Lightning surge design principles consider common-mode differential mode current path factors

Lightning stroke is not only a common physical phenomenon, but also the main source of voltage stress of power adapter. Improper protection will cause power damage or restart, thus affecting the normal operation of electronic equipment. Therefore, the power adapter should meet the lightning voltage level requirements defined by the safety standards. In this issue, we will share the lightning surge standard, lightning surge experimental configuration, differential mode and common mode interference path analysis and design principles.

Lightning surge standard

IEC61000-4-5 is a common lightning surge test standard, and its definition and test procedure are as follows:

Generally speaking, ± 1kV~± 6kV surge voltage is applied on the AC line. The test source is the grounding point between the AC line of the test equipment (EUT) and the system shell. During the test, the EUT is directly exposed to the surge energy and must be intact. After the lightning test, it can still continue to work normally.

Configuration of lightning surge test

图3.差模雷击示意图

Figure 4. Common mode lightning stroke schematic diagram

There are two modules in the lightning surge generator, namely decoupling network and coupling network. The function of decoupling network is to isolate the lightning energy on the EUT phase line from the phase line of power supply. The function of the coupling network is to apply the ideal lightning wave to the phase line of the EUT on the ideal lightning wave through the coupling capacitance.

As shown in Figure 3, the phase lines L and N of the coupling energy EUT of the differential mode lightning strike transfer between them. Through Figure 4, it can be found that the coupling energy of common-mode lightning stroke EUT is transferred between the phase line L (N) and PE.

Fig. 5 Schematic diagram of differential mode lightning current

Since different actual circuit configurations will have different impacts on the differential mode lightning stroke analysis of the system, we will briefly analyze the impact of the circuit above on the differential mode lightning stroke.

Phase lines L and N of differential mode lightning energy EUT, fuse F1, and voltage sensitive MOV1 form loop 1 through coupling network to generate differential mode current 1;

After the differential mode lightning energy is attenuated through circuit 1, circuit 2 is formed through thermistor RT1, rectifier bridge and electrolytic capacitor EC1 to generate differential mode current 2;

After the differential mode lightning energy is attenuated through circuits 1 and 2, the differential mode current 3 is generated through the differential mode inductance L1 and the electrolytic capacitor EC2 to form circuit 3.

Design principles

The addition of MOV1 can absorb the energy of differential mode current 1 in design principle, protect the rectifier bridge BD1 and electrolytic capacitors EC1 and EC2. Due to the large circuit current, circuit 1 is equivalent to the first flood dam of lightning surge energy, and PCB recommends 0 copper foil width of 5mm/kV;

The addition of negative temperature coefficient thermistor RT1 can share the energy BD1 and electrolytic capacitor EC1 and circuit 2 on the differential mode current 2EC protection rectifier bridge, which are equivalent to the second flood dam;

The impedance of the input differential mode inductor can share the energy on the differential mode current 3EC2, and the circuit 3 is equivalent to the third flood dam. Because there is hundreds of residual voltage of voltage energy on the EC2, it is recommended that the high avalanche tolerance power MOSFET be used for the primary power tube.

Experimental result

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

Figure 6.4kV differential mode lightning test waveform

Visible test waveforms: EC1 maximum voltage 756V, EC2 maximum voltage 556V, PN8390 maximum voltage 779V. Therefore, in order to improve the differential mode lightning resistance of the power adapter, in addition to reasonably selecting MOV and NTC, high aluminum foil voltage electrolytic capacitor and high avalanche resistance power MOSFET should also be selected.

Common mode interference path analysis and design principles

Figure 7. Common mode current flow diagram of lightning stroke

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

Common mode current 1: The lightning stroke energy is applied to the output place through output common mode inductance → secondary reference ground → CY1 → positive input electrolytic capacitor → rectifier bridge → input common mode inductance → L line or N line.

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

The design principle considers the common-mode current path factor and optimizes the common-mode current path factor PCB wiring: the input common-mode and Y capacitors are added with discharge pins, the input electrolytic capacitor anode is connected to the original controller ground and transformer ground respectively, and the output electrolytic capacitor anode is connected to the synchronous current chip ground and Y capacitor ground respectively;

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

To prevent the common mode current from interfering with the original master chip, a resistor should be connected in series in the Vdd power supply circuit, placing the Vdd electrolytic capacitor close to the chip pin, and adding 100 nF decoupling capacitors.

The 6.6kV common-mode lightning stroke test of 12V3A adapter based on the experimental results PN8309H is shown in the following figure:

Figure 8.6.6kV Common Mode Lightning Test Waveform

The lightning surge test waveform can be seen. The SW, Vin and Vcc voltages of PN8309H are 161V, 25V and 19V respectively. Therefore, in order to improve the common-mode lightning resistance of the power adapter, in addition to the reasonable layout, in addition to increasing the filter capacitance, the dual power supply and integrated high avalanche tolerance MOSFET synchronous rectifier chip are also given priority.

The design of lightning protection capability of power supply is one of the problems that perplex many power supply engineers. The best design principle is reasonable PCB wiring and better equipment selection. In case of lightning failure, it is necessary to find out the root cause and improve it by combining principle analysis and device characteristics.