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University of Technology Lecture Note 4 Electrical Engineering Department Characteristics Electrical Engineering Division Page 1 of 15 EG 405: Power Dr. Oday A. Ahmed

Thyristors Characteristics

A is the most important type of power devices. They are extensively used in power electronic circuits. They are operated as bi-stable from non-conducting to conducting state.

A thyristor is a four layer, semiconductor of p-n-p-n structure with three p-n junctions. It has three terminals, the , and the gate. The word thyristor is coined from and . It was invented in the year 1957 at . The Different types of Thyristors are o SCR: silicon-controlled o GTO: Gate Turnoff Thyristor o TRIAC: on AC SCR Thyristor SCR is a general class of a four-layer PNPN semiconducting device, as shown below:

Fig.1

► SCRs have the highest power handling capability. They have a rating of 1200V / 1500A with switching ranging from 1 KHz to 20 KHz.

► Used as a latching that can be turned on by the control terminal but cannot be turned off by the gate.

The structure of the Silicon Controlled (SCR also called thyristor) consists of variously doped P and N conducting layers with three external connections named anode A, cathode K and gate G. It can be represented as two series power :

A K

G University of Technology Lecture Note 4 Electrical Engineering Department Thyristors Characteristics Electrical Engineering Division Page 2 of 15 EG 405: Dr. Oday A. Ahmed

The construction of SCR shows that the gate terminal is kept nearer the cathode. The approximate thickness of each layer and densities are as indicated in the Fig.2. In terms of their lateral dimensions, Thyristors are the largest semiconductor devices made. A complete silicon wafer as large as ten centimetre in diameter may be used to make a single high power thyristor.

Fig.2 Structure of a generic thyristor Qualitative Analysis

When the anode is made positive with respect the cathode junctions J1 & J3 are forward biased and junction J2 is reverse biased. With anode to cathode VAK being small, only leakage current flows through the device. The SCR is then said to be in the forward blocking state. If VAK is further increased to a large value, the reverse biased junction J2 will breakdown due to avalanche effect resulting in a large current through the device.

The voltage at which this phenomenon occurs is called the forward VBO. Since the other junctions J1 & J3 are already forward biased, there will be free movement of carriers across all three junctions resulting in a large forward anode current. Once the SCR is switched on, the voltage drop across it is very small, typically 1 to 1.5V. Only the external impedance present in the circuit limits the anode current.

Although an SCR can be turned on by increasing the forward voltage beyond VBO, in practice, the forward voltage is maintained well below VBO and the SCR is turned on by applying a positive voltage between gate and cathode. With the application of positive gate voltage, the leakage current through the junction J2 is increased. This is because the resulting gate current consists mainly of flow from cathode to gate. Since the bottom end layer is heavily doped as compared to the p-layer, due to the applied voltage, some of these reach junction J2 and add to the minority carrier concentration in the p-layer. This raises the reverse leakage current and results in breakdown of junction J2 even though the applied forward voltage is less than the breakdown voltage VBO. With increase in gate current, breakdown occurs earlier. University of Technology Lecture Note 4 Electrical Engineering Department Thyristors Characteristics Electrical Engineering Division Page 3 of 15 EG 405: Power Electronics Dr. Oday A. Ahmed

A typical V-I characteristics of a thyristor is shown Fig.3.

An elementary circuit diagram for obtaining static I-V characteristics of a thyristor.

Fig.3

From SCR characteristic reveals that a thyristor has three basic modes of operation; namely, Reverse blocking mode, forward blocking (off-state) mode and forward conduction (on- state) mode.

In the reverse direction, the thyristor appears similar to a reverse biased , which conducts very little current until avalanche breakdown occurs.

In the forward direction the thyristor has two stable states or modes of operation that are connected together by an unstable mode that appears as a on the V-I characteristics. The low current high voltage region is the forward blocking state or the off state and the low voltage high current mode is the on state. For the forward blocking state the quantity of interest is the forward blocking voltage which is defined for zero gate current. If a positive gate current is applied to a thyristor then the transition or break over to the on state will occur at smaller values of anode to cathode voltage as shown in fig.4. Although not indicated the gate current does not have to be a dc current but instead can be a pulse of current having some minimum time duration. This ability to switch the thyristor by means of a current pulse is the reason for wide spread applications of the device.

