inductive load.


Figure 1: switching pulses generated spikes

  Conducting instant, the primary coil produces a large inrush current, and the emergence of high surge voltage spikes across the primary winding; disconnection moment, due to the leakage flux of the primary coil, resulting in no part of the energy transfer from the primary coil to two secondary coil, with a damped oscillation circuit formed spikes superimposed on the turn-off voltage, forming a turn-off voltage spike.

High-frequency pulse to produce more emissions, periodic signal to produce more emissions. In the LED power supply system, the switching circuit generates a current spikes, and when the load current changes will produce current spikes. This is one of the sources of electromagnetic interference.

    (3) Ground

EMC topics in all, the main problem is caused by improper grounding. There are three signal ground methods: single-point, multi-point and mixing. When the switch circuit frequencies below 1MHz, can be a single point grounding method, but not suitable for high frequency; in high frequency applications, the best multi-point grounding. Mixing ground is low-single-point grounding, and high-frequency multi-point grounding method. Ground layout is the key, the ground circuit high-frequency digital circuits and low-level analog circuits do not mix.

    (4) PCB design

Appropriate printed circuit board (PCB) layout to prevent EMI is essential.

Reset circuit design (5) Intelligent LED power supply

In the LED power supply, LED power supply with a lot of intelligent MCU control, and some LED power supply with single-chip control the duty cycle of the switching circuit, the microcontroller watchdog system to run the entire LED power plays a particularly important role, since All sources of interference can not all be together became effective correction defensive barrier isolation or in addition to, once entered the CPU interfere with the normal operation of the program, then reset the system measures a combination of software processing. Commonly used to reset the system has the following two:

① external reset system. External "watchdog" circuit can also be used to design their own special "watchdog" chip to build. Thus, if the program system stuck in an infinite loop, and the loop happens with "DOG" signal, then the reset circuit can not achieve its proper function.

② Now more and more LED power supplies have reset the system on its own slice, so users can easily use its internal reset timer, but some intelligent LED power supply control circuit reset command is too simple, so there will still be an infinite loop as described above as "DOG" instruction to lose monitoring role.

  Second Hardware Processing Method II of interference measures

  To solve the problem of electromagnetic interference LED drive power is available from the following aspects:

    1. reduce the switching noise on the power itself

① soft-switching technology: increase the inductance and capacitance elements in the original hard switching circuits, use a resonant inductor and capacitor to reduce switching process of du / dt and di / dt, so that the voltage drop when the switching device opened prior to the current rise and fall of current prior to the increase in voltage to eliminate overlapping voltage and current or off.

    ② switching frequency modulation techniques: by modulating the switching frequency fc, the focus fc and its harmonics 2fc, energy 3fc ... spread on the band around them, in order to reduce EMI amplitude of each frequency point.

   ③ components: Choose easy to produce noise, easy to conducted and radiated noise components. Typically particularly noteworthy is that the choice of diodes and transformer windings class components. Reverse recovery current, shorter recovery time is ideal for fast recovery diode rectifier high-frequency switching power supply device portion.

    ④ rational use of electromagnetic interference filters: The main purpose of the EMI filter, a power noise is electromagnetic interference, it belongs to radio frequency interference (RFI), which conducted noise spectrum is substantially 10KHz ~ 30MHz, up to 150MHz.

Under normal circumstances, the differential mode interference amplitude, low frequency interference caused by small; common mode interference amplitude, high frequency, can produce radiation through the wire, the larger the interference caused. To weaken conducted interference, the most effective method is to install EMI filter input and output of the switching power supply circuit.

LED power supply generally use a simple single-stage EMI filters, including common mode choke and a filter capacitor.

