22-10-2012, 03:27 PM
The Thyristor
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• andy chuksA thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as bistable switches, conducting when their gate receives a current pulse, and continue to conduct for as long as they are forward biased (that is, as long as the voltage across the device has not reversed).
Thyristors are mainly used where high currents and voltages are involved, and are often used to control alternating currents, where the change of polarity of the current causes the device to switch off automaticall; referred to as Zero Cross operation. The device can be said to operate synchronously as, once the device is open, it conducts current in phase with the voltage applied over its cathode to anode junction with no further gate modulation being required to replicate; the device is biased fully on. This is not to be confused with symmetrical operation, as the output is unidirectional, flowing only from cathode to anode, and so is asymmetrical in nature.
Thyristors can be used as the control elements for phase angle triggered controllers, also known as phase fired controllers.
They can also be found in power supplies for digital circuits, where they are used as a sort of "circuit breaker" or "crowbar" to prevent a failure in the power supply from damaging downstream components. A thyristor is used in conjunction with a zener diode attached to its gate, and when the output voltage of the supply rises above the zener voltage, the thyristor will conduct, then short-circuit the power supply output to ground (and in general blowing an upstream fuse).
• andy chuksSilicon Controlled Rectifier SCR Functions as a semiconductor, controls small and very large amounts of electrical power by functioning as a switch. This switch function causes the electric current to be rapidly turned off and on by the SCR. This provides overload protection for digital circuits protecting against too much voltage.
Terminals
SCRs contain three terminals, or components: an anode, a cathode and a gate, none of which are moving parts that could wear out. The gate will always contain a positive charge. For current to flow through the SCR the anode must be more positive than the cathode. When the cathode becomes more positive than the anode no current will be conducted, even with a positive gate. When the gate has a lower voltage than the cathode then the SCR is off but when the gate has higher voltage the SCR turns on and the current runs from the anode to the cathode.
High Power
A SCR becomes a valuable tool in high power devices, such as phase control, power switching, battery charging, high voltage conversion and inverters. It can function in the presence of 2,500 volts and 3,000 amps.
Fast Switching
SCRs possess the ability to turn off and on rapidly. In fact, a SCR can switch off and on up to 25,000 times a second. The SCR can switch in microseconds (millionths of a second). This rapid switching regulates the flow of power, or alternating current, through the switch.
• Switching characteristics
V - I characteristics.
• In a conventional thyristor, once it has been switched on by the gate terminal, the device remains latched in the on-state (i.e. does not need a continuous supply of gate current to conduct), providing the anode current has exceeded the latching current (IL). As long as the anode remains positively biased, it cannot be switched off until the anode current falls below the holding current (IH).
• A thyristor can be switched off if the external circuit causes the anode to become negatively biased, a method known as natural, or line, commutation. In some applications this is done by switching a second thyristor to discharge a capacitor into the cathode of the first thyristor. This method is called forced commutation.
• After the current in a thyristor has extinguished, a finite time delay must elapse before the anode can again be positively biased and retain the thyristor in the off-state. This minimum delay is called the circuit commutated turn off time (tQ). Attempting to positively bias the anode within this time causes the thyristor to be self-triggered by the remaining charge carriers (holes and electrons) that have not yet recombined.
• For applications with frequencies higher than the domestic AC mains supply (e.g. 50 Hz or 60 Hz), thyristors with lower values of tQ are required. Such fast thyristors are made by diffusing into the silicon heavy metals ions such as gold or platinum which act as charge combination centres. Alternatively, fast thyristors may be made by neutron irradiation of the silicon.
The functional drawback of a thyristor is that, like a diode, it only conducts in one direction. A similar self-latching 5-layer device, called a TRIAC, is able to work in both directions. This added capability, though, also can become a shortfall. Because the TRIAC can conduct in both directions, reactive loads can cause it to fail to turn off during the zero-voltage instants of the ac power cycle. Because of this, use of TRIACs with (for example) heavily-inductive motor loads usually requires the use of a "snubber" circuit around the TRIAC to assure that it will turn off with each half-cycle of mains power. Inverse parallel SCRs can also be used in place of the triac; because each SCR in the pair has an entire half-cycle of reverse polarity applied to it, the SCRs, unlike TRIACs, are sure to turn off. The "price" to be paid for this arrangement, however, is the added complexity of two separate but essentially identical gating circuits.