Transformer Phasing: The Dot Notation and Dot Convention...

The Dot Notation

Generally, when we study about Transformers, we assume that the primary and secondary voltage and currents are in phase. But, such is not always the case. In Transformer, The phase relation between primary and secondary currents and voltages depends on how each winding is wrapped around the core.

Refer to fig (1) and (2), you may see that the primary sides of both transformers are identical i.e. primary windings of both transformers wrapped in the same direction around the core.

But in fig (2) you may notice that the secondary winding is wound around the core in the opposite direction from the secondary winding in fig (1).

Consequently, the voltage induced in the Secondary winding in fig (2) is 180° out of phase as compared with the induced voltage in secondary in fig (1) and the direction of secondary current (I_S) is opposite from the primary current (I_P)

So we see that

1. The primary and secondary voltage and current are in phase in fig (1)
2. The primary and secondary voltage and current are 180° out of phase in fig (2)

Dot Convention

             To eliminate any confusion in the phase relation between primary and secondary voltage and current, a dot convention has been adopted for transformer schematic diagrams. Dots are placed on the top of primary and secondary terminals as shown in fig (3) and (4)

In fig (3), we see that dots are placed at the top in both primary and secondary terminals. It shows that the primary and secondary current and voltages are in phase. Moreover, the primary and secondary voltages (V_P and V_S) have similar sine wave, also the primary and secondary (I_P and I_S) currents are same in direction.

The story is opposite in fig (4). We can see that one dot is positioned at the top in primary terminal and the other one (dot) is placed at bottom of secondary terminal. It shows that the primary and secondary current and voltages are 180° out of phase. In addition, the primary and secondary voltages (V_P and V_S) sine waves are opposite to each other. Also the primary and secondary currents (I_Pand I_S) are opposite in direction.

 

Source: http://www.electricaltechnology.org/2013/12/transformer-phasing-the-dot-notation-and-dot-convention.html

What is IGBT ???...How it Works ???...

Insulated Gate Bipolar Transistor

The Insulated Gate Bipolar Transistor also called an IGBT for short, is something of a cross between a conventional Bipolar Junction Transistor, (BJT) and a Field Effect Transistor, (MOSFET) making it ideal as a semiconductor switching device.

The IGBT transistor takes the best parts of these two types of transistors, the high input impedance and high switching speeds of a MOSFET with the low saturation voltage of a bipolar transistor, and combines them together to produce another type of transistor switching device that is capable of handling large collector-emitter currents with virtually zero gate current drive.

Typical IGBT

The Insulated Gate Bipolar Transistor, (IGBT) uses the insulated gate (hence the first part of its name) technology of the MOSFET with the output performance characteristics of a conventional bipolar transistor, (hence the second part of its name). The result of this hybrid combination is that the “IGBT Transistor” has the output switching and conduction characteristics of a bipolar transistor but is voltage-controlled like a MOSFET.

IGBTs are mainly used in power electronics applications, such as inverters, converters and power supplies, were the demands of the solid state switching device are not fully met by power bipolars and power MOSFETs. High-current and high-voltage bipolars are available, but their switching speeds are slow, while power MOSFETs may have high switching speeds, but high-voltage and high-current devices are expensive and hard to achieve.

The advantage gained by the insulated gate bipolar transistor device over a BJT or MOSFET is that it offers greater power gain than the bipolar type together with the higher voltage operation and lower input losses of the MOSFET. In effect it is an FET integrated with a bipolar transistor in a form of Darlington configuration as shown.

Insulated Gate Bipolar Transistor

 

We can see that the insulated gate bipolar transistor is a three terminal, transconductance device that combines an insulated gate N-channel MOSFET input with a PNP bipolar transistor output connected in a type of Darlington configuration. As a result the terminals are labelled as: Collector,Emitter and Gate. Two of its terminals (C-E) are associated with a conductance path and the third terminal (G) associated with its control.

The amount of amplification achieved by the insulated gate bipolar transistor is a ratio between its output signal and its input signal. For a conventional bipolar junction transistor, (BJT) the amount of gain is approximately equal to the ratio of the output current to the input current, called Beta.

