By Mathew A. Dirjish, Associate Editor
What are CIRCUIT PROTECTION DEVICES?
Simply stated, circuit protection devices are components strategically placed on and around a circuit board or within a system to protect critical operating circuits from any one of a number of destructive events. These threats to the circuit or system typically include over voltage and current, usually in the form of surges, electrostatic discharge (ESD), electromagnetic interference (EMI), radio frequency interference (RFI), and others. To counteract these potentially destructive occurrences there is a wide range of protection components available, the most common being fuses, circuit breakers, and relays, which are commonly associated with over-voltage and over-current conditions caused by failures within the circuit. For the slightly more esoteric conditions, transient voltage/current surges, ESD, EMI, and RFI, there are also a number of dedicated protection devices on the market. These include semiconductors, transient voltage suppressors (TVSs), EMI and ESD filters, diodes, thyristors, and surge resistors. These devices, in one variation or another, can be found in just about every end product that hits the streets.
For over-voltage conditions, again a fuse can be used to disable a circuit until the fault is addressed. However, this is not always desirable or the best strategy for emerging designs. A number of common over-voltage components in current use include metal oxide varistors (MOVs) and their spin off, the radial-leaded metal oxide varistors (ROVs), thyristors, and gas discharge tubes (GDTs).
Both MOVs and ROVs are non-linear, voltage-dependent devices. As the voltage across them increases, their resistance decreases, allowing current to flow. The voltage-current relationship of a varistor is commonly represented in a V-I characteristic curve. Typically, these devices find employment in high current and line-voltage applications.
A good choice for protecting telecommunication equipment from over-voltage faults, thyristors exhibit high electrical-surge capabilities and high off-state impedance, making them transparent during normal system operation. A typical thyristor is a 4-layer semiconductor consisting of alternating P- and N-type materials (PNPN) with three PN junctions. Usually, three electrodes, anode, cathode, and gate, are on board. The thyristor can be turned on via a current at the gate, but does not require gate current once it is turned on. It will continually conduct until a minimum holding current is no longer maintained between the anode and cathode. Essentially, thyristors act like a mechanical switch with two states, on and off only. The most common type of thyristor is the silicon-controlled rectifier (SCR).
Though not exactly considered to be in the semiconductor category, GDTs (See Figure 1) are routinely employed to protect sensitive telecom power lines, communication lines, signal lines, and data transmission lines from transient surge voltages. In essence, a GDT is a tube that allows an electric current to pass through a gas at low pressure. Metal plates sealed in the ends of a gas discharge tube are employed as the anode and cathode. Once air is pumped out of the tube, the discharge across an induction coil stops and one or more violet streamers connect the anode and cathode.
Another example of a transient voltage protection device is the C-Rated SIDACtor Q2L (See Figure 2) from Littlefuse. Designed for protection chores in subscriber line interface circuit (SLIC) applications where space is very limited, the device is considered to be the industry’s first chip-scale package transient voltage SLIC protector. It comes in a chip-scale QFN package measuring 3.3 mm x 3.3 mm and designed to meet the 500A rating for a 2x10µs GR 1089 waveform. When used in place of programmable SLIC devices, the SIDACtor eliminates the need for additional components such as programming-pin capacitors and series resistors. In addition, the fixed-voltage protector is available in various VDRM/VS values to accommodate varying SLIC interface-card voltages.
Protecting Against ESD
Commonly, a semiconductor is the weapon of choice to protect a circuit against ESD. Zener and TVS diodes, multi-layer varistors, and opto isolators are typical examples, all of which are specified to provide protection that meets certain standards such as the human body model and others. Obviously, these devices are deployed in areas that are susceptible to the effects of ESD.
For protecting high-speed I/O ports, Raychem's PESD devices (See Figure 3) shunt ESD away from sensitive circuitry while exhibiting very low capacitance, specified at 0.25 pF typical. In addition to protecting against ESD, keeping the capacitance low prevents loading of the data signals, thereby eliminating distortion. The PESD devices specify lower trigger and clamping voltages than polymer ESD devices, which results in better protection. Typical applications include USB 2.0 and IEEE 1394 interfaces, portable products, printer ports, satellite radios, DVI and HDMIs, antennas, and GPS systems.
The typical line of defense against over-current conditions is a fuse, which opens, interrupts circuit operation, and needs to be replaced when the fault is corrected. For situations where the designer needs to isolate the circuit, i.e., telecom and network equipment where immediate resumption of operation is critical, another approach is required. One possible approach in this case would be the use of resettable protection components such as PolySwitch polymeric positive temperature coefficient (PPTC) resettable devices from Tyco Electronics. These are essentially non-linear thermistors that limit current in line-voltage applications. Under a fault condition, PTC devices go into a high-resistance state but do not open the circuit. For example, the LVR series, rated at 240 Vac, supports maximum voltages up to 265 Vac and are available in hold currents from 50 mA to 550 mA. An added benefit, the devices are thermally active. Deployed in proximity to heat generating components, they protect against both over-current and over-temperature faults.
Protecting the Circuit From the Wrong Protection Device
With the wide array of fuse sizes with some sizes being almost identical, putting in the wrong component is a distinct possibility. To remedy this, the HESI touch-safe fuse terminal blocks (See Figure 4) from Phoenix Contact accepts Class CC and Class J fuses and will reject 10.3 mm x 38 mm fuses, a.k.a. midget fuses. Class J versions, UK 20.6-J HESI, and UK 27-J HESI terminal blocks accept Class J fuses up to 30A and 60A, respectively. The component carries safety approvals for branch-circuit applications and comes with a short-circuit current rating of 200 kA.
Company: EEPN Magazine
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