A solution to the interference problem of LED driver power supply



 The interference problem of LED drive power supply is not easy to solve, the main reason is that the impact of interference is often unpreventable and unpredictable, and how to do this problem well in the face of various types of interference of different sizes, it is necessary to find the root cause of the interference problem.

First, the cause of interference in the LED drive power supply

  The LED driver power supply first rectifies the power frequency AC to DC, then inverts it into high frequency, and finally outputs it through the rectifier filter circuit to obtain a stable DC voltage, so it contains a large number of harmonic interference. At the same time, electromagnetic interference is caused by the leakage inductance of the transformer and the spikes caused by the reverse recovery current of the output diode. The interference sources in the switching power supply are mainly concentrated in the components with large changes in voltage and current, which are prominently manifested in the switches, diodes, and high-frequency transformers.

  (1) Electromagnetic interference generated by switching circuits

  The switching circuit is one of the main sources of interference in the switching power supply. The switching circuit is the core of the switching power supply (the same is true for the LED street lamp power supply and the LED tunnel light drive power supply), which is mainly composed of a switch tube and a high-frequency transformer. It produces du/dt with large pulses, wide frequency bands and rich harmonics. The main reason for this pulse interference is that the switch load is the primary coil of the high-frequency transformer, which is an inductive load. At the moment when the switch tube is turned on, the primary coil produces a large inrush current, and a high surge spike voltage occurs at both ends of the primary coil; At the moment when the switch tube is disconnected, due to the leakage flux of the primary coil, a part of the energy is not transmitted from the primary coil to the secondary coil, and this part of the energy stored in the inductor will form an attenuation oscillation with a spike with the capacitance and resistance in the collector circuit, which will be superimposed on the shutdown voltage to form a shutdown voltage spike. The interruption of the power supply voltage will produce the same magnetized impulse current transient as when the primary coil is turned on, which is a kind of conductive electromagnetic interference, which not only affects the primary transformer, but also makes the conductive interference return to the distribution system, causing harmonic electromagnetic interference in the power grid, thereby affecting the safety and economic operation of other equipment.

  (2) Electromagnetic interference generated by rectifier circuits

  In the rectifier circuit, there is a reverse current at the cut-off of the output rectifier diode, and the time it takes to return to zero is related to factors such as junction capacitance. Among them, the diode that can quickly restore the reverse current to zero is called the hard recovery characteristic diode, which will produce strong high-frequency interference under the influence of transformer leakage inductance and other distribution parameters, and its frequency can reach tens of MHz. When the rectifier diode in the high-frequency rectifier circuit is turned on by the forward conduction, there is a large forward current flowing, and when it is turned off by the reverse bias voltage, because there is more carrier accumulation in the PN junction, the current will flow in the opposite direction for a period of time before the carrier disappears, resulting in a sharp decrease in the reverse recovery current of the disappearance of the carriers and a large current change.

  (3) High-frequency transformer

  The high-frequency switching current loop composed of the primary coil, switch tube and filter capacitor of the high-frequency transformer may generate large space radiation and form radiated interference. If the capacitor filter capacity is insufficient or the high-frequency characteristics are not good, the high-frequency impedance on the capacitor will cause the high-frequency current to be conducted into the AC power supply in a differential-mode manner, resulting in conducted interference. It should be noted that in the electromagnetic interference generated by the diode rectifier circuit, the di/dt of the rectifier diode reverse recovery current is much larger than that of the freewheeling diode. As a source of electromagnetic interference, the interference intensity and frequency bandwidth formed by the reverse recovery current of the rectifier diode are large. However, the voltage jump generated by the rectifier diode is much smaller than that generated when the power switch is turned on and off. Therefore, the rectifier circuit can also be studied as part of the electromagnetic interference coupling channel, regardless of the │dv/dt│ effect generated by the rectifier diode.

