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in Bookkeeping Terms by (220 points)

Electrostatic discharge has two kinds of interference to electronic circuits, one is conducted interference, and the other is radiated interference. The capacitance in PCB decoupling can well improve the decoupling ability of the signal line and enhance its anti-interference ability.

 

On some power and low-speed signal lines, capacitance in PCB is also used to resist ESD. How much Electrostatic discharge voltage can the capacitor resist? What is the relationship between Electrostatic discharge tolerance and capacitance?

In the manufacturing process of TVS or ESD devices, ESD events may be triggered and damaged. These events can be simulated by three models.

1. Human Body Model, referred to as HBM, the human body model, which simulates the test of the human body during electrostatic discharge.

2. Machine Model, referred to as MM, mechanical model, which simulates the test of mechanical, electrostatic discharge.

3. Charged-Device Model, abbreviated as CDM, is a charging device model, which simulates the test of electrostatic discharge of charged devices.

 

HBM generally has two test specifications, one is the IEC61000-4-2 standard, and the other is AEC-Q200-002.

The IEC61000-4-2 standard is aimed at the Electrostatic discharge immunity experiment. ESD tests are divided into air and contact tests. An electrostatic gun is required. The circuit of the electrostatic gun or electrostatic generator is given below. 

Rc is the charging resistance, Cd is the charging capacity in PCB, and Rd is the discharging resistance. The simple working principle is: the charging switch is on, the discharging switch is off, and the DC high-voltage power supply charges Cd through Rc; the charging switch is off, the discharging switch is on, and the Cd is stored. The charge is released to the device under test.

IEC61000-4-2 will be more commonly used than AEC-Q200-002; the difference lies in the Rd resistance value.

After knowing this basic knowledge, if you play Electrostatic discharge on capacitance in PCB, it is as follows:For a 10KV 150PF model, the energy contained is Q=CV.Assuming that all the energy is released, the instantaneous voltage on Cx=10NF will reach 10KV*150PF/(10nF +150pF)=147.78V. This voltage is still very high and will damage the capacitance in PCB to a certain extent.

Calculate the instantaneous voltage values ​​of 1nF, 10nF, 22nF, 47nF and 100nF under the 10KV 150pF model and the 10KV 330pF model. When the capacitance in PCB reaches 100nF, the voltage on the capacitor is already very low, and the capacitor can withstand it. And if the premise is that all the energy of the model is released, the actual voltage will be lower.

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