Friday, 30 December 2011
Tuesday, 27 December 2011
Introduction to SCR-Silicon Controlled Rectifier
SCR-Schematic-Symbol
As the terminology indicates, the SCR is a controlled rectifier constructed of a silicon semiconductor material with a third terminal for control purposes. Silicon was chosen because of its high temperature and power capabilities. The basic operation of the SCR is different from that of an ordinary two-layer semiconductor diode in that a third terminal called a gate, determines when the rectifier switches from the open-circuit to short-circuit state. It is not enough simply to forward-bias the anode-to-cathode region of the device. In the conduction state the dynamic resistance of the SCR is typically 0.01 to 0.1 ohm and reverse resistance is typically 100 kilo ohm or more. It is widely used as a switching device in power control applications. It can control loads by switching on and off upto many thousand times a second. It can switch on for a variable lengths of time duration, thereby delivering desired amount of power to the load. Thus, it possesses the advantage of a rheostat as well as a switch with none of their drawback. A schematic diagram and symbolic representation of an SCR are shown in figures a & b respectively. As illustrated in fig-a, SCR is a three-terminal four-layer semiconductor device, the layers being alternately of P-type and N-type. The junctions are marked Jj, J2 and J3 (junctions Jj and J3 operate in forward direction while middle junction J2 operates in the reverse direction) whereas the three terminals are anode (A), cathode (C) and gate (G) which is connected to the inner P-type layer. The function of the gate is to control the firing of SCR. In normal operating conditions, anode is positive with respect to cathode.
Construction of an SCR
SCR - construction types
The basic material used for fabrication of an SCR is N-type silicon. It has a specific resistance of about 6 ohm-mm. Silicon is the natural choice as base material because of the following advantages
(i) ability to withstand high junction temperature of the order of 150° C
(ii) high thermal conductivity;
(iii) less variations in characteristics with temperature; and
(iv) less leakage current in P-N junction.
It consists, essentially, of a four layer pellet of P and N type silicon semiconductor materials. The junctions are diffused or alloyed. The material which may be used for P diffusion is aluminium and for N diffusion is phosphorous. The contact with anode can be made with an aluminium foil and through cathode and gate by metal sheet. Diffusion must be carried out at a proper temperature and for necessary duration to provide correct concentration because this decides the properties of the device. Low power SCRs employ the planar construction shown in fig a. Planar construction is useful for making a number of units from a silicon wafer. Here, all the junctions are diffused. The other technique is the mesa construction shown in fig.b. This technique is used for high power SCRs. In this technique, the inner junction J2 is obtained by diffusion, and then the outer two layers are alloyed to it. The PNPN pellet is properly braced with tungsten or molybdenum plates to provide greater mechanical strength and make it capable of handling large currents. One of these plates is hard soldered to a copper or an aluminium stud, which is threaded for attachment to a heat sink. This provides an efficient thermal path for conducting the internal losses to the surrounding medium. The uses of hard solder between the pellet and back-up plates minimises thermal fatigue, when the SCRs are subjected to temperature induced stresses. For medium and low power SCRs, the pellet is mounted directly on the copper stud or casing, using a soft solder which absorbs the thermal stresses set up by differential expansion and provides a good thermal path for heat transfer. For a larger cooling arrangement, which is required for high power SCRs, the press-pack or hockey-puck construction is employed, which provides for double-sided air for cooling.
The salient features to be considered, while designing an SCR, are the diameter and thickness of wafer, composition of the base material, type and amount of the material to be diffused into the wafer, shape, position and contact area of the gate, shape and size of the SCR, type of heat sink etc.
Fabrication technology determines various properties of the device. The voltage rating of a device can be increased by lightly doping the inner two layers and increasing their thickness. But due to this increased resistance, forward voltage drop increases and large triggering currents are required causing greater power dissipation accompanied by smaller current ratings. The heat dissipation of silicon falls from 1.5 W/cm2 at 25° C to 1.25 W/ cm2 at 125° C. A high voltage power device can seldom be used beyond 125° C.
The current carrying capacity and voltage rating of the device can be increased by irradiating silicon with neutrons. The current rating of the device can also be increased by reducing the current density at the junction but this result in a bulky device with large turn-on time.
SCR-Volt-ampere-Character
SCR Characteristics
As already mentioned, the SCR is a four-layer device with three terminals, namely, the anode, the cathode and the gate. When the anode is made positive with respect to the cathode, junctions J1 and J3 are forward biased and junction J2 is reverse-biased and only the leakage current will flow through the device. The SCR is then said to be in the forward blocking state or in the forward mode or off state. But when the cathode is made positive with respect to the anode, junctions J1 and J3 are reverse-biased, a small reverse leakage current will flow through the SCR and the SGR is said to be in the reverse blocking state or in reverse mode.