For a metal oxide semiconductor field effect transistor or MOSFET, there is no input current as the gate is isolated from the main current carrying channel. Therefore, an FET’s gain is equal to the ratio of output current change to input voltage change, making it a transconductance device and this is also true of the IGBT. Then we can treat the IGBT as a power BJT whose base current is provided by a MOSFET.

The Insulated Gate Bipolar Transistor can be used in small signal amplifier circuits in much the same way as the BJT or MOSFET type transistors. But as the IGBT combines the low conduction loss of a BJT with the high switching speed of a power MOSFET an optimal solid state switch exists which is ideal for use in power electronics applications.

Also, the IGBT has a much lower “on-state” resistance, R_ON than an equivalent MOSFET. This means that the I²R drop across the bipolar output structure for a given switching current is much lower. The forward blocking operation of the IGBT transistor is identical to a power MOSFET.

When used as static controlled switch, the insulated gate bipolar transistor has voltage and current ratings similar to that of the bipolar transistor. However, the presence of an isolated gate in an IGBT makes it a lot simpler to drive than the BJT as much less drive power is needed.

An insulated gate bipolar transistor is simply turned “ON” or “OFF” by activating and deactivating its Gate terminal. A constant positive voltage input signal across the Gate and the Emitter will keep the device in its “ON” state, while removal of the input signal will cause it to turn “OFF” in much the same way as a bipolar transistor or MOSFET.

IGBT Characteristics

 

Because the IGBT is a voltage-controlled device, it only requires a small voltage on the Gate to maintain conduction through the device unlike BJT’s which require that the Base current is continuously supplied in a sufficient enough quantity to maintain saturation.

Also the IGBT is a unidirectional device, meaning it can only switch current in the “forward direction”, that is from Collector to Emitter unlike MOSFET’s which have bi-directional current switching capabilities (controlled in the forward direction and uncontrolled in the reverse direction).

The principal of operation and Gate drive circuits for the insulated gate bipolar transistor are very similar to that of the N-channel power MOSFET. The basic difference is that the resistance offered by the main conducting channel when current flows through the device in its “ON” state is very much smaller in the IGBT. Because of this, the current ratings are much higher when compared with an equivalent power MOSFET.

The main advantages of using the Insulated Gate Bipolar Transistor over other types of transistor devices are its high voltage capability, low ON-resistance, ease of drive, relatively fast switching speeds and combined with zero gate drive current makes it a good choice for moderate speed, high voltage applications such as in pulse-width modulated (PWM), variable speed control, switch-mode power supplies or solar powered DC-AC inverter and frequency converter applications operating in the hundreds of kilohertz range.

A general comparison between BJT’s, MOSFET’s and IGBT’s is given in the following table.

IGBT Comparison Table

Device Characteristic	Power Bipolar	Power MOSFET	IGBT
Voltage Rating	High <1kV	High <1kV	Very High >1kV
Current Rating	High <500A	Low <200A	High >500A
Input Drive	Current 20-200 hFE	Voltage VGS 3-10V	Voltage VGE 4-8V
Input Impedance	Low	High	High
Output Impedance	Low	Medium	Low
Switching Speed	Slow (uS)	Fast (nS)	Medium
Cost	Low	Medium	High

We have seen that the Insulated Gate Bipolar Transistor is semiconductor switching device that has the output characteristics of a bipolar junction transistor, BJT, but is controlled like a metal oxide field effect transistor, MOSFET.

One of the main advantages of the IGBT transistor is the simplicity by which it can be driven ON or OFF or in its linear active region as a power amplifier. With its lower on-state conduction losses and its ability to switch high voltages without damage makes this transistor ideal for driving inductive loads such as coil windings, electromagnets and DC motors.

What is Thyrister ??? How it Works ??? How to Control it ???

