The rectification background

The customer's product is a small outdoor communication box with a full metal shell and an exposed antenna. The customer's feedback is that the internal chip is broken during normal use, and the defect rate is close to 10%.

The customer's products are used outdoors, which is easily disturbed by the external environment, especially thunderstorm weather. When lightning strikes around the generation of electromagnetic fields, when outdoor lines through the electromagnetic field, will induce voltage or current in the electromagnetic field; Or the ground current generated by lightning directly conducts to the inside of the product through the shell and causes damage and failure of the product through the voltage difference between the core wire and the antenna. The laboratory uses IEC61000-4-5 lightning surge test standard to simulate this phenomenon.

Simulation test

1.2/50-8/20US combined lightning surge test was carried out on the wire core and shell ground, and the experimental voltage was stepped from low to high:

>  Test 1kV, the product is not abnormal;

>  When 2kV was tested, signal reception was abnormal. After disassembling the shell, it was found that the ESD had exploded and the chip was black, which was close to the abnormal phenomenon of use reported by the customer.

Test the rectification

（1）The metal shell of the product is isolated from the ground of the PCB board by a high voltage capacitor. The high voltage capacitor is replaced by a GSM090D ceramic gas discharge tube. The signal reception is abnormal when tested at 2KV. Apart the shell, the phenomenon has been improved, there is no sign of chip burning black. After testing the device, it is found that the ESD short circuit fails.

（2）According to the analysis, the customer's ESD is directly connected to the discharge tube in parallel, without coupling capacitor to do the decoupling, which leads to ESD acting before GDT, resulting in ESD short circuit failure.

（3）Adjust the circuit, put the ESD TUSD05CB behind the 33pF coupling capacitor, then run the 2KV lightning surge test, and find that the signal reception is still abnormal. After disassembling the shell, no burning phenomenon was found. After testing the device, no ESD damage was found.

（4）Through communication with the customer, it was confirmed that the voltage of the customer's chip was only 8V, and the chip damage caused by the high clamp voltage was analyzed. After ESD, another 0402 package, 22pF patch capacitor is string to inhibit the clamp voltage, and 2KV is tested again, and the test is passed.

（5）The discharge tube used by the customer for the front stage is GSM090D, and the surge flow is 2KA. It can pass the 1.2/50-8/20US combined wave 4KV test, so the protection scheme theory can pass the 4KV lightning surge test. Continue to improve the test requirements, with 3KV lightning surge test, test passed. When the voltage was raised to 4KV for testing, abnormal signal reception was found. When the shell was opened, it was found that the copper skin at the antenna was cracked, which was processed by flying wire and passed the test. The customer needs to widen the wire diameter later.

Conclusion: Through simulation tests and analysis in SPSEMI laboratories, we found that the customer's products did have problems in protection. After selection and rectification, the protection ability of customers' products can be improved, and the scheme and basis can be provided for customers to upgrade and improve the subsequent products.