Cheng Lixin learns with you
For engineers designing LED power supplies, the electromagnetic interference problem should be a key issue that has always existed in the design, especially on July 16 this year, China National Certification and Accreditation Commission has incorporated LED drive power into the category of 3C compulsory certification. The new version of the rules was formally implemented on September 1, 2014. Adjustments to newly added products will begin on September 1, 2015. Unauthorized products may not be manufactured, sold, imported, or used in other business activities. But friends who are familiar with the power supply circuit design know that in the design process of LED power supply, electromagnetic interference (EMI) is a big problem. How can we solve this problem?
First, let's look at several factors that can affect EMI/EMC: the circuit structure of the drive power supply; the switching frequency, grounding, PCB design, and reset circuit design of the smart LED power supply. Since the original LED power supply is a linear power supply, the linear power supply loses a lot of energy in the form of heat during operation. The working mode of the linear power supply makes it necessary to have a pressure device from the high voltage to the low voltage, generally the transformer, and then through the rectified output DC voltage. Although it is bulky and has a large amount of heat, the advantage is that the external interference is small and the electromagnetic interference is small, and it is also easy to solve. Now more LED switching power supplies are used. The LED driving power in the form of PWM is to make the power transistors operate in the on and off states. When turned on, the voltage is low and the current is large; when it is turned off, the voltage is high and the current is small, so the loss generated on the power semiconductor device is also small. The obvious disadvantage is that electromagnetic interference (EMI) is also more serious.
Problems with the electromagnetic compatibility of the LED power supply are generally in the power supply of the switch circuit. The switch circuit is one of the main sources of interference of the switching power supply. The switch circuit is the core of the LED drive power, and the switch circuit is mainly composed of a switch tube and a high-frequency transformer. The du/dt it produces has a larger amplitude pulse, wider frequency band and rich harmonics. The main reason for this high-frequency pulse interference is: the load of the switch tube is the primary coil of the high-frequency transformer, which is an inductive load. In the moment of conduction, the primary coil generates a large inrush current and a high surge voltage appears across the primary coil. At the moment of disconnection, part of the energy is not transmitted from the primary coil to the secondary due to the leakage flux of the primary coil. In the secondary coil, a attenuated oscillation with spikes is formed in the circuit, 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 an LED power system, the switching circuit generates a current spike, and a current spike is also generated when the load current changes. This is one of the sources of electromagnetic interference.
Basically in all the problems of electromagnetic interference problems, it is mainly due to improper grounding. There are three signal grounding methods: single-point, multi-point, 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, it is better to use multi-point grounding. Hybrid grounding is a single-point grounding for low-frequency applications, and multipoint grounding for high-frequency applications. The ground layout is the key. The ground circuit of the high-frequency digital circuit and the low-level analog circuit must not be mixed. It can be said that appropriate printed circuit board (PCB) layout is essential to prevent EMI. In the LED power supply, many smart LED power supplies are controlled by a single-chip microcomputer, and some LED power supplies use a microcontroller to control the duty cycle of the switch circuit. The watchdog system of the single-chip microcomputer plays a particularly important role in the operation of the entire LED power supply. All interference sources may not be completely isolated or removed. Once the CPU interrupts the normal operation of the program, the reset system combined with software processing measures becomes an effective barrier to error correction and defense. Commonly used reset systems have the following two types: 1 External reset system. The external "watchdog" circuit can be designed either by itself or by using a special "watchdog" chip. Thus, if the program system is trapped in an endless loop that happens to have a "feed dog" signal, the reset circuit will not be able to perform its intended function. 2 Now more and more LED power supplies have their own on-chip reset system, so that users can easily use their internal reset timer, but some smart LED power supply control circuit reset command is too simple. There will also be a "feed dog" instruction like the above-mentioned dead loop, which will be lost to monitoring.
To solve the electromagnetic interference problem of LED driving power, we can start from the following aspects:
1. To reduce the interference of the switching power supply itself: soft switching technology, in the original hard-switching circuit to increase the inductance and capacitance components, use the resonance of the inductor and capacitor, reduce the du/dt and di/dt in the switching process, make the switching device open When the voltage drops before the current rises, or when the off current drops before the voltage rises, the voltage and current overlap is eliminated. The switching frequency modulation technique, by modulating the switching frequency fc, disperses the energy concentrated on fc and its harmonics 2fc, 3fc... Into the frequency band around them to reduce the EMI amplitude at each frequency point. The selection of components and components that are not likely to produce noise, conduction, and radiation noise. It is generally worth noting that the choice of windings and 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. Reasonable use of electromagnetic interference filter, one of the main purposes of EMI filter, power grid noise is a kind of electromagnetic interference, it belongs to radio frequency interference (RFI), 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 resulting interference is small; the common mode interference amplitude is large, the frequency is high, and radiation can also be generated through the wire, resulting in greater interference. To weaken the conducted interference, the most effective method is to install an EMI filter in the switching power supply input and output circuits. LED power supplies generally use simple single-stage EMI filters, which mainly include common mode chokes and filter capacitors. The EMI filter can effectively suppress the electromagnetic interference of the switching power adapter.
2. Reduce the electromagnetic interference problem by cutting off the propagation path of the interference signal: The first case is that the power line interference can be filtered using the power line filter. A reasonable and effective switching power supply EMI filter should have a strong inhibitory effect on differential mode and common mode interference on power lines. Improving PCB Electromagnetic Compatibility Design PCBs are support components for circuit elements and devices in LED power systems that provide electrical connections between circuit components and devices. With the rapid development of electronic technology, PCB density is getting higher and higher. The quality of PCB design has a great influence on the electromagnetic compatibility of LED power systems. Practice has confirmed that even if the circuit schematic design is correct, improper design of the printed circuit board will adversely affect the reliability of the LED power supply system. The PCB anti-interference design mainly includes PCB layout, wiring and grounding. Its purpose is to reduce the electromagnetic radiation between the PCB and the crosstalk between the circuits on the PCB. In addition, the commutation frequency caused by the electromagnetic interference of the transformer is generally about 50 Hz, and the hum sound caused by improper grounding wiring is carefully discriminated because the frequency of the frequency multiplication of the rectifier circuit is about 100 Hz. Therefore, when designing a printed circuit board, attention should be paid to adopting the correct method, complying with the general principles of PCB design, and complying with the anti-jamming design requirements.
3. Actively enhance the anti-interference ability of the victim: In the LED power system, the input/output is also the conductive line of the interference source, and the pick-up source of the RF interference signal is received. We usually take effective measures when designing: The necessary common-mode/differential-mode rejection circuits must also take certain filtering and electromagnetic shielding measures to reduce the interference. When conditions permit, various isolation measures (such as opto-isolation or magneto-electric isolation) are adopted as far as possible so as to block the spread of interference. Lightning protection measures, outdoor use of the LED power supply system, or the introduction of indoor power cables and signal cables from the outside, must consider the system's lightning protection. The commonly used lightning protection devices include gas discharge tubes and TVS (Transient Voltage Suppression). When the voltage of the power supply is greater than a certain value, the gas discharge tube is usually several tens of V or several hundred V. The gas is punctured and discharged, and the strong impulse pulse on the power line is led into the earth. TVS can be seen as two Zener diodes connected in parallel and in opposite directions, and turned on when the voltage across them is higher than a certain value. Its characteristic is that it can transiently pass hundreds of thousands of A currents.
Through this article, we can conclude that the main control technologies for LED power EMC/EMI are: circuit measures, EMI filtering, component selection, shielding, and printed circuit board anti-jamming design. If you can correctly and reasonably solve these problems, through the LED drive power passed the 3C certification, not a problem!