Shockingly Good Protection: Designing Circuit Protection

Welcome back to “Shockingly Good Protection,” my ongoing series about electrical transients. In the previous blog, we learned that transient voltages are sudden energy spikes that can significantly damage sensitive equipment and can even put users of that equipment at risk. Fortunately, it’s an avoidable risk as long as you build circuit protection into your design. This post will offer some considerations and proactive steps you can take as a designer to prevent electrical transients from damaging your equipment or its users.

Circuit Protection Considerations

Electromagnetic compatibility (EMC) transient events vary in time. A successful EMC design relies on dynamic performance and the alignment of dynamic characteristics between protection components and the input/output stage of the protected device. Before we dive into how to develop such a successful protection circuit, let us consider the following:

• Neutral or positive design impact | Under normal system operation conditions, the protection circuitry should not interfere with the operation of the product. The protection circuit should be almost invisible to the system, or even enhance the design in more aspects than protection.
• Minimal performance disruption | Protection circuitry must prevent or limit damage caused by transients and allow the system to return to normal operation with minimal impact on performance. Typically, when using transient protection components, the transient voltage is clamped/directed to ground, and it’s worth noting that this can cause momentary disruption to data communication lines.
• Build for intended environment | As we saw in the first blog of this series, there are three different types of transients—electrostatic discharge (ESD), electrical fast transients (EFT), and surges—which vary in speed, duration, and energy. Your protection scheme should be robust enough to withstand the transients and voltage levels the system would be subjected to in its intended environment.
• There’s no such thing as too protected | Design the protection circuit beyond the IEC61000-4-x waveforms if possible, depending on cost. In the real world, transients are not textbook waveforms and can vary drastically. Increased protection may offer higher value to your customers (along with fewer customer returns!)
• Fail safely | In case the protection circuitry itself should fail during overstress, make sure you’ve designed it to fail in a way that protects the system.

Transient Protection Schemes

There are two main types of protection schemes that can help defend a system against transients. Overcurrent protection is used to limit peak current, and overvoltage protection is used to limit peak voltages. Most systems require both types.

The market offers a broad range of overcurrent and overvoltage protection technologies and components, each with its own advantages and disadvantages. We will discuss some of these in the next blog. In this post, we’ll focus specifically on how to develop a hybrid network consisting of multiple devices, although it is worth noting that a single protection component will often suffice. Whether your design calls for singular or hybrid protection depends largely on its application.

Figure 1 shows a typical design architecture for a hybrid network protection scheme. Here we are protecting an Analog Devices (ADI) component from a transient.

The 3 Elements of Hybrid Network Protection

A hybrid network design can be characterized as having primary and secondary protection with a coordinating element in between.

Primary protection diverts most of the transient energy away from the system and is typically located at the interface between the system and the environment where the transients may access the system. For example, transients often access products through cabling or power lines. The primary protection is designed to divert most of the transient energy to ground.

Secondary protection comes into play when transient voltages and/or currents get through the first line of defense. Whereas the primary protection may be more generalized, secondary protection is usually designed to protect one specific part of the system. It should be optimized to both protect against residual transients and allow normal operation of more sensitive parts of the system.

Finally, the primary and secondary designs should be specified to work in conjunction with the system input/output to minimize the stress on the protected circuit. Therefore, these designs typically include a coordinating element, such as a resistance or nonlinear overcurrent protection device, between the primary and secondary protection to ensure optimal coordination of energy dissipation and activation.

As an example, a resistance can provide a voltage drop that ensures the primary protection turns on before the secondary protection becomes damaged, providing the resistor doesn’t interfere with product functionality. A nonlinear overcurrent protection device, such as a resettable fuse, will present a high impedance, isolating the primary protection and transient from sensitive circuitry.

EMC Transient Protection in Action: An Example

Consider a hybrid network system in which the primary overvoltage protector is large enough to divert surge energy to ground, but too slow for ESD and EFT pulses. That means the secondary protection must be fast enough to protect the system from ESD and EFT pulses. However, the primary overvoltage protector must trigger first, or else the secondary protector may be subjected to more energy than it can withstand. The coordinating resistor manages the sequence of response.

Summary of Hybrid Network Scheme Elements

Next: A Deep Dive into Overvoltage Protector Components

In the next blog post, we will take a closer look at overvoltage protectors and the important parameters you must consider when selecting a protection component. Component data sheets may not give you a full picture of the protection that’s needed. Data sheets often only present DC characteristics, and these values can be quite different from the dynamic breakdowns and I/V characteristics that you really need to consider. Meeting EMC standards for transient resistance requires careful design, characterization, and an understanding of the dynamic performance of the input/output stage of the protected device and the protection components.

Then, in part 4, we will synthesize our learnings with some real-world designs. Join me again in two weeks for the next installment of “Shockingly Good Protection!”