Silicon Controlled Rectifier SCR, Introduction:
As the name suggests, a silicon-controlled rectifier (SCR) is made from silicon material. It is a controlled rectifier (i.e. basically a controlled rectifier is one, with which a control element is being used), the conduction of which can be started or also controlled easily. It is an important member of the thyristor family. Silicon Controlled Rectifier SCR is a device that consists of 4 layers and three terminals. In fact, it consists of three diodes connected back to back. A gate connection is also set along with it. It is vastly used as a switching device for controlling power. It can power on or off a load current several thousand times per second via switching. It can be turned on for supplying the proper power to load during different periods of time. Thus, it is a device, which contains the mutual benefits of a rheostat and a switch. It was invented in 1957.
There are two possible modes of an SCR operation. During its off condition, it operates as a typical open circuit between anode and cathode. In such a situation, it offers intense resistance. During its on mode, it operates as an ideal short circuit between anode and cathode. In such a situation, it offers very small forward resistance. Silicon Controlled RectifierSCR is mostly used in motor controllers, time delay circuits, heater controllers and relay controllers, etc.
As compared to other solid-state devices, it has a longer life, is power-efficient, and is small-sized. They have a very high switching speed and tend to conduct immediately (i.e. they do not require any sort of time for heating up). They do not have any mobile component (i.e. they are static). These are physically strongly built, have higher efficiency, and are jerks and jolts-proof. They can be sealed and are quite light in weight. They can pass on/off a maximum current. For covering industrial, commercial as well as military needs, these are available in different sizes, shapes, designs, ranges, and ratings.
Silicon constructed rectifier is a four-layer device, which comprises three terminals and three junctions. These three terminals are called anode, cathode, and gate while the three junctions are J1, J2, and J3 respectively. In figure 6.5, the construction and symbol of an SCR have been illustrated. Its gate terminal is linked with the P region closer to the cathode. This gate terminal is sometimes also called cathode gate, so that gate and SCR gate being used in other 4-layer devices could clearly be differentiated. The function of a gate is to control the firing of an SCR Silicon Controlled Rectifier.
Figure 6.5 – SCR construction
As they can conduct/pass maximum current, therefore SCR junction area is quite large. Normally, stud-mounted units are used. Because its anode is attached directly to the stud for an efficient emission of heat. However, large units are pillow types, in which several units are placed in a series above one another and on which a clamp is mounted via exerting pressure.
In figure 6.6 – a cross-section of a typical SCR pellet
SCR Equivalent Circuit
For understanding the operation of a Silicon Controlled Rectifier SCR, its construction may also be termed as a collection of two transistors. One of these transistors is PNP whereas the other one is NPN (in other words, if four SCR layers are supposed to be divided into two equal parts, one of them converts into SCR and the other one as a transistor) as indicated vide figure 6.7. in this series, N is the base of the upper transistor while the collector of the lower transistor. While the base of the lower transistor is P, while this base is the collector of the upper transistor. In figure 6.8, an equivalent SCR circuit, which comprises two transistors, has been depicted.
Figure 6.7- silicon controlled rectifier
Figure 6.8– the transistor drawn as a circuit diagram
For further elaboration, an SCR has been divided into two equal transistors as shown in figure 6.9. in this diagram, the PNP transistor has been denoted by Q1 whereas the NPN transistor by Q2. This circuit operates just like a 4-layer diode with the exception that gate terminal has been made along the base of lower NPN transistors’ base. If the gate is not used, (or when the gate is an open circuit) SCR operates precisely similar to a four-layer diode. However, instead of enhancing voltages between SCR anode and cathode, the device can be turned ON via gate. Figure 6.9 (b) clearly reveals that
i) Q1 collector current in Q2 base current
ii) Q1 base current in Q2 collector current.
