Four key surge protection methods for RF designs

Article By : Jean-Jaques (JJ) DeLisle

Common RF surge protection employs MOVs, silicon-avalanche suppression diodes, filter-based solutions, and gas discharge tubes.

Transient voltage and transient current events, commonly known as power surges, voltage surges, or current spikes, are a relatively frequent occurrence for many electronics applications. Transient surges can be induced by a variety of man-made or natural factors.

Man-made factors include electromagnetic pulses, high-power transmitters, radars, radar jammers, electronic countermeasures (ECM), destructively failed transformers, switching activities, arcing electronics (welders), industrial inductive loads, and poorly-designed electronics. Natural electromagnetic interference (EMI) phenomena that can lead to transient surges are lighting, solar flares, coronal mass ejections (auroras), and other solar/cosmic weather conditions.

Be it man-made or natural, transient surges can lead to voltage and current conditions that can outright destroy, damage, or degrade electronics that are not properly protected. Overvoltage and overcurrent are one mechanism that can lead to failure of electronics, but with enough sustained high transient voltage/current, overheating and derating can also occur. This is especially true for sensitive RF electronics that are often placed in environments such as defense/aerospace, industrial, automotive and at the top of roofs/tall towers, where these electronics systems may be more exposed to dangerous EMI emissions.

As there are often sensitive analog and digital circuits downstream of the RF electronics near an antenna or in the RF front-end (RFFE) of communication, radar and other sensing equipment, it is critical to protect the RF electronics and prevent transient surges from cascading down the electronics/electrical system and damaging other sensitive equipment.

RF Surge protection

There are a variety of methods and technologies used to protect RF circuits. These methods can differ somewhat from what is used to protect DC or AC electronics, as RF electronic surge protection technologies still need to allow for high frequency signals to pass. Common RF electronic surge protection technologies employ metal-oxide varistors (MOVs), silicon-avalanche suppression diodes (SASDs), filter-based solutions, and gas discharge tubes.

  1. Metal-oxide Varistor (MOV)

MOVs are a semiconductor-based voltage-dependent variable resistor that is typically placed in parallel, or shunt, with a load or the component desired to be protected. At low voltages, MOVs exhibit a high resistance. However, at high voltages, MOVs exhibit a non-linear voltage/current characteristic that results in a much lower apparent resistance.

Four key surge protection methods for RF designs

Figure 1 The above image shows the internal structure of an MOV. Source: JAK Electronics

The result of placing an MOV in shunt with a load is that, at a high-voltage peak, the MOV will transition over a period of microseconds to a low impedance path around the load, possibly to ground. MOVs are bidirectional and can observe high energy amounts. MOVs are useful protection circuits, even though they act slower than other solutions, as they can clamp from either direction, which may be beneficial in a variety of EMI or transient surge inducing events.

  1. Silicon-avalanche suppression diodes (SASDs)

SASDs are true solid-state semiconductor surge suppression devices. These devices are designed to exploit the avalanche breakdown phenomenon, which results in a sudden and rapid increase in current transfer. If a SASD is placed in a shunt with a load or device/component to be protected, a transient overvoltage will trigger this breakdown and result in a shunt to ground. SASDs have extremely fast response times compared to MOVs, which makes them very common in high-speed digital, RF, and DC applications within networked devices.

SASDs are not generally susceptible to thermal runaway, as are MOV devices, and can generally function perpetually unless their critical voltage/current handling is exceeded, which is typically a lower threshold than comparable MOV devices. Hence, there are often hybrid devices that incorporate both MOV and SASDs devices in tandem to realize the advantages of both device types. MOV and SASD devices allow for RF frequencies to a few gigahertz, typically < 3 GHz.

  1. Gas discharge tube transient surge protector/lightning arrestor

Gas discharge tubes are typically constructed with an inert gas captured in a chamber that is electrically connected between a point of potential transient surge ingress and the ground line. When a high enough voltage is apparent across the terminals of a gas discharge tube, the inert gas within ionizes and becomes a highly conductive channel for shunting the path to the downstream electronics instead to ground.

Four key surge protection methods for RF designs

Figure 2 Gas discharge tube is a very fast acting switch having conducting properties that change very rapidly, so when breakdown occurs, it transforms from an open circuit to a quasi-short circuit. Source: Citel

Post surge event, the positive and negative ions dispersed in the gas tube recombine, becoming nonconductive again. Hence, gas discharge tubes are a multi-use and effective transient surge suppression technology, especially for lightning strike events. Gas discharge tubes are also commonly incorporated into hybrid transient surge protection/suppression devices alongside MOV and SASD devices. Gas discharge tubes can be constructed to allow for RF frequency operation to several gigahertz.

  1. Filter-based transient surge protector

Unlike the other transient surge protection/suppression technologies discussed herein, filter-based transient surge suppressors don’t shunt the excess transient energy to ground. Instead, they absorb the energy within the resistive filter elements or reflected from the filter input port. In this way, these surge suppressors are in-line with the RF signal path, as opposed to ground shunts.

Essentially, filter-based transient surge suppressors are band-pass filters that are designed to allow a specific range of frequencies pass through with minimal insertion loss, but present very high attenuation to signals with frequency content outside of that limited bandwidth.

 

This article was originally published on Planet Analog.

Jean-Jaques (JJ) DeLisle, an electrical engineering graduate (MS) from Rochester Institute of Technology, has a diverse background in analog and RF R&D, as well as technical writing/editing for design engineering publications. He writes about analog and RF for Planet Analog.

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