When the anode is positive with respect to cathode i.e. when the SCR is in forward mode, the SCR does not conduct unless the forward voltage exceeds certain value, called the forward breakover voltage, VFB0. In non-conducting state, the current through the SCR is the leakage current which is very small and is negligible. If a positive gate current is supplied, the SCR can become conducting at a voltage much lesser than forward break-over voltage. The larger the gate current, lower the break-over voltage. With sufficiently large gate current, the SCR behaves identical to PN rectifier. Once the SCR is switched on, the forward voltage drop across it is suddenly reduced to very small value, say about 1 volt. In the conducting or on-state, the current through the SCR is limited by the external impedance.
When the anode is negative with respect to cathode, that is when the SCR is in reverse mode or in blocking state no current flows through the SCR except very small leakage current of the order of few micro-amperes. But if the reverse voltage is increased beyond a certain value, called the reverse break-over voltage, VRB0 avalanche break down takes place. Forward break-over voltage VFB0 is usually higher than reverse breakover voltage,VRBO.
From the foregoing discussion, it can be seen that the SCR has two stable and reversible operating states. The change over from off-state to on-state, called turn-on, can be achieved by increasing the forward voltage beyond VFB0. A more convenient and useful method of turn-on the device employs the gate drive. If the forward voltage is less than the forward break-over voltage, VFB0, it can be turned-on by applying a positive voltage between the gate and the cathode. This method is called the gate control. Another very important feature of the gate is that once the SCR is triggered to on-state the gate loses its control.
The switching action of gate takes place only when
(i) SCR is forward biased i.e. anode is positive with respect to cathode, and
(ii) Suitable positive voltage is applied between the gate and the cathode.
Once the SCR has been switched on, it has no control on the amount of current flowing through it. The current through the SCR is entirely controlled by the external impedance connected in the circuit and the applied voltage. There is, however, a very small, about 1 V, potential drop across the SCR. The forward current through the SCR can be reduced by reducing the applied voltage or by increasing the circuit impedance. There is, however, a minimum forward current that must be maintained to keep the SCR in conducting state. This is called the holding current rating of SCR. If the current through the SCR is reduced below the level of holding current, the device returns to off-state or blocking state.
The SCR can be switched off by reducing the forward current below the level of holding current which may be done either by reducing the applied voltage or by increasing the circuit impedance.
Note : The gate can only trigger or switch-on the SCR, it cannot switch off.
Alternatively the SCR can be switched off by applying negative voltage to the anode (reverse mode), the SCR naturally will be switched off.
Here one point is worth mentioning, the SCR takes certain time to switch off. The time, called the turn-off time, must be allowed before forward voltage may be applied again otherwise the device will switch-on with forward voltage without any gate pulse. The turn-off time is about 15 micro-seconds, which is immaterial when dealing with power frequency, but this becomes important in the inverter circuits, which are to operate at high frequency.
the Power Diode
we saw that a semiconductor signal diode will only conduct current in one direction from its anode to its cathode (forward direction), but not in the reverse direction acting a bit like an electrical one way valve. A widely used application of this feature is in the conversion of an alternating voltage (AC) into a continuous voltage (DC). In other words, Rectification. Small signal diodes can be used as rectifiers in low-power, low current (less than 1-amp) rectifiers or applications, but were larger forward bias currents or higher reverse bias blocking voltages are involved the PN junction of a small signal diode would eventually overheat and melt so larger more robust Power Diodes are used instead.
The power semiconductor diode, known simply as the Power Diode, has a much larger PN junction area compared to its smaller signal diode cousin, resulting in a high forward current capability of up to several hundred amps (KA) and a reverse blocking voltage of up to several thousand volts (KV). Since the power diode has a large PN junction, it is not suitable for high frequency applications above 1MHz, but special and expensive high frequency, high current diodes are available. For high frequency rectifier applications Schottky Diodes are generally used because of their short reverse recovery time and low voltage drop in their forward bias condition.
Power diodes provide uncontrolled rectification of power and are used in applications such as battery charging and DC power supplies as well as AC rectifiers and inverters. Due to their high current and voltage characteristics they can also be used as freewheeling diodes and snubber networks. Power diodes are designed to have a forward "ON" resistance of fractions of an Ohm while their reverse blocking resistance is in the mega-Ohms range. Some of the larger value power diodes are designed to be "stud mounted" onto heatsinks reducing their thermal resistance to between 0.1 to 1oC/Watt.
If an alternating voltage is applied across a power diode, during the positive half cycle the diode will conduct passing current and during the negative half cycle the diode will not conduct blocking the flow of current. Then conduction through the power diode only occurs during the positive half cycle and is therefore unidirectional i.e. DC as shown.
