LED we all know, then you know some knowledge of LED driver power supply design? For the design of LED power supply engineers, electromagnetic interference problem should always exist in the design of a key issue, familiar with the power supply circuit design friends know that the design process in the LED power supply, electromagnetic interference EMI is not a small problem, then how can solve this problem?
The article starts with the hardware, introducing three major anti-interference measures.
1、First we look at several factors that can affect the EMI/EMC:
the circuit structure of the driver power supply; switching frequency, grounding, PCB design, intelligent LED power supply reset circuit design.
As the original LED power supply is a linear power supply, but the linear power supply will work in the form of heat loss of a lot of energy. The linear power supply works in such a way that he must have a voltage device from high voltage to low voltage, the general are transformers, and then through the rectification of the output DC voltage. Although bulky, heat generation, the advantage is that the external interference is small, electromagnetic interference is small, but also easy to solve. Now more LED switching power supply is used, are in the form of PWM LED drive power supply is to let the power transistor work in the on and off state. In the on, the voltage is low, the current is high; off, the voltage is high, the current is small, so the power semiconductor devices generated on the loss is also very small. The disadvantage is more obvious, electromagnetic interference (EMI) is also more serious.
LED power supply electromagnetic compatibility problems are generally in the switching circuit of the power supply. The switching circuit is one of the main sources of interference for switching power supplies. Switching circuit is the core of LED driver power supply, switching circuit mainly consists of switching tubes and high frequency transformers. It generates du/dt with large pulse amplitude, wide band and rich harmonics.
The main reason for this high-frequency pulse interference is: the switching tube load is the primary coil of the high-frequency transformer, which 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, due to the leakage flux of the primary coil, resulting in part of the energy not transferred from the primary coil to the secondary coil, the circuit forms a decay oscillation with spikes, superimposed on the turn-off voltage, forming a turn-off voltage spike. High-frequency pulses produce more emissions, and periodic signals produce more emissions. In the LED power system, the switching circuit generates current spike signals, and when the load current changes, it also generates current spike signals. This is one of the root causes of electromagnetic interference.
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Basically, in all topics of electromagnetic interference problems, it is mainly caused by improper grounding. There are three methods of signal grounding: single-point, multi-point and mixed. In the switching circuit frequency below 1MHz, single-point grounding method can be used, but it is not suitable for high frequency; in high frequency applications, it is better to use multi-point grounding.
Hybrid grounding is a single-point grounding method for low frequencies, and a multi-point grounding method for high frequencies. Ground layout is the key, high-frequency digital circuits and low-level analog circuit grounding circuit as far as possible can not be mixed. It can be said that the appropriate printed circuit board (PCB) wiring to prevent EMI is critical.
In the LED power supply, there are many intelligent LED power supply using a single-chip control, and some LED power supply using a single-chip control of the duty cycle of the switching circuit, the single-chip watchdog system plays a particularly important role in the operation of the entire LED power supply, as all sources of interference can not be isolated or to remove, once into the normal operation of the CPU interference program, then the reset system combined with software processing Measures have become an effective error correction defense barrier.
2、The following two types of reset systems are commonly used.
① external reset system. External “watchdog” circuit can be designed by themselves or can use a special “watchdog” chip to build. Thus, if the program system is trapped in a dead loop, and the loop happens to have a “feed dog” signal, then the reset circuit will not be able to achieve its proper function.
② Now more and more LED power supplies come with their own on-chip reset system, so that users can easily use its internal reset timer, but some intelligent LED power control circuit reset instructions are too simple, so there will be like the above dead loop “feed the dog” instructions, so that it is lost to monitor. role.
3、To solve the problem of electromagnetic interference in the LED driver power supply, from the hardware can start from the following aspects:
- Reduce the interference of the switching power supply itself: soft switching technology, in the original hard-switching circuit to add inductors and capacitive components, the use of resonance of inductors and capacitors to reduce the switching process du / dt and di / dt, so that the switching device opens when the voltage drops before the current rises, or off when the current drops before the voltage rises, to eliminate the overlap of voltage and current. Switching frequency modulation technique, by modulating the switching frequency fc, the energy concentrated in fc and its harmonics 2fc, 3fc … on the frequency band around them to reduce the EMI amplitude at each frequency point. The choice of components, choose components that are not easy to generate noise, not easy to conduct and radiation noise. Usually particularly noteworthy is that the diode and transformer winding components such as the choice of components. Reverse recovery current is small, short recovery time of the fast recovery diode is the ideal device for high-frequency rectification part of the switching power supply.
Reasonable use of electromagnetic interference filters, EMI filters, one of the main purposes of the grid noise is a kind of electromagnetic interference, it belongs to the radio frequency interference (RFI), the spectrum of its conducted noise is roughly 10KHz ~ 30MHz, up to 150MHz. In general, the differential mode interference amplitude is small, low frequency, the resulting interference is small; common mode interference amplitude is large, high frequency, but also Through the wire to generate radiation, resulting in greater interference. To weaken the conducted interference, the most effective way is to add electromagnetic interference filters in the input and output circuit of the switching power supply. LED power supply generally uses a simple single-stage EMI filter, mainly including common mode choke and filter capacitors.
2.By cutting off the propagation of interference signals to reduce the problem of electromagnetic interference: the first case is the power line interference can be used to filter out the power line filter. A reasonable and effective switching power supply EMI filter should be on the power line differential mode and common mode interference have a strong suppression effect. Improving the EMC design of the PCB board PCB is the support of the circuit components and devices in the LED power supply system, which provides the electrical connection between the circuit components and devices. With the rapid development of electronic technology, the density of PCB is getting higher and higher.
The PCB design has a great impact on the electromagnetic compatibility of the LED power supply system. PCB anti-interference design mainly includes PCB layout, wiring and grounding, the purpose of which is to reduce the PCB electromagnetic radiation and crosstalk between circuits on the PCB. Also, the general transformer electromagnetic interference caused by AC sound frequency is generally about 50HZ, and ground wiring improperly caused by AC sound, due to the rectifier circuit frequency multiplier effect of about 100HZ, careful distinction can still be detected. Therefore, in the design of the printed circuit board, attention should be paid to the use of the correct method to comply with the general principles of PCB design, and should meet the design requirements of anti-interference.
- active significantly enhance the anti-interference ability of the interfered body: in the LED power supply system input / output is also the source of interference in the transmission line, and receive RF interference signal pickup source, we generally design to take effective measures: the use of the necessary common mode / differential mode suppression circuit, but also to take certain filtering and anti-electromagnetic shielding measures to reduce interference into the. As far as possible under the conditions permitted to take a variety of isolation measures (such as photoelectric isolation or magnetic isolation), so as to block the spread of interference. Lightning protection measures, the outdoor use of LED power system or from the outdoor crowded introduction of indoor power lines, signal lines, to consider the system’s lightning protection topics. Commonly used lightning protection devices are: gas discharge tube, TVS (Transient Voltage Suppression), etc.. Gas discharge tube is when the voltage of the power supply is greater than a certain value, usually tens of V or hundreds of V, gas breakdown discharge, the power line on the strong shock pulse into the earth. TVS can be seen as two parallel and opposite direction of the Zener diode, when the voltage at both ends is higher than a certain value on. It is characterized by the ability to transiently pass hundreds or thousands of A currents.
Through this paper we can summarize the main control techniques for the LED power EMC/EMI are: circuit measures, EMI filtering, component selection, shielding and printed circuit board anti-interference design, etc.. If you can correctly and reasonably solve these problems, smoothly through the 3C certification, is not a problem! The above is some analysis of the LED driver power supply design, I hope to help you.