If the SCR anode is connected with the +ive battery terminal and the cathode with the negative terminal of the battery (as exemplified vide figure 6.10 “a”), then J1 and J3 junctions become forward biased, whereas the J2 junction gets reverse biased. Thus, in such a type of bias, the flow of current through SCR does not occur (except for the leakage current). Inversely, if the SCR anode is connected with a -ive battery terminal and a cathode with a +ive battery terminal (as shown in figure b), then junctions J1 and J3 become reverse biased and junction J2 becomes forward biased. As a result, no current passes through the SCR, once again (that’s under such a situation, SCR does not conduct at all)
Suppose that so many supply voltages parallel to SCR terminals A and C are provided that reverse-biased junction J2 starts to break down. Then, the current passing through the device starts increasing. It means that an increase in IE1 (first transistors’ emitter) has begun. Then,
1). With an increase in IE1, IC1 also increases (remember that IC=αIE)
2). As value of IC1 equals IB2 (i.e. IC1=IB2), therefore, IB2 increases with an increase in IC1.
3). Thus, IC3 also starts increasing (remember that IC=βIB)
4). We know that IC2= IB2, therefore IB1 also increases.
5). Consequently, C1 and IE1 both increase.
Thus, a regenerative action occurs, due to which an initial increase in current generates further increases in the current and its value reaches maximum quite abruptly, which is limited through the application of extrinsic resistance. Both transistors turn fully on and the voltage value parallel to both the transistors becomes minimum. The specific time required for turning an SCR on ranges between 0.1 to 1.0 microseconds.
We know that the Silicon Controlled Rectifier SCR anode is connected with a +ive battery terminal while the cathode with a -ive battery terminal (as shown in figure 6.10 a). thus, no current passes through it owing to the fact that SCR’s J2 junction is reverse biased. However, if anode voltage is increased up to a specific critical value, the junction J2 breakdown and a situation of high conductance suddenly prevail upon SCR (i.e. it starts operating/conducting immediately with the breakdown of J2). The critical voltages supplied on an anode on which junction J2 breaks down, are called forward break over voltages (VBO) (figure 6.11). In such a situation, SCR offers very little forward resistance (value of which ranges between 0.1 to 1.0 ohm). Thus, a very small voltage drop (about 1 volt) occurs across it and the flow of high current within an SCR is controlled through power supply and load resistance.
Figure 6.10 SCR biasing & Figure 6.11
When the anode of an SCR is connected with the -ive terminal and its cathode with the +ive terminal of the battery (figure 6.10 “b”), we know that under such a situation, the supply connection current flowing through SCR is blocked through two reverse-biased junctions J1 and J3. In such a situation, when the supply voltages received on an SCR are increased, a point arrives where Zener breakdown occurs, due to which SCR may become useless or ineffective (fig 6.11). This proves that as compared to a triac (which is a bidirectional device), SCR is a unidirectional device (i.e. the device which operates in one direction)
Firing and Triggering
Normally, an SCR is operated via providing forward bias from anode to cathode (i.e. SCR is operated via connecting its anode with +ive and cathode with -ive terminals of the battery). Thus, fewer voltages are provided to the anode than break over voltage i.e. VBR (F) or VBO. Further, when a low power pulse is provided on its gate, it triggers on and comes into its conduction state (i.e. it turns on). As a consequence of voltage or current supplied on the gate, SCR gets latched in its on-state (i.e. even if gate current or signals are eliminated, SCR remains on instead of being turned off). Thus, a momentary pulse of gate voltage is necessary for turning an SCR on.
In figure 6.12, a positive gate pulse of gate voltage supplied between SCR gate and cathode has been demonstrated, owing to which gate – cathode junction becomes forward biased. When SCR is turned on, the flow of current from anode to cathode becomes limited via series resistance.
