What are the key points of PCB wiring for surge prevention?

1, pay attention to the size of the design surge current in PCB wiring

In the test, it is often encountered that the original design PCB circuit board can not meet the needs of the surge. General engineers only consider the functional design of the system, such as the actual work of the system only needs to carry 1A current, and then design according to this, but there may be surge design required by the system. The transient surge current needs to reach 3KA (1.2/50us&8/20us). Now, if I design according to the actual working current of 1A, can I achieve the above transient surge capability? The actual experienced engineering is telling us that this is impossible, so what do we do? The following calculation method can be used as a basis for PCB wiring to carry instantaneous current:

Example: A 0.36mm wide 1oz copper foil, a 40us rectangle in a 35um thick line, with a maximum surge current of about 580A. If a 5KA(8/20us) guard design is to be made, then a reasonable PCB wiring for the front end would be 2 oz copper foil 0.9mm wide. Safety devices can be extended appropriately.

2. Pay attention to the safe spacing of surge port components layout

In addition to the safe spacing we need to design for normal operating voltage, we also need to consider the safe spacing for transient surges.

We can refer to the relevant specifications of UL60950 for the safety spacing of normal operating voltage design. In addition, UL has stipulated in UL796 standard that the voltage withstand test standard of printed circuit board is 40V/mil or 1.6KV/mm. This data is very useful in guiding the setting of safe spacing between PCB wires that can withstand Hipot's voltage test.

For example, according to Table 5B 60950-1, the conductor between the operating voltage of 500V should meet the voltage test of 1740Vrms, and the peak value of 1740X1.414=2460V. According to the 40V/mil setting standard, it can be calculated that the spacing between the two conductors of the PCB board should not be less than 2460/40=62mil or 1.6mm.

In addition to the above normal attention, we should also pay attention to the size of the surge and the characteristics of the protective device to increase the safety spacing. Calculated with the spacing of 1.6mm, the maximum cutoff creepage voltage is 2460V. If the voltage of the surge is as high as 6KV or even 12KV, So whether the safety spacing is increased depends on the characteristics of the surge overvoltage protection device, which is also the huge sound that our engineers often encounter in the experiment when the surge creepage.

Take ceramic discharge tube as an example, in the requirement of 1740V voltage resistance, we choose the device should be 2200V, and it in the case of the above surge, the peak voltage of the discharge is as high as 4500V, at this time according to the above calculation, our safety spacing is: 4500/1600*1mm=2.8125mm.

3. Pay attention to the position of overvoltage protection devices in printed circuit board.

The position of the protection device is mainly set at the front end of the protected port, especially if the port has multiple branches or loops. If the bypass or the rear position is set, the protection effect will be greatly reduced. In reality, we sometimes lack the location, or for the sake of layout beauty, these problems are often forgotten.

4, pay attention to the large current return path

The return path of large current must be close to the power source or the earth of the shell. The longer the path, the greater the return impedance, and the greater the amplitude of ground level rise caused by transient current. This voltage has a great impact on many chips, and is also the real culprit of system reset and locking.