The solution to the interference problem  photoelectric LED drive power supply

The interference problem of LED drive power is not easy to be solved, mainly because the influence caused by interference is often uncontrollable and unpredictable. Therefore, it is necessary to find out the source of interference problem in the face of various disturbances of different sizes.

I. reasons for interference caused by LED drive power supply

The LED drive power supply first converts the power frequency ac rectifier into dc, then reverses to high frequency, and finally gets a stable dc voltage through the rectifier filtering circuit output, so it contains a lot of harmonic interference. At the same time, electromagnetic interference is formed because of the leakage of transformer and the peak caused by the reverse recovery current of output diode. The interference sources in the switching power supply are mainly concentrated on the components with large changes in voltage and current, which are highlighted in the switching tube, diode and high frequency transformer.

Solenoid interference generated by the switching circuit

Switching circuit is one of the main interference sources of switching power supply. The switching circuit is the core of the switching power supply (same as the LED street lamp power supply and the LED tunnel lamp driving power supply), mainly composed of the switching tube and the high-frequency transformer. It produces du/dt with large pulse, wide frequency band and rich harmonics. The main reason for this kind of pulse interference is that the switching tube load is the primary coil of the high frequency transformer and the inductive load. At the moment when the switch tube is on, the primary coil generates a large current and a high surge peak voltage occurs at both ends of the primary coil. At the moment when the switch tube is disconnected, due to the leakage magnetic flux of the primary coil, a part of the energy is not transmitted from the primary coil to the secondary coil. The energy stored in the inductor will form a attenuation oscillation with a peak of attenuation with capacitance and resistance in the collector circuit, which will be superimposed on the cut-off voltage to form the cut-off voltage peak. Power supply voltage interruption will produce the same magnetized shock current transient as when the primary coil is connected. This transient is a kind of conductive electromagnetic interference, which not only affects the primary transformer, but also causes the conduction interference to return to the distribution system, causing the power grid harmonic electromagnetic interference, thus affecting the safety and economic operation of other equipment.

Electromagnetic interference generated by the rectifier circuit

In rectifier circuit, there is a reverse current at the end of the output rectifier diode. Among them, the diode that can quickly restore the reverse current to zero is called the hard recovery characteristic diode. Under the influence of the transformer leakage inductance and other distribution parameters, the diode will generate strong high-frequency interference with a frequency up to dozens of MHz. To the high-frequency rectifier circuits of rectifier diode is often larger forward current through the, in the reverse bias voltage and when to stop, because the more carrier accumulation in the PN junction, so before the carrier disappear a period of time, current will flow, the carrier of reverse recovery current decreased dramatically and produce large current changes.

Quadrature voltage transformer

The high frequency switching current loop composed of primary coil, switch tube and filter capacitor of high frequency transformer may generate large space radiation and form radiation interference. If the capacitance filtering capacity is insufficient or the high frequency characteristic is not good, the high frequency impedance on the capacitance will cause the high frequency current to conduct to the ac power supply in the mode of differential mode and form the conduction interference. It should be noted that in the electromagnetic interference caused by diode rectifier circuit, the di/dt of rectifier diode reverse-restoring current is much larger than that of continuous diode reverse-restoring current. As the electromagnetic interference source, the rectifier diode reversely recovers the current form the interference intensity is large, the frequency band is wide. However, the voltage jump produced by rectifier diode is much smaller than the voltage jump produced when power switch tube is on and off. Therefore, also can be produced without rectifier diode │ dv/dt │ influence, the rectifier circuit as part of the electromagnetic interference coupling channels to study.

The interference caused by the distributed capacitance

The switching power supply operates at high frequency, so its distributed capacitance cannot be ignored. On the one hand, the contact area between the heat sink and the switch tube collector is large, and the insulation piece is thin, so the distribution capacitance between the two cannot be ignored at high frequency. The high frequency current will flow to the radiator through distributed capacitance and then to the chassis, causing common mode interference. On the other hand, there is distributed capacitance between the initial stages of the pulse transformer, which can directly couple the original side voltage to the side of the side and generate common mode interference on the two power lines that output dc at the side of the side.

