Electronic devices are designed to operate at certain maximum supply voltages. Anything higher than maximum allowable voltage may cause considerable damage involving wires that melt, devices that stop working, packages that bulge, physical holes, cracking, or even devices that burst into flame when the circuit receives the over voltage.
What Causes Overvoltage?
Voltage that surpasses the limitations specified for a device is the most common cause of failure. When the voltage is raised above its design limit, there exists an over voltage. Power surges, incorrect input supply, excess temperature, mechanical shock, electrostatic discharge, low resistance between the supply and ground pins, pin short, device failure and more, can all trigger an over voltage situation. Depending on the duration of the event, it is called a transient, voltage spike, or power surge.
Safety devices—such as fuses—exist that protect electronics against excess current damage. They can be ineffective, however, when transients and spikes of high voltage on the power supply take place. Transistors, too, can offer protection and act as switches and amplifiers simultaneously. Typically, however, they handle small electronic currents in the milliamp range.
Figure 1: Thyristors are simple devices, but provide substantial protection in over voltage situations. (Source: BBC)
Thyristors, in comparison, deal with electronic currents in the several hundred volts and up to 10 amps range and are particularly useful where fluctuating current and overcurrent situations exist. They are used in factory power switches, car ignitions, surge protectors, thermostats, electric motor speed controls and solid-state relays, and are commonly found in telecommunications equipment. In an over-current situation, the thyristor turns on and remains in that state until a circuit is reset.
Bidirectional Thyristors
An example of a thyristor family includes ON Semiconductor’s NP1800SAT3G NP Series thyristor surge protectors, which are high-voltage bidirectional thyristor surge protector devices (TSPD) used in telecommunication circuits including central office, access and customer premises equipment, protecting them from over voltage conditions.
As bidirectional devices, they feature the functionality of two devices in one package, which saves valuable board space. The devices act as a crowbar when over voltage occurs, diverting energy away from a circuit or device that is being protected. The use of ON Semiconductor’s NP Series is designed to help manufacturers meet a variety of regulatory requirements.
관련 상품 참조
The Bourns TISP400H1BJR-S thyristors are designed to limit over voltage on digital telecommunication lines, which are normally caused by AC power system or lightning flash disturbances that are induced or conducted onto a telephone line. A single device provides 2-point protection of transformer windings and low-voltage electronics.
The protector consists of a symmetrical voltage-triggered bidirectional thyristor. During operation, over voltage is initially clipped by breakdown clamping until the voltage rises to a level that causes the device to crowbar into a low-voltage on-state condition. The low-voltage on-state causes the current resulting from the over voltage to be safely diverted through the device. The device switches off when the diverted current falls below the holding current value.
관련 상품 참조
STMicroelectronics’ SMTPA62 Trisil™ thyristor is also used for telecommunications equipment protection. Features include bidirectional crowbar protection, a voltage range from 62 V to 320 V, low capacitance from 12 pF to 20 pF at 50 V, low leakage current: IR= 2 μA maximum, and a holding current of = 150 mA minimum.
Applications in the telecom area include analog and digital line cards—xDSL, T1/E1, ISDN, terminals (phone, fax, modem) and central office equipment.
The Trisil series from STMicroelectronics is designed to protect equipment against lightning and transient induced by AC power lines. They are available in SMA, SMB and DO-15 packages. Thyristors in the Trisil series are not subject to aging and provide a fail-safe mode in short circuit for a better protection. They are used to help equipment meet various standards such as UL1950, IEC950 / CSA C22.2, UL1459 and FCC part 68.
Another example is the SDP1800Q38CB thyristor SIDAC 170 V 30 A 8-pin QFN device from Littelfuse, which provides over voltage protection for VDSL2, ADSL2 and ADSL2+, and has a minimal effect on data signals. The design results in a capacitive loading characteristic, compatible with high-bandwidth applications. The surface-mount package provides a surge capacity that exceeds most worldwide standards and recommendations for lighting surges.
The device features balanced over voltage protection, low distortion and insertion loss and a low profile.
What is a Thyristor?
Thyristors are solid-state semiconductors that feature four alternating layers of N and P material—(P-N-P-N). The device conducts when the gate receives a current trigger and continues to conduct as long as the voltage is not reversed. While there are many types, the most common include Diacs (Diode ACs), Triacs (Triode ACs) and silicon-controlled rectifiers. Thyristors simply are electronic components featuring three leads—an anode, cathode and gate.
When the voltage across a resistor rises, the thyristor switches on and the power rails are short-circuited for just a few milliseconds before the fuse blows. The faster the thyristor, the faster the response time. The job of the gate is to control current flow between the anode and the cathode. A fuse is 1,000 times slower than a thyristor, which, in comparison, takes only a few microseconds to trigger.
How Does a Thyristor Work in a Circuit?
Thyristors are unidirectional, meaning that they only conduct current in one direction when a triggering current is applied to the gate. A small gate current controls a larger anode current and the anode current must be greater than the holding current to maintain the conduction.
Thyristors acts as rectifying diodes once triggered “on.” When triggered on, it will be latched “on,” conducting even when a gate current is no longer applied, provided that the anode current is above latching current. When current is no longer flowing into a gate, the device is switched off and current cannot flow from the anode to the cathode. Thyristors have no moving parts, do not arc on contact, react to corrosion or dirt, and can be made to control the mean value of an AC load current without dissipating large amounts of power.
Thyristor Circuit Applications
Thyristors are commonly used in AC circuits where the forward current drops to zero during each cycle, so there is always a turn-off feature. This means that the gate must be triggered during every cycle just to turn it back on. The thyristor’s most important job, however, is in the timing of these functions to provide sufficient power control and protection to today’s sometimes-fragile electronics.