The cornerstone of transient protection design is the selection of appropriate protective components. Each component has its unique characteristics and suitability depending on the specific transient parameters like energy, voltage, and duration. To improve the electromagnetic compatibility (EMC) performance of a design, engineers must carefully analyze the transient specifications and select components that provide adequate protection without compromising the circuit's performance.

So far in the “Shockingly Good Protection” blog series, we’ve learned about the different types of transients and the damage they can cause, as well as considerations and proactive steps you can take as a designer to protect your equipment and its users. Now it’s time to get into the weeds of it. In this post, we’ll take a detailed look at some commonly used overvoltage protection components: What they are, how they work, and when to use them.

Choose Your Overvoltage Protection Type

There are two main types of overvoltage protectors on the market: Clamping and crowbar/snap-back types. Both of these devices remain high-impedance until triggered—ideally, up to or beyond the normal operating voltage of the system. What differs is their behavior when an overvoltage occurs.

A clamping type component protects the victim circuitry by, you guessed it, clamping the voltage or holding it at a steady level that won’t harm the device. One example of a clamping device is a transient voltage suppression (TVS) diode. A TVS diode is ideal when a fast response is needed to address transients such as electrostatic discharge (ESD) and electrical fast transients (EFT).

With a crowbar type component, on the other hand, the protection device “snaps” back in the event of an overvoltage. This reduces the voltage so the system can withstand more power, such as you’d get from a surge. Examples include gas discharge tubes (GDTs) and thryistors.

Figure 1. Comparison of clamping and crowbar type component responses to an overvoltage event

Current Response Considerations

When choosing protection components, you must also consider current response. Clamping and crowbar type devices have very different current handling capabilities, so choose wisely based on the level of transient protection needed for your application. It may help to refer back to the calculation for power, which is based off voltage and current:

P = V x I

A clamping protection device can have restricted power handling capabilities since it clamps the voltage at a higher level than the system level voltage.

With a crowbar type device, the voltage level is reduced when it snaps back to protect the victim circuitry. That enables the device to handle more current, or put another way, the device has better power handling capabilities. Therefore for higher levels of surge a snap-back device may be more applicable.

Beware that the product may continue to function albeit with some errors in the clamping scenario, whereas the crowbar scenario may result in the product dipping below the Normal Operation Voltage in Figure 1, i.e. powering down temporarily or resetting. The choice of type should also be considered against the expected product behavior during exposure.

A Brief Comparison of Overvoltage Protection Components

Now that we understand the two main types of protection component, let’s look a little more closely at some of the most common options and when to use them.

Metal Oxide Varistors (MOV) MOVs are clamping devices. They have slower response times than TVS diodes but can handle higher currents, making them more suited to medium surge levels rather than ESD or EFT.	Gas Discharge Tubes (GDT) GDTs are crow-bar style devices with excellent current handling capabilities but very slow response times. They are excellent for high surge voltages but are bulky and expensive.
Transient Voltage Suppression (TVS) diodes TVS devices are clamping devices. They have the fastest response time, making them ideal for handling electrostatic discharge (ESD) and electrical fast transients (EFT). Low capacitance makes TVS diodes well suited for use in high-speed communication lines, and their small size makes them ideal for space-constrained applications. TVS devices can also handle low levels of surge.	Thyristors These snap-back crow bar style devices offer superior current handling capabilities. Thyristors are excellent for space-constrained applications that require surge immunity, as they offer good levels of surge protection while taking up relatively small PCB area. In a moment, we’ll take a closer look at when and how to use these two smaller components in your space-constrained design.

Overvoltage Protection Components at a Glance

Figure 2. This table came from Henry Ott’s book "Electromagnetic Compatibility." It nicely summarizes the various overvoltage components on the market.

Clamping Device 1: Transient Voltage Suppression (TVS) Diodes

Figure 3. On the left is the circuit diagram symbol for a TVS. On the right is an I/V characteristic curve showing that the TVS is a clamping device. For simplicity, I have only shown a unidirectional device characteristic curve.

A TVS is a clamping type overvoltage protector whose main function is to divert the energy away from the victim circuitry. It automatically resets and goes high impedance when the overvoltage goes away. TVS components come in surface mount packages, making them ideal for space-constrained applications. The are excellent as both primary and secondary overvoltage protectors. Let’s take a closer look at some parameters to consider when designing with a TVS.

Working Stand-Off Reverse Voltage (VWM)

The VWM indicates the maximum voltage of the TVS when not in breakdown. This is a important parameter as the protection device must not affect the signals and voltages normally occurring in a system. This value should be above the normal operating voltages of the pin it is trying to protect.

Breakdown Voltage (VBR)

This is the voltage at which the device goes into avalanche breakdown, and is measured at a specified current on the datasheet.

Maximum Breakdown Voltage or Clamping Voltage (Vc)

The clamping voltage appears across the TVS at the specified peak pulse current rating. It is important to know at what voltage the TVS will clamp for a given peak pulse current. Always know the corresponding Vc for the maximum peak pulse current in a transient that may occur in a system, as it must never exceed the victim pin breakdown voltage.

Clamping Device 2: Thyristors

Figure 4. On the left is the circuit diagram symbol for a thyristor, in this instance a TISP. On the right is an I/V characteristic curve showing that the TISP is a crowbar type device. Notice that it has a discontinuous voltage-current characteristic, the discontinuity being caused by the switching action between high voltage and low-voltage regions.

Thyristors are snapback or crowbar type devices whose main function is to divert the energy away from the victim circuitry. In this example, we are using a totally integrated surge protector (TISP). TISPs come in a variety of packages, perhaps most importantly in surface mount packages—making them very suitable for densely populated PCBs. There are many specifications used to describe the characteristic curve of a TISP thyristor, but here we will focus on three of the most important ones.

Maximum Repetitive Off-State Voltage (V_DRM)

As we discussed earlier, the protection device  must not affect the signals and voltages normally occurring in a system. When choosing a TISP you must ensure that the V_DRM value is higher than the maximum normal operating voltage on the pin of the victim circuitry.

Impulse Breakover Voltage (V_(BO))

As we move along the characteristic curve, we enter the breakdown region of the TISP. This region triggers a small clamping action, during which the victim circuitry will briefly be exposed to a high voltage before the system switches into a low-voltage state. The V_(BO) is the maximum voltage that the TISP will reach in this region. It is important that the V_(BO) is below the breakdown voltage of the pin you are protecting.

Holding Current (I_H)

Leaving the breakdown region, the TISP “snaps back” and enters a low voltage state. In this state the TISP conducts the current to ground. When the diverted current falls below a critical value, the holding current (I_H), the TISP device switches off and allows normal system operation to resume.

Shockingly Good Protection in Action

Over the last three blog posts, we’ve learned about transient voltages and how you can begin to design transient protection. Today we looked at some of the main overvoltage protectors that can be used in these protection schemes. In the next and final post of the series, we’ll bring all this information together and look at some real-world examples of circuit protection for industrial communication buses. Join me again in two weeks for the conclusion of “Shockingly Good Protection!”