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How to solve the EMC/EMI problem in LED power supply design

2019-07-22

Electromagnetic compatibility (EMC) is the study of the generation, propagation, and reception of accidental electromagnetic energy in electricity, and the detrimental effects of such energy. The goal of electromagnetic compatibility is to be able to operate normally in different environments involving electromagnetic phenomena in the same environment, without the ability to generate unacceptable electromagnetic interference to any device in the environment. It is customary to say that EMC includes both EMI (electromagnetic interference) and EMS (electromagnetic sensitivity).

Electromagnetic Interference (EMI) refers to any electromagnetic phenomenon that occurs when a conductive or electromagnetic field is accompanied by a voltage or current that can degrade the performance of a device, device, or system, or cause adverse effects.

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

For engineers designing LED power supplies, the problem of electromagnetic interference is a key issue that has always existed in the design. How can I solve this problem? Let's first look at several factors that affect electromagnetic compatibility.

A number of factors affecting EMC

(1) Circuit structure of the driving power supply

The original LED power supply was a linear power supply, but the linear power supply would consume a lot of energy in the form of heat when it was working. The linear power supply works in such a way that he must have a pressure device from high voltage to low voltage, which is generally a transformer, and then rectified to output DC voltage. Although it is bulky and has a large amount of heat, the advantage is that the external interference is small, the electromagnetic interference is small, and it is easy to solve.

Nowadays, more LED switching power supplies are used, and the LED driving power supply in the form of PWM is to make the power transistor work in the on and off states. When turned on, the voltage is low and the current is large; when turned off, the voltage is high and the current is small, so the loss generated on the power semiconductor device is also small. The disadvantage is that electromagnetic interference (EMI) is also more serious.

(2) Switching frequency

The problem of electromagnetic compatibility of the LED power supply is generally in the power supply of the switching circuit. The switching circuit is one of the main sources of interference for the switching power supply.

The switching circuit is the core of the LED driving power supply, and the switching circuit is mainly composed of a switching tube and a high frequency transformer. It produces a du/dt with a large amplitude pulse, a wide frequency band and a rich harmonic.

The main reason for this high-frequency pulse interference is that the switching tube load is the primary coil of the high-frequency transformer and is an inductive load.

At the moment of 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, part of the energy is not transmitted from the primary coil to the second due to the leakage flux of the primary coil. In the secondary coil, a decaying oscillation with a spike is formed in the circuit, superimposed on the turn-off voltage to form a turn-off voltage spike.

High frequency pulses produce more emissions and periodic signals produce more emissions. In an LED power system, the switching circuit produces a current spike, which also produces a current spike when the load current changes. This is one of the root causes of electromagnetic interference.

(3) Grounding

In all EMC topics, the main topic is caused by improper grounding. There are three methods of signal grounding: single point, multiple points, and hybrid. When the switching circuit frequency is lower than 1MHz, a single-point grounding method can be used, but it is not suitable for high frequency; in high-frequency applications, multi-point grounding is preferred. Hybrid grounding is a method in which the low frequency is grounded at a single point and the high frequency is grounded at multiple points. The ground layout is the key, and the grounding circuit of the high-frequency digital circuit and the low-level analog circuit cannot be mixed.

(4) PCB design

Proper printed circuit board (PCB) wiring is critical to preventing EMI.

(5) Reset circuit design of intelligent LED power supply

In the LED power supply, there are many intelligent LED power supplies that are controlled by single-chip microcomputers, and some LED power supplies use a single-chip microcomputer to control the duty cycle of the switching circuit. The watchdog system of the single-chip microcomputer plays a particularly important role in the operation of the entire LED power supply, due to All sources of interference cannot be isolated or removed. Once the CPU interferes with the normal operation of the program, the reset system combined with software processing becomes an effective barrier to error correction. There are two commonly used reset systems:

1 external reset system. The external "watchdog" circuit can be designed by itself or by a special "watchdog" chip. Thus, if the program system is stuck in an infinite loop, and the loop happens to have a "feed dog" signal, then the reset circuit cannot achieve its proper function.

2 Now more and more LED power supplies have their own on-chip reset system, so users can easily use their internal reset timer, but some intelligent LED power supply control circuit reset instructions are too simple, so There will also be a "feeding dog" command like the above-mentioned infinite loop, which will cause it to lose its monitoring role.

Two pairs of interference measures hardware processing methods

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

1. Reduce the interference of the switching power supply itself

1 soft switching technology: increase the inductance and capacitance components in the original hard switching circuit, use the resonance of the inductor and capacitor to reduce the du/dt and di/dt in the switching process, so that the voltage drop before the switching device is turned on before the current The rise in current, or the drop in current, precedes the rise in voltage to eliminate voltage and current overlap.