Monday, 26 December 2011
Sunday, 25 December 2011
Bipolar Junction Transistor, or BJT
In the Diode tutorials we saw that simple diodes are made up from two pieces of semiconductor material, either silicon or germanium to form a simple PN-junction and we also learnt about their properties and characteristics. If we now join together two individual signal diodes back-to-back, this will give us two PN-junctions connected together in series that share a common P or N terminal. The fusion of these two diodes produces a three layer, two junction, three terminal device forming the basis of a Bipolar Junction Transistor, or BJT for short.
Transistors are three terminal active devices made from different semiconductor materials that can act as either an insulator or a conductor by the application of a small signal voltage. The transistor's ability to change between these two states enables it to have two basic functions: "switching" (digital electronics) or "amplification" (analogue electronics). Then bipolar transistors have the ability to operate within three different regions:
- 1. Active Region - the transistor operates as an amplifier and Ic = β.Ib
- 2. Saturation - the transistor is "fully-ON" operating as a switch and Ic = I(saturation)
- 3. Cut-off - the transistor is "fully-OFF" operating as a switch and Ic = 0
Typical Bipolar Transistor
The word Transistor is an acronym, and is a combination of the words Transfer Varistor used to describe their mode of operation way back in their early days of development. There are two basic types of bipolar transistor construction, PNP and NPN, which basically describes the physical arrangement of the P-type and N-type semiconductor materials from which they are made.
The Bipolar Transistor basic construction consists of two PN-junctions producing three connecting terminals with each terminal being given a name to identify it from the other two. These three terminals are known and labelled as the Emitter ( E ), the Base ( B ) and the Collector ( C ) respectively.
Bipolar Transistors are current regulating devices that control the amount of current flowing through them in proportion to the amount of biasing voltage applied to their base terminal acting like a current-controlled switch. The principle of operation of the two transistor types PNP and NPN, is exactly the same the only difference being in their biasing and the polarity of the power supply for each type.
Bipolar Transistor Construction
 |
The construction and circuit symbols for both the PNP and NPN bipolar transistor are given above with the arrow in the circuit symbol always showing the direction of "conventional current flow" between the base terminal and its emitter terminal. The direction of the arrow always points from the positive P-type region to the negative N-type region for both transistor types, exactly the same as for the standard diode symbol.
Bipolar Transistor Configurations
As the Bipolar Transistor is a three terminal device, there are basically three possible ways to connect it within an electronic circuit with one terminal being common to both the input and output. Each method of connection responding differently to its input signal within a circuit as the static characteristics of the transistor vary with each circuit arrangement.
- 1. Common Base Configuration - has Voltage Gain but no Current Gain.
- 2. Common Emitter Configuration - has both Current and Voltage Gain.
- 3. Common Collector Configuration - has Current Gain but no Voltage Gain.
The Common Base (CB) Configuration
As its name suggests, in the Common Base or grounded base configuration, the BASE connection is common to both the input signal AND the output signal with the input signal being applied between the base and the emitter terminals. The corresponding output signal is taken from between the base and the collector terminals as shown with the base terminal grounded or connected to a fixed reference voltage point. The input current flowing into the emitter is quite large as its the sum of both the base current and collector current respectively therefore, the collector current output is less than the emitter current input resulting in a current gain for this type of circuit of "1" (unity) or less, in other words the common base configuration "attenuates" the input signal.
The Common Base Transistor Circuit
 |
This type of amplifier configuration is a non-inverting voltage amplifier circuit, in that the signal voltages Vin and Vout are in-phase. This type of transistor arrangement is not very common due to its unusually high voltage gain characteristics. Its output characteristics represent that of a forward biased diode while the input characteristics represent that of an illuminated photo-diode. Also this type of bipolar transistor configuration has a high ratio of output to input resistance or more importantly "load" resistance (RL) to "input" resistance (Rin) giving it a value of "Resistance Gain". Then the voltage gain (Av) for a common base configuration is therefore given as:
Common Base Voltage Gain
Where: Ic/Ie is the current gain, alpha (α) and RL/Rin is the resistance gain.
The common base circuit is generally only used in single stage amplifier circuits such as microphone pre-amplifier or radio frequency (Rf) amplifiers due to its very good high frequency response.
The Common Emitter (CE) Configuration
In the Common Emitter or grounded emitter configuration, the input signal is applied between the base, while the output is taken from between the collector and the emitter as shown. This type of configuration is the most commonly used circuit for transistor based amplifiers and which represents the "normal" method of bipolar transistor connection. The common emitter amplifier configuration produces the highest current and power gain of all the three bipolar transistor configurations. This is mainly because the input impedance is LOW as it is connected to a forward-biased PN-junction, while the output impedance is HIGH as it is taken from a reverse-biased PN-junction.
