» Power Electronics

Gate Protection in Thyristors

arjun — Fri, 17 Apr 2009 20:29:28 +0000

Gate circuit should also be protected against overvoltages and over currents. Overvoltages across the gate circuit can cause false triggering of the SCR. Overcurrent may raise junction temperature beyond specified limit leading to its damage. Protection against over-voltages is achieved by connecting a zener diode ZD across the gate circuit. A resistor R2 connected in series with the gate circuit provides protection against overcurrents. A common problem in thyristor circuits is that they suffer from spurious, or noise, firing. Turning-on or turning-off of an SCR may induce trigger pulses in a nearby SCR. Sometimes transients in a power circuit may also cause unwanted signal to appear across the gate of a neighbouring SCR. These undesirable trigger pulses may turn on the SCR leading to read more

Electronic crowbar protection

arjun — Fri, 17 Apr 2009 20:27:22 +0000

As thyristor possesses high surge current capability, it can be used in an electronic crowbar circuit for overcurrent protection of power converters using SCRs. An electronic crowbar protection provides rapid isolation of the power converter before any damage occurs Fig. 4.28 illustrates the basic principle of electronic crowbar protection. A crowbar thyristor is connected across the input dc terminals. A current sensing resistor detects the value of converter current. If it exceeds preset value, gate circuit provides the signal to crowbar SCR and turns it on in a few microseconds. The input terminals are then short-circuited by crowbar SCR and it shunts away the converter overcurrent. The crowbar thyristor current depends upon the source voltage and its impedance. After some time, main read more

Overcurrent Protection in Thyristors

arjun — Fri, 17 Apr 2009 20:24:14 +0000

Thyristors have small thermal time constants. Therefore, if a thyristor is subjected to overcurrent due to faults, short circuits or surge currents ; its junction temperature may exceed the rated value and the device may be damaged. There is thus a need for the overcurrent protection of SCRs. As in other electrical systems, overcurrent protection in thyristor circuits is achieved through the use of circuit breakers and fast-acting fuses as shown in Fig. 4.29. The type of protection used against overcurrent depends upon whether the supply system is weak or stiff. In a weak supply network, fault current is limited by the source impedance below the multi-cycle surge current rating of the thyristor. In machine tool and excavator drives, if the motor stalls due to overloads, the current is read more

Overvoltage Protection in Thyristors

arjun — Fri, 17 Apr 2009 20:19:38 +0000

Thyristors are very sensitive to overvoltages just as other semi-conductor devices are. Overvoltage transients are perhaps the main cause of thyristor failure. Transient overvoltages cause either maloperation of the circuit by unwanted turn-on of a thyristor or permanent damage to the device due to reverse breakdown. A thyristor may be subjected to internal or external overvoltages ; the former is caused by the thyristor operation whereas the latter comes from the supply lines or the load circuit. (i) Internal overvoltages. Large voltages may be generated internally during the commutation of a thyristor. After thyristor anode current reduces to zero, anode current reverses due to stored charges. This reverse recovery current rises to a peak value at which time the SCR begins to block. read more

Design of Snubber Circuits for Thyristor Protection

arjun — Fri, 17 Apr 2009 18:45:38 +0000

A snubber circuit consists of a series combination of resistance Rs and capacitance Cs in parallel with the thyristor as shown in Fig. 4.25. Strictly speaking, a capacitor Cs in parallel with the device is sufficient to prevent unwanted dv/dt triggering of the SCR. When switch S is closed, a sudden voltage appears across the circuit. Capacitor Cs behaves like a short circuit, therefore voltage across SCR is zero. With the passage of time, voltage across Cs builds up at a slow rate such that dv/dt across Cs and therefore across SCR is less than the specified maximum dv/dt rating of the device. Here the question arises that if Cs is enough to prevent accidental turn-on of the device by dv/dt, what is the need of putting Rs in series with Cs ? The answer to this is as under. Before SCR is read more

THYRISTOR PROTECTION

arjun — Fri, 17 Apr 2009 18:20:18 +0000

Reliable operation of a thyristor demands that its specified ratings are not exceeded. In practice, a thyristor may be subjected to overvoltages or overcurrents. During SCR turn-on, di/dt may be prohibitively large. There may be false triggering of SCR by high value of dv/dt. A spurious signal across gate-cathode terminals may lead to unwanted turn-on. A thyristor must be protected against all such abnormal conditions for satisfactory and reliable operation of SCR circuit and the equipment. SCRs are very delicate devices, their protection against abnormal operating conditions is, therefore, essential. The object of this section is to discuss various techniques adopted for the protection of SCRs. (a) di/dt protection. When a thyristor is forward biased and is turned on by a gate pulse, read more

TWO-TRANSISTOR MODEL OF A THYRISTOR

arjun — Tue, 14 Apr 2009 19:14:32 +0000

The principle of thyristor operation can be explained with the use of its two-transistor model (or two-transistor analogy). Fig. 4.15 (a) shows schematic diagram of a thyristor. From this figure, two-transistor model is obtained by bisecting the two middle layers, along the dotted line, in two separate halves as shown in Fig. 4.15 (b). In this figure, junctions J1 – J2 and J2 -J3 can be considered to constitute pnp and npn transistors separately. The circuit representation of the two-transistor model of a thyristor is shown in Fig. 4.15 (c). In the off-state of a transistor, collector current Ic is related to emitter current IE as IC = αIE + ICBO where α is the common-base current gain and ICB0 is the common-base leakage current of collector-base junction of a transistor. For read more

THYRISTOR GATE CHARACTERISTICS

arjun — Mon, 13 Apr 2009 21:19:47 +0000

The forward gate characteristics of a thyristor are shown in Fig. 4.9 in the form of a graph between gate voltage and gate current. Here positive gate to cathode voltage Vg and positive gate to cathode current Ig represent dc values. As gate-cathode circuit of a thyristor is a p-n junction, gate characteristics of the device are similar to that of a diode. For a particular type of SCRs, Vg-Ig characteristic has a spread between two curves 1 and 2 as shown in Fig. 4.9. This spread, or scatter, of gate characteristics is due to difference in the low doping levels of p and n layers. The gate trigger circuitry must be suitably designed to take care of this unavoidable scatter of characteristics. In Fig. 4.9, curve 1 represents the lowest voltage values that must be applied to turn-on the read more

SWITCHING CHARACTERISTICS OF THYRISTORS DURING TURN OFF

arjun — Thu, 09 Apr 2009 19:50:33 +0000

Static and switching characteristics of thyristors are always taken into consideration for economical and reliable design of converter equipment. Static characteristics of a thyristor have already been examined. In this part of the section; switching, dynamic or transient, characteristics of thyristors are discussed. During turn-on and turn-off processes, a thyristor is subjected to different voltages across it and different currents through it. The time variations of the voltage across a thyristor and the current through it during turn-on and turn-off processes give the dynamic or switching characteristics of a thyristor. Here, first switching characteristics during turn-on are described and then the switching characteristics during turn-off Switching Characteristics during read more

SWITCHING CHARACTERISTICS OF THYRISTORS DURING TURN-ON

arjun — Thu, 09 Apr 2009 19:04:12 +0000