                         In many ways the Silicon Controlled Rectifier, or the Thyristor as it is more commonly known, is similar to the transistor. It is a multi-layer semiconductor device, hence the “silicon” part of its name. It requires a gate signal to turn it “ON”, the “controlled” part of the name and once “ON” it behaves like a rectifying diode, the “rectifier” part of the name. In fact the circuit symbol for the thyristor suggests that this device acts like a controlled rectifying diode.

Thyristor Symbol

However, unlike the diode which is a two layer ( P-N ) semiconductor device, or the transistor which is a three layer ( P-N-P, or N-P-N ) device, theThyristor is a four layer ( P-N-P-N ) semiconductor device that contains three PN junctions in series, and is represented by the symbol as shown.

Like the diode, the Thyristor is a unidirectional device, that is it will only conduct current in one direction only, but unlike a diode, the thyristor can be made to operate as either an open-circuit switch or as a rectifying diode depending upon how the thyristors gate is triggered. In other words, thyristors can operate only in the switching mode and cannot be used for amplification.

The silicon controlled rectifier SCR, is one of several power semiconductor devices along with Triacs (Triode AC’s), Diacs (Diode AC’s) and UJT’s (Unijunction Transistor) that are all capable of acting like very fast solid state AC switches for controlling large AC voltages and currents. So for the Electronics student this makes these very handy solid state devices for controlling AC motors, lamps and for phase control.

The thyristor is a three-terminal device labelled: “Anode”, “Cathode” and “Gate” and consisting of three PN junctions which can be switched “ON” and “OFF” at an extremely fast rate, or it can be switched “ON” for variable lengths of time during half cycles to deliver a selected amount of power to a load. The operation of the thyristor can be best explained by assuming it to be made up of two transistors connected back-to-back as a pair of complementary regenerative switches as shown.

A Thyristors Two Transistor Analogy

 

The two transistor equivalent circuit shows that the collector current of the NPN transistor TR₂feeds directly into the base of the PNP transistor TR₁, while the collector current of TR₁ feeds into the base of TR₂. These two inter-connected transistors rely upon each other for conduction as each transistor gets its base-emitter current from the other’s collector-emitter current. So until one of the transistors is given some base current nothing can happen even if an Anode-to-Cathode voltage is present.

When the thyristors Anode terminal is negative with respect to the Cathode, the centre N-P junction is forward biased, but the two outer P-N junctions are reversed biased and it behaves very much like an ordinary diode. Therefore a thyristor blocks the flow of reverse current until at some high voltage level the breakdown voltage point of the two outer junctions is exceeded and the thyristor conducts without the application of a Gate signal.

This is an important negative characteristic of the thyristor, as Thyristors can be unintentionally triggered into conduction by a reverse over-voltage as well as high temperature or a rapidly risingdv/dt voltage such as a spike.

If the Anode terminal is made positive with respect to the Cathode, the two outer P-N junctions are now forward biased but the centre N-P junction is reverse biased. Therefore forward current is also blocked. If a positive current is injected into the base of the NPN transistor TR₂, the resulting collector current flows in the base of transistor TR₁. This in turn causes a collector current to flow in the PNP transistor, TR₁ which increases the base current of TR₂ and so on.

Typical Thyristor

Very rapidly the two transistors force each other to conduct to saturation as they are connected in a regenerative feedback loop that can not stop. Once triggered into conduction, the current flowing through the device between the Anode and the Cathode is limited only by the resistance of the external circuit as the forward resistance of the device when conducting can be very low at less than 1Ω so the voltage drop across it and power loss is also low.

Then we can see that a thyristor blocks current in both directions of an AC supply in its “OFF” state and can be turned “ON” and made to act like a normal rectifying diode by the application of a positive current to the base of transistor, TR₂ which for a silicon controlled rectifier is called the “Gate” terminal.

The operating voltage-current I-V characteristics curves for the operation of a Silicon Controlled Rectifier are given as:

Thyristor I-V Characteristics Curves

 

Once the thyristor has been turned “ON” and is passing current in the forward direction (anode positive), the gate signal looses all control due to the regenerative latching action of the two internal transistors. The application of any gate signals or pulses after regeneration is initiated will have no effect at all because the thyristor is already conducting and fully-ON.