Figure 6.12– turning on an SCR with a gate pulse
Signals provided on the SCR gate are
1). DC firing signals (figure 6.13 a)
2). Pulse signals (fig b)
When circuit switch S as shown in figure (a) is open, SCR does not conduct at all and as such, the lamp does not illuminate. When switch S is closed for a brief moment, we get +ive voltages on the SCR gate due to which mid-SCR junction PN becomes forward biased. In this way, Silicon Controlled Rectifier SCR starts conducting and the lamp gets lightened. SCR conducts or remains ON until supply voltage is reversed (or it is turned off). In figure b, the triggering mechanism has been illustrated via time pulses obtained through a pulse source. Triggering can also be undertaken through an increase in the temperature of forward-biased junction (until reverse bias breakdown occurs) and increasing voltage values supplied across SCR anode and cathode (due to which the breadth of depletion layers decreases and ultimately reverse bias junction disappears). However, the most common method of an SCR triggering is the gate triggering method.
Turning the SCR ON
When the value of gate current provided on the gate is zero, SCR remains OFF (as shown in figure 6.14 c). in such a condition, large resistance exists between anode and cathode which is represented by an open switch. When a positive current pulse or trigger is provided on a gate, both transistors turn ON (however, the anode should be more positive with respect to the cathode). This process has been illustrated via figure “b”, IB2 turns transistor Q2 ON, due to which IB2 gets passage to enter Q2 transistor collector. In this way, Q1 powers on. Q1 collector current provides additional current to Q2 base so that Q2 could conduct continuously even if trigger pulse is disposed of the gate. As a result of this regenerative process, Q2 retains the conduction of Q1 by means of letting IB1 a passage. In response, Q1 retains Q2 conduction via letting IB2 a passage. Thus, once triggered, an SCR is always ON (or it remains in latching mode) as has been denoted by figure “c”. In such a situation, a very small resistance exists between an anode and cathode, and SCR is depicted in the shape of a closed switch.
Figure 6.14 – a). SCR OFF b). SCR triggered on c). SCR stays ON after triggering pulse
Turning the SCR OFF
As described above, once SCR is fired and turned ON, it remains ON even if the triggering pulse is removed. This characteristic of an SCR staying ON despite depleting the gate current is called latching. This SCR is also called a kind of latching device.
The following different methods are used for turning an SCR OFF.
i). Anode current operation
ii). Reversing anode-cathode voltage polarity. This operation is called forced commutation
iii). Reducing the current value passing through an SCR than the value of holding current. This method is called low current dropout.
In the anode current interruption method, a switch is fitted parallel to the anode or its series for thwarting the anode current (as shown in figure 6.15) In figure (a) a switch fitted on the anode series can be seen. The value of the anode current is kept zero by turning this switch closed, due to which, SCR turns off. In figure b, a switch parallel to the anode has been fixed. Through the closing of this switch, the anode current is divided into two parts or two parallel paths, owing to which value of current or the anode current leading towards SCR gets less relative to the holding current (IB). Resultantly, SCR becomes OFF (remember, a current which is required to keep an SCR ON, is called a holding current)
Figure 6.15 – SCR turns off by anode current interruption
In the forced commutation method, a forcing current is passed momentarily through the opposite direction of SCR forward conduction (i.e. the direction in which SCR conducts, a current of inverse polarity is passed from the opposite direction of SCR). Thus, the value of total forward current passing through the SCR lessens as compared to the holding value and consequently, SCR becomes OFF.
This type of basic circuit has been shown in figure 6.16. This circuit consists of a switch and a battery, which are mounted parallel to SCR. The switch is retained open at the time when SCR has been conducted as depicted vide figure “a”. When there is an urgency to turn an SCR off, the switch is closed. Thus, the battery gets paralleled to SCR and a forcing current starts passing in the opposite direction of SCR’s forward current. This has been shown in figure “b”. Normally, the off-time of the SCR ranges from a few micro-seconds to 30 micro-seconds.