The Common Emitter Amplifier Circuit
 |
In this type of configuration, the current flowing out of the transistor must be equal to the currents flowing into the transistor as the emitter current is given as Ie = Ic + Ib. Also, as the load resistance (RL) is connected in series with the collector, the current gain of the common emitter transistor configuration is quite large as it is the ratio of Ic/Ib and is given the Greek symbol of Beta, (β). As the emitter current for a common emitter configuration is defined as Ie = Ic + Ib, the ratio of Ic/Ie is called Alpha, given the Greek symbol of α. Note: that the value of Alpha will always be less than unity.
Since the electrical relationship between these three currents, Ib, Ic and Ie is determined by the physical construction of the transistor itself, any small change in the base current (Ib), will result in a much larger change in the collector current (Ic). Then, small changes in current flowing in the base will thus control the current in the emitter-collector circuit. Typically, Beta has a value between 20 and 200 for most general purpose transistors.
By combining the expressions for both Alpha, α and Beta, β the mathematical relationship between these parameters and therefore the current gain of the transistor can be given as:
Where: "Ic" is the current flowing into the collector terminal, "Ib" is the current flowing into the base terminal and "Ie" is the current flowing out of the emitter terminal.
Then to summarise, this type of bipolar transistor configuration has a greater input impedance, current and power gain than that of the common base configuration but its voltage gain is much lower. The common emitter configuration is an inverting amplifier circuit resulting in the output signal being 180o out-of-phase with the input voltage signal.
The Common Collector (CC) Configuration
In the Common Collector or grounded collector configuration, the collector is now common through the supply. The input signal is connected directly to the base, while the output is taken from the emitter load as shown. This type of configuration is commonly known as a Voltage Follower or Emitter Follower circuit. The emitter follower configuration is very useful for impedance matching applications because of the very high input impedance, in the region of hundreds of thousands of Ohms while having a relatively low output impedance.
The Common Collector Transistor Circuit
 |
The common emitter configuration has a current gain approximately equal to the β value of the transistor itself. In the common collector configuration the load resistance is situated in series with the emitter so its current is equal to that of the emitter current. As the emitter current is the combination of the collector AND the base current combined, the load resistance in this type of transistor configuration also has both the collector current and the input current of the base flowing through it. Then the current gain of the circuit is given as:
The Common Collector Current Gain
This type of bipolar transistor configuration is a non-inverting circuit in that the signal voltages of Vin and Vout are in-phase. It has a voltage gain that is always less than "1" (unity). The load resistance of the common collector transistor receives both the base and collector currents giving a large current gain (as with the common emitter configuration) therefore, providing good current amplification with very little voltage gain.
Bipolar Transistor Summary
Then to summarise, the behaviour of the bipolar transistor in each one of the above circuit configurations is very different and produces different circuit characteristics with regards to input impedance, output impedance and gain whether this is voltage gain, current gain or power gain and this is summarised in the table below.
Bipolar Transistor Characteristics
The static characteristics for a Bipolar Transistor can be divided into the following three main groups.
| Input Characteristics:- | Common Base - | ΔVEB / ΔIE |
| Common Emitter - | ΔVBE / ΔIB | |
| Output Characteristics:- | Common Base - | ΔVC / ΔIC |
| Common Emitter - | ΔVC / ΔIC | |
| Transfer Characteristics:- | Common Base - | ΔIC / ΔIE |
| Common Emitter - | ΔIC / ΔIB | |
with the characteristics of the different transistor configurations given in the following table:
| Characteristic | Common Base | Common Emitter | Common Collector |
| Input Impedance | Low | Medium | High |
| Output Impedance | Very High | High | Low |
| Phase Angle | 0o | 180o | 0o |
| Voltage Gain | High | Medium | Low |
| Current Gain | Low | Medium | High |
| Power Gain | Low | Very High | Medium |
In the next tutorial about Bipolar Transistors, we will look at the NPN Transistor in more detail when used in the common emitter configuration as an amplifier as this is the most widely used configuration due to its flexibility and high gain. We will also plot the output characteristics curves commonly associated with amplifier circuits as a function of the collector current to the base current.
What is Electronics ?
Electronics is the study of the flow of electrons through materials and devices. Devices could be semiconductors, resistors, inductors, capacitors and vacuum tubes. An electronic component is any physical entity in an electronic system which affects the electrons. Current flows from +ve to -ve
Increase in voltage increases current. Decrease in resistance will increase current flow.The advantage that alternating current provides for the power outlet is the fact that it is relatively easy to change the voltage of the power, using a device called a transformer .V=IR
Volts * Amps = Watts ::Volts-Potential difference
If you allow electrons to move through a wire, they will create a magnetic field around the wire.Similarly, if you move a magnet near a wire, the magnetic field will cause electrons in the wire to move.
Subscribe to:
Posts (Atom)