Unlike the transistor, the SCR can not be biased to stay within some active region along a load line between its blocking and saturation states. The magnitude and duration of the gate “turn-on” pulse has little effect on the operation of the device since conduction is controlled internally. Then applying a momentary gate pulse to the device is enough to cause it to conduct and will remain permanently “ON” even if the gate signal is completely removed.

Therefore the thyristor can also be thought of as a Bistable Latch having two stable states “OFF” or “ON”. This is because with no gate signal applied, a silicon controlled rectifier blocks current in both directions of an AC waveform, and once it is triggered into conduction, the regenerative latching action means that it cannot be turned “OFF” again just by using its Gate.

So how do we turn “OFF” the thyristor?. Once the thyristor has self-latched into its “ON” state and passing a current, it can only be turned “OFF” again by either removing the supply voltage and therefore the Anode (I_A) current completely, or by reducing its Anode to Cathode current by some external means (the opening of a switch for example) to below a value commonly called the “minimum holding current”, I_H.

The Anode current must therefore be reduced below this minimum holding level long enough for the thyristors internally latched PN-junctions to recover their blocking state before a forward voltage is again applied to the device without it automatically self-conducting. Obviously then for a thyristor to conduct in the first place, its Anode current, which is also its load current, I_L must be greater than its holding current value. That is I_L > I_H.

Since the thyristor has the ability to turn “OFF” whenever the Anode current is reduced below this minimum holding value, it follows then that when used on a sinusoidal AC supply the SCR will automatically turn itself “OFF” at some value near to the cross over point of each half cycle, and as we now know, will remain “OFF” until the application of the next Gate trigger pulse.

Since an AC sinusoidal voltage continually reverses in polarity from positive to negative on every half-cycle, this allows the thyristor to turn “OFF” at the 180^o zero point of the positive waveform. This effect is known as “natural commutation” and is a very important characteristic of the silicon controlled rectifier.

Thyristors used in circuits fed from DC supplies, this natural commutation condition cannot occur as the DC supply voltage is continuous so some other way to turn “OFF” the thyristor must be provided at the appropriate time because once triggered it will remain conducting.

However in AC sinusoidal circuits natural commutation occurs every half cycle. Then during the positive half cycle of an AC sinusoidal waveform, the thyristor is forward biased (anode positive) and a can be triggered “ON” using a Gate signal or pulse. During the negative half cycle, the Anode becomes negative while the Cathode is positive. The thyristor is reverse biased by this voltage and cannot conduct even if a Gate signal is present.

So by applying a Gate signal at the appropriate time during the positive half of an AC waveform, the thyristor can be triggered into conduction until the end of the positive half cycle. Thus phase control (as it is called) can be used to trigger the thyristor at any point along the positive half of the AC waveform and one of the many uses of a Silicon Controlled Rectifier is in the power control of AC systems as shown.

Thyristor Phase Control

 

At the start of each positive half-cycle the SCR is “OFF”. On the application of the gate pulse triggers the SCR into conduction and remains fully latched “ON” for the duration of the positive cycle. If the thyristor is triggered at the beginning of the half-cycle ( θ = 0^o ), the load (a lamp) will be “ON” for the full positive cycle of the AC waveform (half-wave rectified AC) at a high average voltage of 0.318 x Vp.

As the application of the gate trigger pulse increases along the half cycle ( θ = 0^o to 90^o ), the lamp is illuminated for less time and the average voltage delivered to the lamp will also be proportionally less reducing its brightness.

Then we can use a silicon controlled rectifier as an AC light dimmer as well as in a variety of other AC power applications such as: AC motor-speed control, temperature control systems and power regulator circuits, etc.

Thus far we have seen that a thyristor is essentially a half-wave device that conducts in only the positive half of the cycle when the Anode is positive and blocks current flow like a diode when the Anode is negative, irrespective of the Gate signal.

But there are more semiconductor devices available which come under the banner of “Thyristor” that can conduct in both directions, full-wave devices, or can be turned “OFF” by the Gate signal.