Figure 6.16 – SCR turn off by forced commutation
A Simple Circuit for Turning SCR OFF
In figure 6.17, a simple method of turning an SCR OFF has been illustrated, which consists an NPN transistor, which has been biased in a simple way. Whenever there is a need to turn an SCR off, a pulse is provided on the base of this transistor. The working mechanism of this circuit is very simple. During the time period, during which we want to keep SCR ON, no pulse is supplied on the base of a transistor. It means that the transistor’s base current value and collector current values both are zero (i.e. IB=0, IC=0). In such a situation, transistors’ collector reflects the status of emitters open circuit and thus no intrusion occurs in SCR operated. Whenever SCR is desired to be turned OFF, a pulse is supplied on the transistors’ gate, as has been shown in the figure. Due to this pulse, the transistor turns on, and the current flow starts from SCR in a direction that is opposite from the one under normal conditions. Thus, SCR holding current decreases and SCR turns OFF. Remember, the value of this current must be equal to or higher compared to SCR holding current so that SCR could be turned off successfully. This circuit is considered very widely used for turning an SCR off.
Figure 6.17 – a simple turn–off circuit of an SCR
SCR Characteristics Curves
A Silicon Controlled Rectifier SCR can be turned on very easily by providing a substantial gate pulse of a suitable current on its gate. When SCR is turned on, its break-over voltage decreases with an increase in the values of gate current. The characteristics curves, which are drawn on different values of gate current between the output current and (IA) and output voltage (VAK) are called SCR characteristics curves. Remember that holding current decreases due to an increase in the values of gate current.
In figure 6.18 (a), an outer (external) resistor has been placed alongside SCR, the function of which is to limit current in the event of a break over. When the value of VAK is very small, and SCR is off, the value of the leakage current also diminishes. Thus, a very negligible voltage drop occurs parallel to R.
Thus, under such a situation, most of the supply voltages VA appear paralleled to SCR in the form of VAK and here SCR is within its forward blocking region. When the value of VAK equals VBR (F), SCR turns on abruptly. Under such a situation, SCR tends to operate like a closed switch and the voltage drop parallel to it suddenly becomes quite small. From the figure, it is clearly evident that the point at which VAK and VBR(F) are equal, a loop of the characteristic curves retracts from this point. Further, the value of IA is very low at this point. If the value of VA is made negative up to a suitable limit, a reverse breakdown occurs, which can be seen from the figure.
Figure 6.18 – the current-voltage characteristics of an SCR with IG=0
Figure 6.19- SCR characteristic curves
An SCR can also be turned on without the operation of gate triggering by increasing its anode to cathode forward break over-voltage, which can be seen through the characteristics curves are shown in figure 6.19 (a) (these characteristics curves are obtained when gate current is equal to zero). In other words, an SCR can also be turned on without providing a pulse on its gate. In this method, voltages parallel to the SCR anode and cathode are increased gradually, until their value exceeds forward break-over voltage VBF (F). under such a scenario, SCR powers on immediately. If the value of gate current IG is enhanced above zero, as can be seen via several curves given in figure (b), forward break over voltages start reducing. Finally, a value of IG reaches (2- volts), at which SCR turns on via a very low anode to cathode voltage. Thus, gate current controls the value of forwarding voltage for keeping an SCR ON. That’s the reason, it is called a silicon-controlled rectifier.
Although, anode to cathode voltage exceeds forward break overvoltages, however, if the current is limited, such voltages do not inflict any damage on the device. This type of situation must always be refrained from because the control of a normal SCR ceases under such a situation. It has to be remembered that SCR should normally be triggered through a pulse on the gate only.
The reverse bias section of the SCR characteristics curves is just analogous to a PN diode. A very small amount of leakage current flows via the action of reverse biasing on SCR. When reverse voltages are increased gradually, reverse breakdown occurs in a reverse direction at a specific break-over voltage, and reverse current starts increasing very sharply. However, the gate terminal has a very negligible impact on the reverse bias region. Different characteristics and ratings of an SCR are as follows:
Forward Breakover Voltage (VBR(F)
Such voltages, through which SCR enters into its forward conduction region, are called forward Breakover voltages. When the value of the gate current IG is zero, the value of these voltages at that time is maximum. As the value of IG is increased, forward Breakover voltage decreases in the same ratio. As it is clear from SCR characteristics curves, these voltages are represented either by VBR (F) or VB (RF).
That value of an anode current, under which an SCR switches or enters into the forward blocking region from its forward conduction region, is called holding current I(H). The value of IH is always opposite or inverse to IG. (That’s when the value of IH increases, the value of IG decreases), until the value of holding current IH becomes maximum once gate current IG comes gets zero.
