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Lightning strikes are a common physical phenomenon and the main source of voltage stress for power adapters. Improper protection can cause power damage or restart, thus affecting the normal operation of electronic equipment. Therefore, power adapters must meet the lightning voltage level requirements defined by safety standards.

In this issue, the Xinpengwei technical team will share with you lightning strike standards, lightning strike experiment configuration, differential mode and common mode interference path analysis and design principles.

Lightning strike standards

IEC61000-4-5 is a commonly used lightning test standard. Its definition and experimental procedures are as follows:

Generally, a surge voltage of ±1kV~±6kV is applied to the AC line. The test source is the AC line of the test equipment (EUT) and the grounding point of the system housing. During the test, the EUT is directly exposed to the surge energy and must be intact and continue to work normally after the lightning test is completed.

Configuration of lightning strike experiment

Figure 3 Schematic diagram of differential mode lightning configuration

Figure 4 Schematic diagram of common mode lightning configuration

There are two modules inside the lightning generator, namely the decoupling network and the coupling network. The function of the decoupling network is to isolate the lightning energy applied to the EUT phase line by the coupling network from the power supply phase line. The function of the coupling network is to apply the ideal lightning generation wave to the EUT phase line through the coupling capacitor.

As shown in Figure 3, the coupled energy of differential mode lightning is transferred between the phase lines L and N of the EUT. From Figure 4, it can be found that the coupled energy of common mode lightning is transferred between the phase line L (N) and PE of the EUT.

Differential Mode Interference Path Analysis and Design Principles

Path analysis

Figure 5 Schematic diagram of differential mode lightning current

Since different actual circuit configurations will have different effects on the system differential mode lightning strike analysis, we will make a brief analysis of the impact of the circuit in the figure above on differential mode lightning strikes.

The differential mode lightning energy passes through the coupling network, inputs into the phase lines L and N of the EUT, fuse F1, and varistor MOV1 to form loop 1, generating differential mode current 1;

After the differential mode lightning energy is attenuated through loop 1, it forms loop 2 through thermistor RT1, rectifier bridge, and electrolytic capacitor EC1, generating differential mode current 2;

After the differential mode lightning energy is attenuated through loops 1 and 2, it forms loop 3 through differential mode inductor L1 and electrolytic capacitor EC2, generating differential mode current 3.

Design principles

The addition of MOV1 can absorb the energy of differential mode current 1 and protect the rectifier bridge BD1 and electrolytic capacitors EC1 and EC2. Loop 1 is equivalent to the first flood control dam for lightning surge energy. Due to the large current in this loop, the PCB copper foil width is recommended to be 0.5mm/kV;

The addition of high-resistance negative temperature coefficient thermistor RT1 can share the energy applied to EC1 by differential mode current 2, protecting rectifier bridge BD1 and electrolytic capacitor EC1. Loop 2 is equivalent to the second flood control dam.

The impedance of the input differential mode inductor can share the energy applied to EC2 by the differential mode current 3. Loop 3 is equivalent to the third flood control dam. Since there is a residual voltage of several hundred volts of lightning energy on EC2, it is recommended to use a high avalanche withstand power MOSFET as the primary power tube.

[Expert Post] How to improve the avalanche reliability of MOSFET switching power supply chips?

Experimental Results

The 12V1.5A adapter based on PN8390, 4kV (90°) differential mode lightning test is shown in the figure below:

Figure 6 4kV differential mode lightning test waveform

From the test waveform, it can be seen that the maximum voltage of EC1 is 756V, the maximum voltage of EC2 is 556V, and the maximum voltage of PN8390 is 779V. Therefore, in order to improve the power adapter's ability to resist differential mode lightning strikes, in addition to reasonably selecting MOV and NTC resistors, high aluminum foil voltage electrolytic capacitors and high avalanche withstand power MOSFETs should be selected.

Common mode interference path analysis and design principles

Path analysis

Figure 7 Flow diagram of lightning common mode current

When common-mode lightning strikes, there are two main common-mode current paths (taking negative voltage as an example):

Common mode current 1: Lightning energy is applied to the output ground, through the output common mode inductor → secondary reference ground → CY1 → positive of the input electrolytic capacitor → rectifier bridge → input common mode inductor → L line or N line.

Common mode current 2: Lightning energy is applied to the output ground, through the output common mode inductor → secondary reference ground → positive of output electrolytic → transformer → ground of auxiliary winding → negative of input electrolytic capacitor → rectifier bridge → input common mode inductor → L line or N line.

Design principles Consider the common-mode current path factor and optimize PCB wiring: add discharge pins to the input common-mode and Y capacitors, connect the ground of the primary controller and the ground of the transformer to the negative electrode of the input electrolytic capacitor, and connect the ground of the synchronous rectifier chip and the ground of the Y capacitor to the negative electrode of the output electrolytic capacitor.

In order to prevent common-mode current from interfering with the synchronous rectifier chip, dual-power supply synchronous rectifier chips, such as PN8309H, are preferred, and a 10~22Ω resistor is connected in series to the Vin pin;

In order to prevent common-mode current from interfering with the primary main control chip, a resistor should be connected in series in the Vdd power supply loop, the Vdd electrolytic capacitor should be placed close to the chip pin, and a 100nF decoupling capacitor should be added.

The experimental results based on the PN8309H 12V3A adapter 6.6kV common mode lightning test are shown in the figure below:

Figure 8 6.6kV common mode lightning test waveform

From the test waveform, it can be seen that the SW, Vin, and Vcc voltages of PN8309H are 161V, 25V, and 19V respectively. Therefore, in order to improve the power adapter's ability to resist common-mode lightning strikes, in addition to reasonable layout and increased filter capacitors, dual power supplies and synchronous rectification chips with integrated high avalanche tolerance MOSFETs are preferred.

Conclusion

The design of power supply lightning resistance is one of the problems that bothers many power supply engineers. The best design principle is reasonable PCB routing, plus better device selection. Once a lightning failure occurs, it is necessary to start with the transient operating waveform of the failure, combine principle analysis and device characteristics, find out the root cause and improve it.

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