Such devices include “Gate Turn-OFF Thyristors” (GTO), “Static Induction Thyristors” (SITH), “MOS Controlled Thyristors” (MCT), “Silicon Controlled Switch” (SCS), “Triode Thyristors” (TRIAC) and “Light Activated Thyristors” (LASCR) to name a few, with all these devices available in a variety of voltage and current ratings making them attractive for use in applications at very high power levels.

Thyristor Summary

Silicon Controlled Rectifiers known commonly as Thyristors are three-junction PNPN semiconductor devices which can be regarded as two inter-connected transistors that can be used in the switching of heavy electrical loads. They can be latched-“ON” by a single pulse of positive current applied to their Gate terminal and will remain “ON” indefinitely until the Anode to Cathode current falls below their minimum latching level.

Static Characteristics of a Thyristor

• Thyristors are semiconductor devices that can operate only in the switching mode.
• Thyristor are current operated devices, a small Gate current controls a larger Anode current.
• Conducts current only when forward biased and triggering current applied to the Gate.
• The thyristor acts like a rectifying diode once it is triggered “ON”.
• Anode current must be greater than holding current to maintain conduction.
• Blocks current flow when reverse biased, no matter if Gate current is applied.
• Once triggered “ON”, will be latched “ON” conducting even when a gate current is no longer applied providing Anode current is above latching current.

Thyristors are high speed switches that can be used to replace electromechanical relays in many circuits as they have no moving parts, no contact arcing or suffer from corrosion or dirt. But in addition to simply switching large currents “ON” and “OFF”, thyristors can be made to control the mean value of an AC load current without dissipating large amounts of power. A good example of thyristor power control is in the control of electric lighting, heaters and motor speed.

Source : electronics-tutorials

Why do we use a diac for triggering a triac ???

Why do we use a diac for triggering a triac?

               It is possible to give a gate pulse on triac directly by using something that produces gate pulses. A DIAC is just such a device and is doing just what you suggest. A TRIAC gate is not designed to switch at a tightly controlled voltage or current and if a rising voltage source is connected to the gate the TRIAC will turn on in a "soft" and ill defined manner and time location. The DIAC allows input voltage to rise to a wellish defined level and then dumps a more than high enough voltage, energy source (capacitor) into the gate.

What is Diac... & Quadrac...??? How it Works...???

 

               The DIode AC switch, or Diac for short, is another solid state, three-layer, two-junction semiconductor device but unlike the transistor the Diac has no base connection making it a two terminal device, labelled A₁ and A₂. Diacs have no control or amplification but act much like a bidirectional switching diode as they can conduct current from either polarity of a suitable AC voltage supply.

In our tutorial about SCR’s and Triacs, we saw that in ON-OFF switching applications, these devices could be triggered by simple circuits producing steady state gate currents as shown.

When switch, S1 is open no gate current flows and the lamp is “OFF”. When switch S1 is closed, gate current I_Gflows and the SCR conducts on the positive half cycles only as it is operating in quadrant Ι.

We remember also that once gated “ON”, the SCR will only switch “OFF” again when its supply voltage falls to a values such that its Anode current, I_A is less than the value of its holding current, I_H.

If we wish to control the mean value of the lamp current, rather than just switch it “ON” or “OFF”, we could apply a short pulse of gate current at a pre-set trigger point to allow conduction of the SCR to occur over part of the half-cycle only. Then the mean value of the lamp current would be varied by changing the delay time, T between the start of the cycle and the trigger point. This method is known commonly as “phase control”.

But to achieve phase control, two things are needed. One is a variable phase shift circuit (usually an RC passive circuit), and two, some form of trigger circuit or device that can produce the required gate pulse when the delayed waveform reaches a certain level. One such solid state semiconductor device that is designed to produce these gate pulses is the Diac.

The diac is constructed like a transistor but has no base connection allowing it to be connected into a circuit in either polarity. Diacs are primarily used as trigger devices in phase-triggering and variable power control applications because a diac helps provide a sharper and more instant trigger pulse (as opposed to a steadily rising ramp voltage) which is used to turn “ON” the main switching device.