Figure 2 is a conventional LED power filter, L, C1 and C2 are used to filter out common mode interference, C3 and C4 series mode filter out interference. When the common-mode interference occurs, magnetic flux due to the same direction in the two coils L, after the total inductance coupling increases rapidly, so the common mode signal exhibits a large inductance, so that is not easy to pass, so called common mode choke coil. Its two coils are wound on ferrite toroid low loss, high permeability. R is the discharge resistance can be accumulated charge on the C3 vent let go, to avoid the charge accumulation and influence the filter characteristic, but also to power after the end of the power line L, N is not charged, to ensure the safety of use.


                                    Figure 2 conventional LED power filter

  ⑤ EMI filter can effectively suppress electromagnetic interference Switching power adapter

Figure 3 is a curve without EMI filter when switching on the power adapter 0.15MHz ~ 30MHz conducted noise waveform.

Curve b is a waveform added after EMI filter, it will be electromagnetic interference attenuation 50 db (Uv) ~ 70 分贝 (uV). Clearly, better EMI filter insertion.

Set before and after the transmission of electromagnetic interference filter was added to the noise voltage across the load, respectively U1 and U2, the calculation formula is 20lgU1 / U2.

Expressed in decibels dB insertion loss, the greater the decibel value, indicating a stronger ability to suppress noise.

Added loss measurement circuit shown in Figure 3. e is the noise signal generator, Zi is the internal impedance of the signal source, ZL is the load impedance, and generally 50 ohms. Optional noise frequency range 10KHz ~ 30MHz. First were measured at different frequencies of noise around U1 join EMI filter pressure drop across the load, U2, and then into the formula 20lgU1 / U2 insertion loss values calculated for each frequency point, and finally export insertion loss curves.


                  Figure 3: Add situation EMI filter before and after

  2. cut off the route of transmission interference signals

   ① power line disturbances can use power line filter to remove. A reasonable and effective switching power supply EMI power line filters should be differential mode and common mode interference has a strong inhibitory effect.

    ② improve the electromagnetic compatibility design PCB board

PCB is LED power supply system support components and devices, which provide electrical circuit connection between components and devices. With the rapid development of electronic technology, PCB density is increasing. PCB design quality great influence on electromagnetic compatibility LED power supply systems.

Practice has proved that, even if the circuit schematic design is correct, the printed circuit board design properly, will adversely affect the reliability of the LED power supply.

PCB anti-jamming design includes PCB layout, wiring and grounding, which aims to reduce crosstalk and electromagnetic radiation PCB on PCB circuit.

Also, general transformer electromagnetic interference caused by the hum frequency is generally about 50HZ, improper wiring and grounding hum, since the effect of frequency doubling rectifier circuit is about 100HZ, carefully distinguish still perceptible.

The correct wiring method is to select the main filter capacitor pin as a centralized ground, strong and weak signal ground strictly separated, the summary of the total ground.

Therefore, in the design of printed circuit boards, we should pay attention to the correct approach, comply with the general principles of PCB design, and shall conform to the requirements of anti-jamming design.

    3. The enhanced anti-jamming capability disturbed body

In the LED power supply system input / output also interference sources conductive lines, and receiving radio frequency interference signal source Pick up, we generally take effective measures designed to:

    ① adopt the necessary common mode / differential mode suppression circuit, but also to take some of filtering and anti-electromagnetic shielding to reduce interference to enter.

    ② take as conditions permit in the various quarantine measures (such as optically isolated or magnetic isolation), thereby blocking the spread of interference.

    ③ lightning protection measures

LED power supply systems used outdoors or indoors from the outdoor crowd out the introduction of power lines, signal lines, to consider the system of lightning problem. Commonly used anti-lightning devices include: gas discharge tube, TVS (Transient Voltage Suppression) and the like. Gas discharge tube when the power supply voltage is greater than a certain value, typically tens or hundreds V V, gas breakdown discharge, the power line strong impulse conduction into the earth. TVS can be seen as two parallel and opposite direction Zener diode, when the voltage across the conduction above a certain value. Its characteristics can be transient by hundreds of thousands of A is current.