A Spike’s Mortal Enemy
Disturbances of the power supply are common
Disturbances of the power supply are common – in the automotive space just as much as in classic AC grids. Additional demands imposed by various forms of alternate energy will only make the issue worse, thereby ensuring that surge protection will remain a topic.
Metal Oxide Varistors, also known as MOVs, are a knee-jerk reaction to designs facing line-related problems. In principle, a MOV is a variable resistor: as the supply voltage raises, its resistance falls.
GDT – the term is short for gas discharge tube, but the parts are also known as gas plasma arresters – takes a slightly different approach to the problem. A GDT is a tube filled with a special gas waiting for an opportunity to become conductive.
If only you could drill into the part
When the voltage across the two terminals reaches a set point, the gas becomes ionized. From that moment onward, current can flow in the gas until it deionizes – a process which takes place when the current falls low enough.
Classic MOVs tend to have a high capacity – a problem not known to GDTs. We recently saw a lot of search interest in the Bourns 2049 family of diodes. It consists of 12 different parts able to clamp DC voltages from 75 to a whooping 600V.
In addition to their robustness – a spike of 20000A can be handled once, while the 15000A will be survived for a minimum 10 times – the parts are very low in capacitance. When dealing with a switching frequency of 1 MHz, Bourns promises a capacity of less than 1.5pF. Due to this extremely low capacity, GDTs find frequent use in telecom applications.
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GDTs really comes alone: in many cases, manufacturers combine them with a MOV or a set of zener diodes. This is caused by a physical effect: establishing the gas connection is a process which takes a bit of time. During this span, the spike can travel further down into the circuit, thereby giving all kind of grief to the system.
Given that GDTs are not particularly common. First of all, Bourns provides an excellent overview of the technology here. Chinese vendor Bencent has a great introduction into the concept as whole – it can be found here.
Littelfuse -a competitor, whose parts we also price-compare for you at oemsecrets.com – cover combined MOV-GDT applications in a whitepaper available here.