Gate Trigger Current (IGT)
The value of gate current which is required under certain circumstances for bringing an SCR from the forward blocking region into the forwarding conduction region is called gate trigger current (IGT)
Average Forward Current IF(AVG)
That continuous and maximum value of anode current (DC) which an SCR can withstand in its conduction mode under special conditions, is called average forward current.
Forward Conduction Region
A region (or that part or region of characteristics curves) that represents the ON or conduction state of an SCR, is called the forward conduction region. Remember that in this region SCR operates as a typical short circuit. As a result of this the resistance of SCR in this region is excessively low (approx. zero)
Forward and Reverse Blocking Regions
These are such reasons, which represent the OFF status of an SCR (that’s in this SCR remains off) therefore, these regions reflect an open circuit. In such a situation flow of current from anode to cathode stops.
Reverse Breakdown Voltages VBR(R)
That value of the reverse voltage between an anode and cathode, in which SCR breaks down and enters a breakdown or avalanche region, is shown in the figure. Remember that in such a situation, SCR conducts very quickly just like an ordinary PN junction diode. Thus, Silicon Controlled Rectifier SCR can render useless or ineffective due to the flow of an excessive reverse current. Thus, SCR is not operated in this region.
1). Following supplying rated voltage between anode and cathode when positive voltages are provided on SCR gate, then this operates as a diode and it turns on.
2). Providing negative voltages on an SCR gate, does not turn it on.
3). SCR does not conduct normally in case no voltages are provided on its gate.
4). Once SCR turns on, it remains ON under full load even if these positive gate voltages are removed
5). Provision of positive voltages on the gate during conduction does not cast any influence
6). Provision of negative voltages during conduction does not turn SCR off
7). Voltages provided between SCR’s anode and cathode for keeping an SCR ON should not exceed Breakover voltage.
8). SCR is very sensitive to heat, therefore they should be protected from intense temperature
9). When an SCR conducts, its anode to cathode circuit should be opened so as to turn it off
10). Turning an SCR off, its operating current value should be lowered as compared to its holding current value.
SCRs are commonly available normally between less than one-ampere current rating to 1400 amperes current ratings and 15 volts to 1500 volts range. Their turn-on gate current is about 50 Mille amperes and the anode holds a voltage value of approximately 1 volt.
In figure 6.20, different types of SCRs have been illustrated.
Power FET versus SCR
Although both FET and SCR can turn on or off current excessively, however even then both the devices are basically different. The basic difference between the two is the methods by which they turn off (i.e. both devices turn off in different ways). A power FET can be turned on or off by providing gate voltages on the gate. Whereas SCR does not turn off in this manner, because gate voltages of an SCR can just turn it on.
In figure 6.21, this difference has been illustrated. When inputs of a power FET are high, its output voltages are low at that time as can be seen in figure (a). When input voltages are low, its output voltages are high. In other words, an inverted rectangular output pulse generates from a rectangular input pulse.
Figure 6.21 (Power FET Versus SCR)
When input voltages of SCR Silicon Controlled Rectifier are high (as can be seen in figure b) its output voltages are low. However, when input voltages are low, out voltage also remains low. If a rectangular input pulse is provided on an SCR, a negative-going output produces. Further,
SCR does not reset.
As both the devices reset in different ways, therefore, their usage is also different. Power FETs operate like a push-button switch, while an SCR acts as a single pole single throw (SPST) switch. As it is easy to control power FETs, therefore is mostly used in heavy loads and digital ICs, whereas SCR is used in places where latching assumes importance. A Silicon Controlled Rectifier SCR is mostly used as a power control device. As has been described earlier, when SCR is off, its current is almost negligible. And when it is on, its voltages are almost equal to nothing. Resultantly, it consumes a very negligible amount of power (i.e. it does not consume even a suitable quantity of power). For example, an SCR requires just 150mA in order to control the 2500A load current.
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