# Capacitor Selection Tips For Transient Voltage Protection

Transient voltage protection is not just something that is “nice to have,” but rather it is required in many devices. Power systems, industrial systems, networking equipment, and even simpler devices with some connectors can all function without transient protection, but there is a risk that devices fail when transients occur. Including transient protection requires a few simple strategies that conform to your product or industry standards, and this will improve the reliability of your devices.

Most designers may not think of capacitors as viable components for ESD protection or transient voltage protection. However, there is a long-standing model that describes how a capacitor could provide protection against ESD, power surges, and slow transient voltages associated with switching. In this article, we’ll examine how this works and where capacitors should be used in relation to protection from transient voltages.

## Capacitors and Transients

Transient voltages come in several forms, with different peak voltages, time constants, decay rates, and associated currents. Transient voltages are often talked about in terms of underdamped oscillations as seen in switching circuits (such as switching DC/DC converters), but they are also observable as pulses. The types of transients seen in electronics include:

• ESD - high voltage, moderate current, very fast (nanosecond rise time)
• Inrush from switching - high voltage, moderate current, slow (microsecond to millisecond rise time)
• Power surges - varies greatly but can create up to 1000V overvoltage with slow rise time

As shown above, transients can be created from many different sources, with different peak voltages and rise times. The duration around the peak voltage can also be quite long. For example, when switching excites an underdamped oscillation in a power system, the oscillation can persist for a significant period of time, including all the way up to the next switching event. Some examples are shown in the image below.

### Calculating the Required Capacitance

The calculation of capacitance required for transient protection requires a few inputs:

• The energy (voltage) of the transient voltage source (Vsource)
• The output capacitance of the source (C1)
• The maximum voltage that should reach the protected circuit (Vcircuit)

Modeling the transient voltage source as a capacitance accounts for several types of transient voltage sources, such as the output buffer from integrated circuits, FETs, and the human body model (HBM) for understanding ESD. When the voltage source capacitance (C1) interacts with the protection capacitor (CP), the required value of (CP) is:

Protection capacitance needed to withstand high voltages from a transient source

If you know the voltage you need to withstand, and you know the capacitance you need, will you be able to find an actual capacitor that hits these specifications?

### High or Low Voltages?

If some example numbers are plugged into the above formula, one finds that high capacitance is needed to get a lower protection voltage. However, the capacitor may still need to withstand very large voltages during transient events. Larger voltage capacitors come in larger case sizes, which will have slower responses due to the resistance along the transient voltage path.

Because of the slower response of high voltage capacitors, they are best used to reduce transient voltages from switching events, such as you would find during inrush and when a switching element draws current into a circuit. They are not appropriate for lightning strikes, ESD from touch, or fast power surges.

## Capacitors Fail at More Extreme Transient Voltages

From the above graph, it should be clear that capacitors are only useful for a specific subset of transient voltages. When you get beyond the typical voltage ratings provided by capacitors, you enter the realm of electrostatic discharge (ESD), where peak transient voltages can reach into the kV range and have very fast edge rates.

In this range, capacitors will be inadequate for protecting against transient voltages, particularly associated with ESD. This is due to the fact that the capacitors that could be capable of having high voltage ratings will have larger capacitor case sizes. However, the larger case sizes can have higher ESR and ESL values, so they may not respond fast enough to a transient voltage and they can fail as a result.

For more extreme situations that are encountered in practice, traditional forms of circuit protection are needed. Typically this would involve:

These are often used together, with the reason being that they will address multiple possible transient voltage sources for a single circuit. Make sure that you simulate the full range of possible responses if you plan to use multiples of these protection methods on a single component.

Whenever you need to implement circuit protection from transient voltages in your PCB design, make sure to design your system with the complete set of simulation tools in PSpice from Cadence. PSpice users can access a powerful SPICE simulator as well as specialty design capabilities like model creation, graphing and analysis tools, and much more.