Solving the problem of electromagnetic interference of LED power supply is a loop that cannot be bypassed by 3C certification. Those who are familiar with the design of power supply circuits know that electromagnetic interference EMI is a big problem in the design process of LED power supply. 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!
First, several factors affecting EMC1. The 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.
3. Switching pulse spikes
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.
Figure 1 shows the switching pulse 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.
4. 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.
5.PCB design
Proper printed circuit board (PCB) wiring is critical to preventing EMI.
Reset circuit design of intelligent LED power supply
Enhance the anti-interference ability of the interfered body.
In the LED drive 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:
Use the necessary common mode / differential mode suppression circuit, and also take certain filtering and anti-electromagnetic shielding measures to reduce the interference.
Whenever possible, take various isolation measures (such as optical isolation or magnetoelectric isolation) to block the spread of interference.
6. 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 tubes 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.
Second, the hardware processing method for interference measuresTo 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.
Figure 2 Common LED Power Filters
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.
Figure 3: Before and after adding an EMI filter
2. Cut off the propagation path 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.
Figure 4: Correct selection of the main filter capacitor pin as a concentrated ground point
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 as shown in Figure 4. The main filter capacitor pin is selected as the concentrated grounding point. 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.
Therefore, LED power supply electromagnetic electromagnetic interference control technology mainly includes: circuit measures, EMI filtering, component selection, shielding and printed circuit board anti-interference design.
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Operating temperature range:
vibration motor should be used at a temperature of -10~60℃.
The figures stated in the catalog specifications are based on use at ordinary room temperature catalog specifications re based on use at ordinary room temperature (approximately20~25℃.
If a vibration motor is used outside the prescribed temperature range,the grease on the gearhead area will become unable to function normally and the motor will become unable to start.Depending on the temperature conditions ,it may be possible to deal with them by changing the grease of the motor's parts.Please feel free to consult with us about this.
Storage temperature range:
vibration motor should be stored ta a temperature of -15~65℃.
In case of storage outside this range,the grease on the gearhead area will become unable to function normally and the motor will become unable to start.
Service life:
The longevity of vibration motor is greatly affected by the load conditions , the mode of operation,the environment of use ,etc.Therefore,it is necessary to check the conditions under which the product will actually be used .The following conditions will have a negative effect on longevity.Please consult with us should any of them apply.
â—Use with a load that exceeds the rated torque
â—Frequent starting
â—Momentary reversals of turning direction
â—Impact loads
â—Long-term continuous operation
â—Forced turning using the output shaft
â—Use in which the permitted overhang load or the permitted thrust load is exceeded
â—A pulse drive ,e.g.,a short break,counter electromotive force,PWM control
â—Use of a voltage that is nonstandard as regards the rated voltage
â—Use outside the prescribed temperature or relative-humidity range,or in a special environment.
â—Please consult with us about these or any other conditions of use that may apply,so that we can be sure that you select the most appropriate model.
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