The diac symbol and the voltage-current characteristics curves of the diac are given below.

Diac Symbol and I-V Characteristics

 

We can see from the above diac I-V characteristics curves that the diac blocks the flow of current in both directions until the applied voltage is greater than V_BR, at which point breakdown of the device occurs and the diac conducts heavily in a similar way to the zener diode passing a sudden pulse of voltage. This V_BR point is called the Diacs breakdown voltage or breakover voltage.

In an ordinary zener diode the voltage across it would remain constant as the current increased. However, in the diac the transistor action causes the voltage to reduce as the current increases. Once in the conducting state, the resistance of the diac falls to a very low value allowing a relatively large value of current to flow. For most commonly available diacs their breakdown voltage typically ranges from about ±25 to 35 volts.

This action gives the diac the characteristic of a negative resistance as shown above. As the diac is a symmetrical device, it therefore has the same characteristic for both positive and negative voltages and it is this negative resistance action that makes the Diac suitable as a triggering device for SCR’s or triacs.

Diac Applications

As stated above, the diac is commonly used as a triggering device for other semiconductor switching devices, mainly SCR’s and triacs. Triacs are widely used in applications such as lamp dimmers and motor speed controllers and as such the diac is used in conjunction with the triac to provide full-wave control of the AC supply as shown.

Diac AC Phase Control

 

As the AC supply voltage increases at the beginning of the cycle, capacitor, C is charged through the series combination of the fixed resistor, R1 and the potentiometer, VR1 and the voltage across its plates increases. When the charging voltage reaches the breakover voltage of the diac (about 30 V), the diac breaks down and the capacitor discharges through the diac, producing a sudden pulse of current, which fires the triac into conduction. The phase angle at which the triac is triggered can be varied using VR1, which controls the charging rate of the capacitor.

Once the triac has been fired into conduction, it is maintained in its “ON” state by the load current flowing through it, while the voltage across the resistor–capacitor combination is limited by the “ON” voltage of the triac and is maintained until the end of the present half-cycle of the AC supply.

At the end of the half cycle the supply voltage falls to zero, reducing the current through the triac below its holding current, I_H turning it “OFF” and the diac stops conduction. The supply voltage then enters its next half-cycle, the capacitor voltage again begins to rise (this time in the opposite direction) and the cycle of firing the triac repeats over again.

Triac Conduction Waveform

 

Then we have seen that the Diac is a very useful device which can be used to trigger triacs and because of its negative resistance characteristics this allows it to switch “ON” rapidly once a certain applied voltage level is reached. However, this means that whenever we want to use a triac for AC power control we will need a separate diac as well. Fortunately for us, some bright spark somewhere replaced the individual diac and triac with a single switching device called a Quadrac.

The Quadrac

The Quadrac is basically a diac and triac fabricated together within a single package and as such are also known as “internally triggered triacs”. This all in one bi-directional device is gate controlled using either polarity of the main terminal voltage which means it can be used in full-wave phase-control applications such as heater controls, lamp dimmers, and AC motor speed control, etc.

Like the triac, quadracs are a three-terminal semiconductor switching device labelled MT2 for main terminal one (usually the anode), MT1 for main terminal two (usually the cathode) and G for the gate terminal.

The quadrac is available in a variety of package types depending upon their voltage and current switching requirements with the TO-220 package being the most common as it is designed to be an exact replacement for most triac devices.

Diac Tutorial Summary

In this diac tutorial we have seen that the diac is a two-terminal voltage blocking device that can conduct in either direction. Diacs posses negative resistance characteristics which allows them to switch “ON” rapidly once a certain applied voltage level is reached.

Since the diac is a bidirectional device, it makes it useful for the triggering and firing of triacs and SCR’s in phase control and general AC circuits such as light dimmers and motor speed controls.

Quadracs are simply triacs with an internally connected diac. As with triacs, quadracs are bidirectional AC switches which are gate controlled for either polarity of main terminal voltage.

Source : http://www.electronics-tutorials.ws/power/diac.html