However once the thyristor is in the on state the gate cannot be used to turn the device off. The only way to turn off the thyristor is for the external circuit to force the current through the device to be less than the holding current for a minimum specified period. University of Technology Lecture Note 4 Electrical Engineering Department Thyristors Characteristics Electrical Engineering Division Page 4 of 15 EG 405: Power Electronics Dr. Oday A. Ahmed

Fig.4 Effects on gate current on forward blocking voltage

Holding and Latching Currents

Holding Current IH This is the minimum anode current required to maintain the thyristor in the on state. To turn off a thyristor, the forward anode current must be reduced below its holding current for a sufficient time for mobile charge carriers to vacate the junction. If the anode current is not maintained below IH for long enough, the thyristor will not have returned to the fully blocking state by the time the anode-to-cathode voltage rises again. It might then return to the conducting state without an externally applied gate current.

Latching Current IL This is the minimum anode current required to maintain the thyristor in the on-state immediately after a thyristor has been turned on and the gate signal has been removed. If a gate current, greater than the threshold gate current is applied until the anode current is greater than the latching current IL then the thyristor will be turned on or triggered.

Example 1: The SCR shown has the latching current of 20mA and is fired by the pulse of width 50µs. Determine whether the SCR triggers or not.

Solution: When the SCR T1 is turned on, a step of voltage is applied to the RL load. Thus, the current via RL can be obtained as: University of Technology Lecture Note 4 Electrical Engineering Department Thyristors Characteristics Electrical Engineering Division Page 5 of 15 EG 405: Power Electronics Dr. Oday A. Ahmed

() = () − By applying Laplace transform,

= − Then, = = − (1 − ) By applying inverse Laplace transform, i(t) can be obtained as: () = (1 − ) Here observe that the SCR will be latched if i(t) is greater than latching current when gate triggering pulse is removed after 50µsec. Hence,

100 () = 1 − × ×. = 10 20 Hence the SCE will not be triggered since:

() = 10 < = 20 Example 2: A SCR is connected in series with a 0.5H and 20Ω resistance. A 100V DC voltage is applied to this circuit. If the latching current is 4mA, find the minimum width of the gate trigger pulse required to properly turn-on the SCR.

Solution: The equivalent circuit is shown aside:

= 4 () = (1 − )

When () is equal to latching current , SCR must be turned ON.Hence, if () = = (1 − ) 100 20 4 × 10 = (1 − ) 20 0.5 University of Technology Lecture Note 4 Electrical Engineering Department Thyristors Characteristics Electrical Engineering Division Page 6 of 15 EG 405: Power Electronics Dr. Oday A. Ahmed

Solving above equation by taking the ln of two sides, the required width to trigger the SCR is equal to: = 20 Thyristor Gate Characteristics

Fig. 5 shows the gate trigger characteristics.

The gate voltage is plotted with respect to gate current in the above characteristics. Ig(max) is the maximum gate current that can flow through the thyristor without damaging it Similarly Vg(max) is the maximum gate voltage to be applied. Similarly Vg (min) and Ig(min) are minimum gate voltage and current, below which thyristor will not be turned-on. Hence to turn- on the thyristor successfully the gate Fig.5 current and voltage should be

Ig(min) < Ig < Ig(max)

Vg (min) < Vg < Vg (max)

The characteristic of Fig. 5 also shows the curve for constant gate power (Pg). Thus for reliable turn-on, the (Vg, Ig) point must lie in the shaded area in Fig. 5. It turns-on thyristor successfully. Note that any spurious voltage/current spikes at the gate must be less than Vg(min) and Ig(min) to avoid false triggering of the thyristor. The gate characteristics shown in Fig. 5 are for DC values of gate voltage and current. Pulsed Gate Drive