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 to reduce the EMI amplitude at each frequency point.

3 component selection: select components that are not easy to produce noise, difficult to conduct and radiate noise. Of particular note is the selection of winding components such as diodes and transformers. The fast recovery diode with low reverse recovery current and short recovery time is an ideal device for the high-frequency rectification part of the switching power supply.

4 Reasonable use of electromagnetic interference filter: One of the main purposes of EMI filter, grid 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.

Under normal circumstances, the differential mode interference amplitude is small, the frequency is low, and the interference caused by the difference 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 by the interference is large. The most effective way to reduce conducted interference is to install an electromagnetic interference filter in the switching power supply input and output circuits.

LED power supplies generally use a simple single-stage EMI filter, which mainly includes common mode chokes and filter capacitors.

Figure 2 shows a common LED power supply filter. L, C1 and C2 are used to filter out common mode interference, and C3 and C4 are used to filter out serial mode interference. When common mode interference occurs, since the magnetic flux directions of the two coils in L are the same, the total inductance increases rapidly after coupling, so that the common mode signal exhibits a large inductive reactance, making it difficult to pass, so it is called Common mode choke. Its two coils are wound around a low-loss, high-permeability ferrite ring. R is a bleeder resistor, which can discharge the accumulated charge on C3, avoiding the influence of the charge accumulation and affecting the filter characteristics. After the power is turned off, the incoming end L and N of the power supply can be de-charged to ensure the safety of use.

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

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

Curve b is the waveform after adding the EMI filter, which attenuates electromagnetic interference by 50 decibels (Uv) to 70 decibels (uV). Obviously, the effect of inserting an EMI filter is better.

The noise voltages transmitted to the load before and after the EMI filter are added are U1 and U2, respectively, and the calculation formula is 20lgU1/U2.

The insertion loss is expressed in decibels dB, and the larger the decibel value, the stronger the ability to suppress noise interference.

The circuit for measuring the loss of insertion is shown in Figure 3. e is the noise signal generator, Zi is the internal impedance of the signal source, and ZL is the load impedance, which is generally 50 ohms. The noise frequency range is from 10KHz to 30MHz. Firstly, the noise voltage drops U1 and U2 at both ends of the load before and after the EMI filter are added are measured at different frequencies, and then the insertion loss value of each frequency point is calculated by substituting into the formula 20lgU1/U2, and finally the insertion loss curve is derived.

2. Cut off the transmission route of the interference signal

1 Power line interference can be filtered out using the power line filter. A reasonable and effective switching power supply EMI filter should have strong suppression of differential mode and common mode interference on the power line.

2 improve the electromagnetic compatibility design of PCB

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

Practice has confirmed that even if the schematic design of the circuit is correct and the printed circuit board is not properly designed, it will adversely affect the reliability of the LED power system.

The PCB anti-interference design mainly includes PCB layout, wiring and grounding, and its purpose is to reduce the electromagnetic interference between the PCB and the crosstalk between the circuits on the PCB.

In addition, the frequency of the ac vibration caused by the electromagnetic interference of the general transformer is generally about 50HZ, and the humming sound caused by the improper wiring of the grounding wire, because the frequency multiplication frequency of the rectifying circuit is about 100HZ, careful distinction can still be detected.

The correct wiring method is to select the main filter capacitor pin as the concentrated grounding point, and the strong and weak signal ground lines are strictly separated and summarized at the total grounding point.

Therefore, when designing a printed circuit board, you should pay attention to the correct method, follow the general principles of PCB design, and should meet the anti-interference design requirements.

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

In the LED power system, the input/output is also the transmission line of the interference source, and the detection source for receiving the RF interference signal. We generally take effective measures when designing:

1 Use the necessary common mode / differential mode suppression circuit, and also take certain filtering and anti-electromagnetic shielding measures to reduce the interference.

2 Whenever possible, take various isolation measures (such as optical isolation or magnetoelectric isolation) to block the spread of interference.

3 lightning protection measures

The LED power supply system used outdoors or the power line and signal line that is introduced into the room from the outside should be considered for the lightning protection of the system. Commonly used lightning protection devices are: gas discharge tube, TVS (Transient Voltage Suppression) and the like. The gas discharge tube is a gas breakdown discharge when the voltage of the power source is greater than a certain value, usually tens of V or hundreds of V, and the strong shock pulse on the power line is guided into the earth. The TVS can be viewed as two parallel and opposite-direction Zener diodes that conduct when the voltage across the terminals is above a certain value. It is characterized by the ability to transiently pass hundreds of thousands of currents.

Anterior: LED driver design 5 key points attention

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