  (4) Interference caused by distributed capacitance

  The switching power supply works in a high-frequency state, so its distributed capacitance cannot be ignored. On the one hand, the contact area of the insulating sheet between the heat sink and the collector of the switch tube is large, and the insulating sheet is thin, so the distributed capacitance between the two cannot be ignored at high frequencies. The high-frequency current will flow to the heat sink through the distributed capacitance, and then to the chassis ground, resulting in common-mode interference. On the other hand, there is a distributed capacitance between the primary stage of the pulse transformer, which can directly couple the voltage of the primary side to the secondary side, and generate common-mode interference on the two power lines of the secondary side as the DC output.

  (5) Spurious parameters affect the characteristics of the coupling channel

  In the conducted interference frequency band (< 30MHz), most of the coupling channels of switching power supply interference can be described in terms of circuit networks. However, any actual component in the switching power supply, such as resistors, capacitors, inductors, and even switches and diodes, contains spurious parameters, and the wider the frequency band studied, the higher the order of the equivalent circuit. As a result, the equivalent circuitry of a switching power supply, including the spurious parameters of each component and the coupling between components, will be much more complex. At high frequencies, spurious parameters have a great influence on the characteristics of the coupling channel, and the presence of distributed capacitance becomes a channel for electromagnetic interference. In addition, when the power of the switch tube is large, the collector generally needs to add a heat sink, and the distributed capacitance between the heat sink and the switch tube can not be ignored at high frequency, which can form the radiation interference facing the space and the common mode interference of power line conduction.

Second, the control technology of electromagnetic interference of switching power supply

  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 propagation path of the interference signal; 3) Enhance the anti-interference ability of the victim body. Therefore, the control technology of electromagnetic electromagnetic interference of switching power supply mainly includes: circuit measures, EMI filtering, component selection, shielding and anti-interference design of printed circuit boards.

  (1) Reduce the interference of the switching power supply itself

  Soft switching technology: add inductance and capacitance components in the original hard switching circuit, use the resonance of inductance and capacitance to reduce the du/dt and di/dt in the switching process, so that the voltage drop when the switching device is turned on is before the rise of current, or the drop of current when the switch is turned off is before the rise of voltage, so as to eliminate the overlap of voltage and current.

  Switching frequency modulation technology: by modulating the switching frequency FC, the FC and its harmonics 2FC, 3FC... to reduce the EMI amplitude at each frequency. This method does not reduce the total amount of interference, but the energy is dispersed to the baseband of the frequency so that the individual frequencies do not exceed the limits specified by the EMI. In order to achieve the purpose of reducing the peak value of the noise spectrum, there are usually two processing methods: random frequency method and modulated frequency method. 

  Component selection: Select components that are not prone to noise, conduction and radiated noise. Of particular note is the selection of winding components such as diodes and transformers. Fast recovery diodes, with small reverse recovery current and short recovery time, are ideal devices for the high-frequency rectification part of switching power supplies.

  Active Suppression of Common-Mode Interference: Attempts to take a compensated EMI noise voltage from the main loop that is completely inverted to 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 frequency band range of 150kHz~30MHz, but there is no attenuation of the power frequency signal with a frequency of 50Hz, so that the rated voltage and current can pass smoothly, and at the same time, it must meet certain size requirements. Conducted interference signals on any power line can be represented by both differential-mode and common-mode signals. In general, the amplitude of differential mode interference is small, the frequency is low, and the interference caused is small; The common-mode interference amplitude is large, the frequency is high, and the radiation can also be generated through the wire, which causes a large interference. Therefore, the most effective way to reduce conducted interference and control EMI signals below the limit level specified in the relevant EMC standards is to install electromagnetic interference filters in the input and output circuits of switching power supplies.

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

  Buffer circuit to reduce electromagnetic interference: It is composed of a linear impedance stabilization network, which is used to eliminate potential interference in the power supply power line, including power line interference, electrical fast transients, power surges, voltage changes and power line harmonics. These interferences do not have a great impact on the general regulated power supply, but the impact on the high-frequency switching power supply is significant.

  (2) Cut off the propagation path of interference signals - common mode and differential mode power line filter design

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

  (3) Enhance the anti-interference ability of sensitive circuits