Transaction stray parameters affect the characteristics of the coupling channel

In the conduction interference frequency band (<30MHz), most coupling channels of switching power interference can be described by circuit network. However, any practical component of a switching power supply, such as resistance, capacitance, inductance and even switching tube and diode, contains stray parameters, and the wider the frequency band studied, the higher the order of equivalent circuit. Therefore, the equivalent circuit of switching power supply, including the stray parameters of components and the coupling between components, will be much more complicated. At high frequencies, the stray parameters have great influence on the characteristics of the coupled channel, and the distributed capacitance becomes the channel of electromagnetic interference. In addition, when the power of the switching tube is large, the collector is generally required to add a radiator fin. The distributed capacitance between the radiator fin and the switching tube cannot be ignored at high frequencies.

Control technology of switching power source electromagnetic interference

To solve the electromagnetic interference problem of switching power supply, we can start from three aspects: 1) reduce the interference signal generated by the interference source; 2) cut off the transmission channel of the interference signal; 3) enhance the anti-interference ability of the subject. Therefore, the control technology of switching power electromagnetic interference mainly includes: circuit measures, EMI filtering, component selection, shielding and printed circuit board anti-interference design.

Solenoid reduces the interference of the switching power supply itself

Soft switching technology: add inductor and capacitor elements in the original hard switching circuit, reduce the du/dt and di/dt in the switching process by using the resonance of inductor and capacitor, and make the voltage drop before the current rise when the switching device is on, or the current drop before the voltage rise when it is off, to eliminate the voltage and current overlap.

Switching frequency modulation technology: by modulating switch frequency fc, focusing on fc and its harmonics 2fc, 3fc... The energy on the band is dispersed around them to reduce the EMI amplitude at each frequency point. This method cannot reduce the total amount of interference, but the energy is dispersed to the baseband of the frequency point, so that each frequency point does not exceed the EMI limit. In order to reduce the peak value of noise spectrum, there are usually two methods: random frequency method and modulation frequency method.

Component selection: select components that are not prone to noise, conduction and radiation. Especially noteworthy is the selection of winding components such as diodes and transformers. Fast recovery diode with low reverse recovery current and short recovery time is an ideal device for high frequency rectifier of switching power supply.

Active suppression technology for common-mode interference: try to take out a compensated EMI noise voltage from the main circuit that is completely inversed from the main switching voltage waveform causing electromagnetic interference and use it to balance the original switching voltage.

Filtering: one of the main purposes of EMI filter is to obtain high insertion loss in the range of 150kHz ~ 30MHz frequency band, but there is no attenuation of 50Hz power frequency signal, so that rated voltage and current can pass smoothly, and certain size requirements must also be met. Any transmission interference signal on the power line can be represented by difference mode and common mode signal. In general, the range of differential mode interference is small, the frequency is low, and the interference is small. The interference range of common mode is large, the frequency is high, and the radiation can be generated through the wire, resulting in greater interference. Therefore, in order to weaken the conduction interference, the most effective way to control EMI signal below the limit level stipulated by relevant EMC standards is to install electromagnetic interference filter in the input and output circuit of switching power supply.

PCB design: PCB anti-interference design mainly includes PCB layout, wiring and grounding. Its purpose is to reduce the electromagnetic radiation of PCB and the crosstalk between circuits on PCB. The best way to layout a switching power supply is similar to its electrical design. After determining the size and shape of PCB, the position of special components (such as various generators, crystal vibration, etc.) is determined. Finally, all components of the circuit are arranged according to the functional unit of the circuit.

Buffer circuit to reduce electromagnetic interference: it is composed of linear impedance stabilization network, which can eliminate potential interference in power line, including power line interference, power fast transient, power surge, voltage fluctuation and power line harmonics. These interferences have little influence on the general stable voltage power supply, but have significant influence on the high frequency switching power supply.

Design of common-mode and different-mode power line filters designed to shut off interference signals

Power line interference can be filtered by a power line filter. A reasonable and effective EMI filter for switching power supply should have strong inhibitory effect on both differential mode and common mode interference on power line.

Torque enhances the anti-interference capability of sensitive circuits