ESD Case Sharing-- GDT Wonderful Use
In the most gateway products, RJ45 need surge protection and ESD protection, but occasionally meet the situation that surge and ESD protection are both difficult to balance.
The Ethernet port protection circuit is roughly shown as (Figure 1). 90V GDT-BZ091N is used for common mode surge at the primary side, and low-capacitance BV03C-H is used for differential mode protection at the secondary side to suppress surge or ESD voltage spikes.
Customer feedback common mode surge 10/700us 6kV test can pass, port ESD air 15KV test equipment crash need to restart. Through communication, it is known that the PCB board area is small and the space is limited, the CPU and phy are close to the RJ45 port, and the PCB layout is roughly as follows (Figure 2).
Firstly, the possible paths of ESD are analyzed, ①From transformer primary coupling to secondary coupling to phy and CPU; ②From the centre tap to the GDT to PE through the coupling capacitance to the system Gnd. ③Other (such as RJ45 metal shell to the Gnd...) .
Next is the rectification, we remove the GDT, completely PASS, but without GDT protection, the surge comes and the PCB board blows up. What do we do?
At this time, it is time to show the wonderful of GDT, change the GDT to 400V BZ401M, surge and ESD test directly through, it and BZ091N exactly the same package, pin to pin.
Next, explain why the 90V GDT ESD test fails and the 400V PASS. The breakdown voltage difference between the two components at 1KV/us is only 150V, and there is no perfect replacement for the 6kV surge. But ESD test 90V GDT excited more charged particles, unit time (di/dt) transferred to the PCBA system naturally more charge, resulting in greater ground jitter (u=L(di/dt)), the system\'s sequential logic circuit is greatly affected. In contrast, 400V GDT excites fewer charged particles.
This is mainly determined by the GDT gas formulation and process differences. The complete conduction of GDT requires the electric field to excite the active electrons in the gas molecules and atoms and then collide with other electrons in the gas molecules and atoms to form a large current, and this process takes time, ESD belongs to nanosecond discharge (about 100ns).
Therefore, the GDT conduction time (excitation of more charged particles) of different processes and formulations has a great difference, and the surge discharge is more than tens of microseconds, and GDT has enough time to stimulate more charged particles conduction.
As mentioned earlier, the board is small, the area is limited, the signal route will be incomplete, and the incomplete ground has parasitic inductance, so the ground will jitter. Moreover, the CPU is closer to the edge of the board to receive stronger interference.
GDT cannot be replaced in high speed Ethernet port common mode protection due to its advantages of low capacitance, high flow and low cost. However, when using GDT as surge and ESD protection, we should pay attention to the surge test method required by the project (whether it is necessary to start from low-voltage surge, such as 500V), voltage resistance test, etc., whether there is a blind area of GDT that is not broken down, or insufficient voltage resistance test.
Finally, it should be mentioned that ESD protection often uses the basic idea of choke and block, and pays attention to more practical cases of Bencent Electronics to share with you.
