TF cards are widely used in many portable devices. Although they have a metal casing, they are not exposed, so they mainly perform air discharge during ESD testing.
1、 Phenomena before rectification:
Before the rectification, an 8KV air discharge was applied to the TF card port in the TF card mode, causing the speaker to switch modes and unable to connect back on its own. This does not meet the Class B requirements of the IEC/EN61000-4-2 standard.
2、 Rectification method:
At the beginning, only ASIM ESD (model: CV0402VT6030T) was connected to the ground in parallel with the signal line connected to the TF card and MCU. The test showed significant improvement, but it still did not meet the standard. Analysis idea: Since ESD protection devices have been installed on the signal line, eliminating static electricity is directly transmitted to the signal line through ground crosstalk. Analyzing the schematic diagram again, it is known that there are several suspended pins on this TF card. The most likely interference path for static electricity is the formation of a high potential difference between the ground and the suspended pins, and this strong interference voltage directly interferes with the TF card from these pins.
Rectification measures: Use ASIM ESD (model: CV0402VT6030T) to string the suspended pins with the ground, as shown in the following figure. Make the potential difference between the suspended pin and the ground within the range borne by the TF card. Performing ESD testing
3、 Test results after rectification:
In TF card mode, perform 8KV air discharge on the TF card port, and the speaker operates normally, which meets the Class B requirements of IEC/EN61000-4-2 standard.
4、 Essence of ESD Immunity Testing
From the configuration description of ESD testing, it can be seen that when conducting ESD testing again, the grounding wire of the electrostatic gun needs to be connected to the reference ground plate, and the EUT needs to be placed on the reference ground plate. The discharge gun head of the electrostatic gun points towards various parts of the EUT that may be touched by hands or the horizontal and vertical coupling plates. This determines that ESD testing is a common mode based immunity test, as the ESD current ultimately flows to the reference ground plate.
5、 Analysis of ESD pulse waves:
From the analysis of the electrostatic discharge waveform, the rise time is 0.7ns to 1ns, and according to the Fourier transform, the spectrum width can reach 300MHZ. The response time of ESD devices is generally PS level, so using ESD devices to protect against static electricity is an effective measure.
From the pulse width of static electricity, it can be seen that the pulse duration of static electricity is very short, and its energy is much smaller than that of surges. Therefore, the vast majority of low-power ESD devices are sufficient to discharge it.
6、 Usage of ESD devices:
ESD devices are mainly connected in parallel on the line, and when an electrostatic event occurs, the ESD device can instantly conduct, providing a low impedance circuit for the static electricity. Here are some points to note:
1、 The discharge site of ESD devices must be complete, defined as those with a aspect ratio less than 3 (without gaps or openings)
2、 ESD devices must be placed