Instead of applying a continuous (DC) gate drive (see Fig.6 a), the pulsed gate drive is used (see Fig.6 b&c). The gate voltage and current are applied in the form of high pulses. The frequency of these pulses is up to l0 kHz. Hence, the width of the pulse can be up to 100 microseconds. The pulsed gate drive is a applied for following reasons (advantages): b i) The thyristor has small turn-on time i.e. up to 5 microseconds. Hence, a pulse of gate drive is sufficient to turn-on the thyristor. c ii) Once thyristor turns-on, there is no need of gate drive. Hence, gate drive in the form of pulses is suitable. Fig.6 University of Technology Lecture Note 4 Electrical Engineering Department Thyristors Characteristics Electrical Engineering Division Page 7 of 15 EG 405: Power Electronics Dr. Oday A. Ahmed iii) The DC gate voltage and current increases losses in the thyristor. Pulsed gate drive has reduced losses. iv) The pulsed gate drive can be easily passed through isolation to isolate thyristor and trigger circuit.

Usually, a train of pulses is used rather than single pulse as shown in Fig.6b. This is to insure the SCR turned-on. If the first pulse fails to turn on the SCR, then the second and successive pluses are available to turn on the SCR. This is can be clarified as shown in fig.7

Fig.7

Requirement of Gate Drive

The gate drive has to satisfy the following requirements: i) The maximum gate power should not be exceeded by gate drive, otherwise thyristor will be damaged. ii) The gate voltage and current should be within the limits specified by gate characteristics (Fig. 5) for successful turn-on. iii) The gate drive should be preferably pulsed. In case of pulsed drive the following relation must be satisfied: (Maximum gate power (Pgmax) x pulse width (Tp)) x (Pulse frequency (f)) ≤ Allowable average gate power (Pav), iv) The width of the pulse should be sufficient to turn-on the thyristor successfully TP>>tON. v) The gate drive should be isolated electrically from the thyristor. This avoids any damage to the trigger circuit if in case thyristor is damaged. vi) The gate drive should not exceed permissible negative gate to cathode voltage, otherwise the thyristor is damaged. vii) The gate drive circuit should not sink current out of the thyristor after turn-on. University of Technology Lecture Note 4 Electrical Engineering Department Thyristors Characteristics Electrical Engineering Division Page 8 of 15 EG 405: Power Electronics Dr. Oday A. Ahmed

Example 3: A SCR has a linearized gate-cathode characteristic of slope 25 V/A. A gate current of 200mA turns the thyristor on in 16µs. The gate source voltage is 10V. The manufacturer’s average maximum power for the gate is 400mW. Pulse firing is used. Calculate: (a) the value of the gate series resistance; (b) the gate power dissipation during turn-on; (c) the frequency of the gate pulses.

Solution

Example 4: The range of spread of gate-cathode characteristics for a certain thyristor can be linearized to between 15V/A and 10V/A. The manufacturer's data gives the maximum gate power dissipation as 5W. Sketch the characteristic up to Vcc = 15V and Ic = 1.5A, and insert the PG(max av) line. With the gate firing circuit as shown in Fig. aside, a 1:1 isolating , Vp amplitude of 20V, and Rl = R2 = 20Ω, determine the possible range of VGC and IG.

Solution The characteristic is sketched in Fig aside

Load line AB can be inserted.

This gives an operating region between C and D, i.e. about 5-7V for VGC and 0.4-0.5A for IG University of Technology Lecture Note 4 Electrical Engineering Department Thyristors Characteristics Electrical Engineering Division Page 9 of 15 EG 405: Power Electronics Dr. Oday A. Ahmed

Switching Characteristics (Dynamic characteristics)

When the SCR is turned on with the application of the gate signal, the SCR does not conduct fully at the instant of application of the gate trigger pulse. In the beginning, there is no appreciable increase in the SCR anode current, which is because, only a small portion of the silicon pellet in the immediate vicinity of the gate electrode starts conducting. The duration between 90% of the peak gate trigger pulse and the instant, the forward voltage has fallen to 90% of its initial value is called the gate controlled / trigger delay time tgd . It is also defined as the duration between 90% of the gate Fig.8 trigger pulse and the instant at which the anode current rises to 10% of its peak value. tgd is usually in the range of 1µsec.

