Interference sources and solutions for switching power adapters

The advantages of switching power adapters are small size and high conversion efficiency, but because it works in a high-frequency switching state, it will generate high-frequency harmonic components, and these harmonic components will radiate to external circuits and spaces through circuits and spaces, interfering with the normal operation of other electronic devices.

 

There are two main aspects of interference:

1. The impact of high-frequency interference signals generated by the switching power adapter itself on the normal operation of other electronic devices;

2. The ability of the switching power adapter itself to resist interference from external interference signals and ensure its normal operation, that is, anti-interference. A switching power adapter with good interference and anti-interference performance will have better working stability.

 

According to the form of interference, the interference of the switching power adapter can be divided into electromagnetic radiation interference (EMI) and radio frequency interference (RFI). There are many factors that cause interference sources in the switching power adapter. The following are several main sources of interference.

 

1. Interference generated by the power switch tube when it is in the switching working state.

The power switch tube in the switching power adapter works in the switching state, and it will generate large pulse voltage and pulse current when working. Because the pulse current and pulse voltage contain rich high-order harmonic components, and because the leakage inductance of the switching transformer and the recovery characteristics of the rectifier diode when the power switch tube is turned on will form current oscillation, and the surge voltage generated on the rectifier diode, and the surge voltage generated by the leakage inductance of the transformer when the power switch tube is turned off, these are all noise sources of the switching power supply adapter.

 

2. Interference caused by the recovery characteristics of the diode.

When the diode performs high-frequency rectification, due to the junction capacitance of the diode, the charge stored in the forward current cannot disappear immediately when the reverse voltage is applied, which will form the inherent reverse current of the diode. This period of time is called the reverse recovery time. At this time, due to the large reverse voltage applied to the diode, it will produce large losses and form a large source of interference.

 

If the current change rate di/dt of the diode is large when the reverse current recovers, a large peak voltage will be generated due to the inductance, which is the recovery noise of the diode. When Di/dt is large, it is called hard recovery, and when Di/dt is small, it is called soft recovery. Soft recovery can be achieved through absorption circuits or resonant switching technology. Soft recovery is of great benefit to improving the working reliability of the switching power supply adapter and reducing interference. Since Schottky diodes have no carrier accumulation effect, the recovery noise is very small.

3. Interference generated by high-frequency transformer windings.

The current in the high-frequency transformer windings forms magnetic flux, most of which passes through the high-permeability magnetic core, but a small part of the magnetic flux radiates through the winding gap, becoming the so-called leakage flux, which will form electromagnetic interference.

 

4. Interference generated by the rectifier filter circuit.

The AC input end of the switching power supply adapter is connected to the rectifier filter circuit. The conduction angle of the rectifier diode is very small, which makes the peak value of the rectifier current very large. This pulse-shaped diode rectifier current will also cause interference.

 

Interference and solution of switching power supply adapter

 

According to the factors that generate electromagnetic compatibility, solving the electromagnetic compatibility of the switching power supply adapter 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 interfered body

 

For external interference generated by the switching power supply adapter, such as power line harmonic current, power line conduction interference, electromagnetic field radiation interference, etc., can only be solved by reducing interference. On the one hand, the design of input/output filter circuit can be enhanced, the performance of active power factor compensation (APFC) circuit can be improved, the voltage and current change rate of switch tube and rectifier and freewheeling diode can be reduced, and various soft switch circuit topology structures and control methods can be adopted; on the other hand, the shielding effect of the casing can be strengthened, the gap leakage of the casing can be improved, and good grounding treatment can be performed.

 

For external anti-interference ability, such as surge and lightning strike, the lightning protection ability of AC input and DC output ports should be optimized. For lightning strike, a combination of zinc oxide varistor and gas discharge tube can be used to solve it. For electrostatic discharge, TVS tube and corresponding grounding protection can be used, the distance between small signal circuit and casing can be increased, or devices with anti-static interference can be selected to solve it. To reduce the internal interference of the power adapter, we should start from the following aspects: pay attention to the single-point grounding of digital circuits and analog circuits, and the single-point grounding of high-current circuits and low-current circuits, especially current and voltage sampling circuits, to reduce common impedance interference and reduce the impact of ground loops; pay attention to the spacing between adjacent lines and signal properties when wiring to avoid crosstalk; reduce ground line impedance; reduce the area surrounded by high-voltage and high-current lines, especially the primary side of the transformer and the switch tube, power supply filter capacitor circuit; reduce the area surrounded by the output rectifier circuit and the freewheeling diode circuit and the DC filter circuit; reduce the leakage inductance of the transformer and the distributed capacitance of the filter capacitor; use filter capacitors with high resonant frequency, etc.

 

In terms of transmission paths, appropriately increase TUS with high anti-interference ability and high-frequency capacitors, ferrite beads and other components to improve the anti-interference ability of small signal circuits; small signal circuits close to the casing should be properly insulated and withstand voltage treated; the heat sink of the power device and the electromagnetic shielding layer of the main transformer should be properly grounded; the large area grounding between the control units should be shielded with a grounding plate; on the rectifier rack, the electromagnetic coupling between the rectifiers and the grounding layout of the whole machine should be considered to improve the stability of the internal operation of the power adapter.

 

We have established our own electromagnetic compatibility laboratory and have been committed to the research of electromagnetic compatibility in the early stage of the development of switching power adapters. Through professional power input and output filter design and lightning protection design, as well as the safety of the whole machine, the anti-static design of the digital interface circuit and the anti-fast transient pulse group design, the electromagnetic shielding design of the whole machine structure is just right, so that the electromagnetic environment inside the whole machine is good, the operation is stable, and the reliability is improved. The wide AC input voltage range enables the switching power adapter to work normally after the interference of voltage drop, voltage transient and short-term voltage interruption of the whole machine.