ESD (Electrostatic Disharge) is the rapid transfer of electrostatic charge. When one object is positive and another negative and they come into contact, they are going to want to balance their electrons. The rush of electrons from one object to the other is ESD. ESD can be considered an enemy to electronic circuits and their components. The transfer of charge to electronic components can easily damage them leaving them useless. The sad part is most of the time you will not know until it is too late.
This is where an ESD protection capacitor comes into play. ESD capacitor are put into circuits to absorb the unwanted ESD that the circuit may come into contact with. Here is a great post on ESD capacitors: What are ESD capacitors?
This post is going to focus more on how to choose an ESD protection Capacitor. Also, keep in mind there are many different ways to protect a circuit from ESD, a capacitor is an economical solution.
There are 3 main parameters you should use when choosing a ESD protective capacitor:
Device Under Test
The DUT effect is the effective resultant voltage across the capacitor in a ESD test circuit. The circuit can be seen in figure 1.
Figure 1.
Vx = Resultant Voltage
Cx = DUT (Capacitor Under Test)
Co = Charge Capacitor
Vo = Source Voltage
This equation shows the relationship between Vx and Cx. If you keep Vo and Co constant, then Vx and Cx are inversely proportional. So, the higher value Cx the more Vx decreases.
Here is an example a 6kV (VO) ESD requirement for a 2000pF capacitor (CX). We are using the AEC-Q200 test method which has Co = 150pF. The DUT effect relationship shows that with 6kV applied, CX only sees 418.6V (VX). Below is the step through math.
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Vx = (Co / Co + Cx) Vo
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Vx = (150pF / 150pf + 2000pF) x 6kV
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Vx = (.00000000015 / .00000000015 + .000000002) x 6000 Volts
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Vx = (.00000000015 / .00000000215) x 6000 Volts
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Vx = (.0697674418604651) x 6000 Volts
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Vx = 418.6 Volts
Voltage Breakdown
Voltage breakdown is a method used to find the voltage level strength of a capacitor. You can find the max sustained voltage the capacitor can handle by placing DC voltage across the capacitor at an increasing rate until it fails. We make our capacitor selection by choosing a capacitor with the voltage breakdown level greater than Vx.
DC Bias
When you apply DC voltage to a ceramic capacitor the effective and nominal capacitance may be different. With that DC Bias is expressed as a percent capacitance change from the nominal. Below is the equation to find DC Bias.
The dielectric material plays a big role in DC Bias. Different classes of capacitors also play a role. For Class I capacitors the change is flat, while Class II the capacitance increases at first and then has a steady decrease as the rated capacitance is reached. Class II typical will have a bias of -10% to -70%. An increase in applied voltage will decrease the effective capacitance.
ESD test rating alone is not the best way to select a capacitor value. If you look at the DUT, voltage breakdown and DC Bias affects, you can prevent over-design and more important under-designing the protection circuit.
For more on the classes of capacitors check this post out: Understanding Ceramic Capacitor Temp-Coefficients
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