Once tgd has lapsed, the current starts rising towards the peak value. The period during which the anode current rises from 10% to 90% of its peak value is called the rise time. It is also defined as the time for which the anode voltage falls from 90% to 10% of its peak value. The summation of tgd and tr gives the turn on time ton of the thyristor.

Variations of iA and VAK with time are shown in Fig.9, where Vo is the initial voltage before triggering and Vf is the steady voltage drop of SCR.

Practically td depends on the amplitude and the slope of the trigger pulse Ig. Higher and steep trigger pulse Ig, reduces td. (td depend on the parameters of the trigger circuit). ts depend on anode circuit, if anode circuit inductive, the current rises slowly, if the anode circuit capacitive the current rises very sharp and this can be dangerous. (This is due to that when the gate pulse is applied to the SCR, Fig.9 conduction spread across the cathode area, if the rate of increase of anode current diA/dt is > the rate at which the conduction area is increasing, there will be high power density in this area resulting in excessively high temperature and possible leading to permeant damage to SCR. University of Technology Lecture Note 4 Electrical Engineering Department Thyristors Characteristics Electrical Engineering Division Page 10 of 15 EG 405: Power Electronics Dr. Oday A. Ahmed

A small external Lext in connected in series with SCR to reduce diA/dt. If IG is suddenly increased, the anode current IA will immediately increase which resulting to undesirable turn-on of thyristor.

During spread time ton, the conduction spread over the complete cross-section area of SCR. The IA reach to its maximum value. And the VAK falls to the lowest value. The dissipation in the SCR is reduce. Thyristor Turn OFF Characteristics

When an SCR is turned on by the gate signal, the gate loses control over the device and the device can be brought back to the blocking state only by reducing the forward current to a level below that of the holding current.

In AC circuits, however, the current goes through a natural zero value and the device will automatically switch off. But in DC circuits, where no neutral zero value of current exists, the forward current is reduced by applying a reverse voltage across anode and cathode and thus forcing the current through the SCR to zero. As in the case of diodes, the SCR has a reverse recovery time trr which is due to charge storage in the junctions of the SCR. These excess carriers take some time for recombination resulting in the gate recovery time or reverse recombination time tgr.

Thus, the turn-off time tq is the sum of the durations for which reverse recovery current flows after the application of reverse voltage and the time required for the recombination of all excess carriers present. At the end of the turn off time, a depletion layer develops across J2 and the junction can now withstand the forward voltage. The turn off time is dependent on the anode current, the magnitude of reverse Vg applied ad the magnitude and rate of application of the forward voltage. To ensure that SCR has successfully turned off before re-applied the forward votlage, it is required that the circuit off time tc be greater than SCR turn off time tq . Thyristor Turn ON

Thermal Turn on: If the temperature of the thyristor is high, there will be an increase in charge carriers which would increase the leakage current. This would cause an increase in α1 & α2 and the thyristor may turn on. This type of turn on many cause thermal run away and is usually avoided. University of Technology Lecture Note 4 Electrical Engineering Department Thyristors Characteristics Electrical Engineering Division Page 11 of 15 EG 405: Power Electronics Dr. Oday A. Ahmed

Light: If light be allowed to fall on the junctions of a thyristor, concentration would increase which may turn on the SCR.

LASCR: Light activated SCRs are turned on by allowing light to strike the silicon wafer. When the intensity of light becomes more than a normal value, SCR starts conducting. The wavelength of the light waves can be guided by an optic fiber. This type of SCR is called Activated Silicon Controlled Rectifier (LASCR) and are built with both light and gate triggering arrangement.

High Voltage Triggering: This is triggering without application of gate voltage with only application of a large voltage across the anode- cathode such that it is greater than the forward breakdown voltage VBO . This type of turn on is destructive and should be avoided.

Gate Triggering: Gate triggering is the method practically employed to turn-on the thyristor. Gate triggering will be discussed in detail later. dv/dt Triggering: Under transient conditions, the capacitances of the p-n junction will influence the characteristics of a thyristor. If the thyristor is in the blocking state, a rapidly rising voltage applied across the device would cause a high current to flow through the device resulting in turn-on. If ij2 is the current through the junction j2 and Cj2 is the junction capacitance and Vj2 is the voltage across j2, then

From the above equation, we see that if dv/dt is large, ij2 will be large. A high value of charging current may damage the thyristor and the device must be protected against high dv/dt. The manufacturers specify the allowable dv/dt.

