While designing a circuit to pass EMI/EMC or any other compliance, the major consideration would be selecting components which would help to absorb radiation or restrict entry of any parasitics into the design. We always design the I/O interfaces with TVS diodes placed to ground, which helps us to eliminate the ESD that can enter due to static discharge. We classify this models as Human body model (HBM) or Machine Model (MM) or charged Device Model (CDM). There are standards defined in each electronics segment which define the amount of ESD circuit must withstand. As mentioned earlier, TVS diode, Transil diode, sometimes Varistor are the major devices used for ESD protection. One has to remember that ESD is a fast rising short duration transient.
We always think of compliance, which is post-design. But during the protoype Assembly or during Production, the yield goes down due to ESD events. In such cases, KEMET has come up with capacitors which are ESD rated that can be used on the exposed interfaces which improves the production yield reducing ESD failures. The ESD capacitors from KEMET, perform both the capacitive action as well as ESD protection. These capacitors from KEMET are HBM - AEC Q200–002 rated (+/-8kV Direct contact & +/-25kV Air Discharge). As this capacitor is ESD rated, there is very less chance that either the capacitor or the circuit gets damaged during the Assembly. These capacitors also help for RF filtering protecting from EMI.
One thing to remember here is that we always think of damage to ICs due to ESD, but the damage to discretes has been a less concern. But with modern sophisticated equipment, there are very densed, low foot print and also stringent in compliance. In such cases, considering discretes for ESD protection is important. More than any other component, capacitor is the front facing for amost every I/O interface and this capacitor having ESD protection helps reduce any damage.
Some of the important specifications of these capacitors from KEMET are:
1. Available upto 1608 size
2. Available in C0G or X7R dielectric
3. Available in the votlage range of 25V to 200V
4. Available in tolerances from +/1% to +/-20%
5. −55°C to +125°C operating temperature range
6. The capacitor range from 1nF to 220nF for the MLCC Type (X7R Dielectric)
7. The capacitor range from 1nF to 15nF for the MLCC Type (C0G Dielectric)
One observation from the above is that Ceramic capacitors do withstand transients which heps in circuit reliability.
What happens if capacitors doesn't have the high voltage transient handling capacity?
Dielectric breakdown may occur, which results in short circuit, that meant very high sudden current flowing through the board. This breakdown can happen in low size capacitors.
How to select capacitors in application?
1. When you are selcting cpacitors near the I/O application, better to have bigger size capacitors (like 0805 or above)
2. Prefer to use high voltage handling capacity capacitors.
3. Prefer to use C0G capacitors over X7R, Y5V type capacitors.
4. The DC bias effects where the capacitance gets reduced is very important consideration during the capacitor selection
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