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Solutions for Interference Problems Of Led Drivers (Part 1)

May. 26, 2020


The interference problem of CE LED driver is not easy to solve. The main reason is that the impact of interference is often unpredictable and unpredictable. How can this problem be done well in the face of various interferences of different sizes, You need to find the source of the interference problem.

1. Reasons for interference caused by LED driver

Constant voltage led driver first rectifies the power frequency AC to DC, and then inverts to high frequency, and then outputs it through the rectifier filter circuit to obtain a stable DC voltage, so it contains a lot of harmonic interference. At the same time, due to the leakage inductance of the transformer and the spike caused by the reverse recovery current of the output diode, electromagnetic interference is formed. The interference sources in the switching power supply are mainly concentrated on the components with large changes in voltage and current, which are prominently displayed on the switch tube, diode, and high-frequency transformer.

Constant Voltage Led Driver

Constant Voltage Led Driver

① Electromagnetic interference generated by the switching circuit

The switching circuit is one of the main interference sources of 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 lamp driving power supply), which is mainly composed of a switch tube and a high-frequency transformer. The du / dt it produces has a larger amplitude pulse, a wider frequency band and rich harmonics. The main reason for this pulse interference is: the switch tube load is the primary coil of the high-frequency transformer, which is an inductive load. At the instant when the switch is turned on, the primary coil generates a large inrush current, and a high surge peak voltage appears at both ends of the primary coil; when the switch is turned off, due to the leakage flux of the primary coil, part of the energy is not From the primary coil to the secondary coil, this part of the energy stored in the inductor will form a peaked attenuation oscillation with the capacitor and resistance in the collector circuit, superimposed on the turn-off voltage, forming a turn-off voltage spike. The interruption of the power supply voltage will produce the same magnetizing inrush current transient as when the primary coil is turned on. This transient is a conductive electromagnetic interference, which not only affects the transformer primary, but also returns the conducted interference to the distribution system, causing grid harmonics. Electromagnetic interference, which affects the safety and economic operation of other equipment.

② Electromagnetic interference generated by the rectifier circuit

In the rectifier circuit, there is a reverse current when the output rectifier diode is cut off. The time for it to return to zero is related to factors such as junction capacitance. Among them, the diode that can quickly recover the reverse current to zero is called a hard recovery characteristic diode. This diode will produce strong high-frequency interference under the influence of transformer leakage inductance and other distributed parameters, and its frequency can reach tens of MHz . A large forward current flows when the rectifier diode in the high-frequency rectifier circuit is turned on in the forward direction. When it is turned off by the reverse bias voltage, there are more carriers accumulated in the PN junction. A period of time before the vanishes, the current will flow in the reverse direction, causing the reverse recovery current for the vanishing of the carriers to decrease sharply and a large current change will occur.

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 spatial radiation, which forms radiation interference. If the filtering capacity of the capacitor 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 in a differential mode to form conducted interference. It should be noted that in the electromagnetic interference generated by the diode rectifier circuit, the di / dt of the reverse recovery current of the rectifier diode is much larger than that of the reverse recovery current of the freewheeling diode. Researched as a source of electromagnetic interference, the reverse recovery current formed by the rectifier diode has large interference intensity and frequency bandwidth. However, the voltage jump generated by the rectifier diode is much smaller than the voltage jump generated when the power switch is turned on and off. Therefore, the influence of the rectifier diode can be ignored, and the rectifier circuit can be considered as part of the electromagnetic interference coupling channel.

④Interference 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. High-frequency current will flow to the heat sink through the distributed capacitor and then to the chassis ground, causing common mode interference; on the other hand, there is a distributed capacitor between the primary and secondary of the pulse transformer, which can directly couple the primary voltage to the secondary In the above, the common-mode interference is generated on the two power lines for DC output on the secondary side.

⑤ Stray 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 by circuit networks. However, any actual component in the switching power supply, such as resistance, capacitance, inductance and even the switch tube and diode, contains stray parameters, and the wider the frequency band studied, the higher the order of the equivalent circuit. Therefore, the equivalent circuit of the switching power supply including the stray parameters of each component and the coupling between the components will be much more complicated. At high frequencies, stray 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. The distributed capacitance between the heat sink and the switch tube cannot be ignored at high frequencies. It can form space-oriented radiation interference and power line conduction. Common mode interference.