How to read Thyristor Datasheet

A sample of Thyristor Datasheet is shown below should be discussed in the class.

University of Technology Lecture Note 4 Electrical Engineering Department Thyristors Characteristics Electrical Engineering Division Page 12 of 15 EG 405: Power Electronics Dr. Oday A. Ahmed

University of Technology Lecture Note 4 Electrical Engineering Department Thyristors Characteristics Electrical Engineering Division Page 13 of 15 EG 405: Power Electronics Dr. Oday A. Ahmed

GTO

A gate turn-off thyristor (GTO) is a thyristor which is turned on or off by the gate. Like a SCR, GTO can be triggered by into the conducting state by a pulse of positive gate current. However, unlike the SCR, a pulse of negative current at the gate terminal can cause its turn-off. This feature lead to use it of more compact inverter and chopper circuits since no commutation circuits are required.

There are three significant differences between a GTO and a conventional thyristor.

1- The gate and cathode structures are highly interdigitated, with various types of geometric forms being used to layout the gates and . The basic goal is to maximize the periphery of the cathode and minimize the distance from the gate to the centre of a cathode region. 2- The cathode areas are usually formed by etching away the silicon surrounding the cathodes so that they appear as islands or mesas. 3- The n+ regions are overlaid with the same metallization that contacts the p-type anode resulting in a so-called anode short. The anode-short structure is used to speed up the turn-off of the GTO. The i-v characteristic of a GTO, as shown in the figure below, in the forward direction is identical to that of a conventional thyristor. However, in the reverse direction, the GTO has virtually no blocking capability because of the anode-short structure. The only junction that blocks in the reverse direction is junction J3, and it has a rather low breakdown voltage (20- 30 V typically) because of the large doping densities on both sides of the junction. University of Technology Lecture Note 4 Electrical Engineering Department Thyristors Characteristics Electrical Engineering Division Page 14 of 15 EG 405: Power Electronics Dr. Oday A. Ahmed

Triac

SCR can be used to control lamps, motors, or heaters etc. However, one of the problems of using a SCR for controlling such circuits is that like a diode, the “SCR” is a unidirectional device, meaning that it passes current in one direction only, from Anode to Cathode.

Circuits like shown below can be used to obtain full-wave power control in two-directions but this increases both the complexity and number of components used in the switching circuit. a “Triode AC Switch” or Triac for short which is also a member of the thyristor family that be used as a solid state power switching device but more importantly it is a “bidirectional” device. In other words, a Triac can be triggered into conduction by both positive and negative applied to its Anode and with both positive and negative trigger pulses applied to its Gate terminal making it a two-quadrant switching Gate controlled device.

A Triac behaves just like two conventional thyristors connected together in inverse parallel (back-to- back) with respect to each other and because of this arrangement the two thyristors share a common Gate terminal all within a single three-terminal package.

A “Triac” is a 4-layer, PNPN in the positive direction and a NPNP in the negative direction, three-terminal bidirectional device that blocks current in its “OFF” state acting like an open-circuit switch, but unlike a conventional thyristor, the Triac can conduct current in either direction when triggered by a single gate pulse.

Four modes in which a Triac can be operated are shown using the I- V characteristics curves.

 Ι + Mode = MT2 current positive (+ve), Gate current positive (+ve) University of Technology Lecture Note 4 Electrical Engineering Department Thyristors Characteristics Electrical Engineering Division Page 15 of 15 EG 405: Power Electronics Dr. Oday A. Ahmed

 Ι – Mode = MT2 current positive (+ve), Gate current negative (-ve)  ΙΙΙ + Mode = MT2 current negative (-ve), Gate current positive (+ve)  ΙΙΙ – Mode = MT2 current negative (-ve), Gate current negative (-ve)

In Quadrant Ι, the Triac is usually triggered into conduction by a positive gate current, labelled above as mode Ι+. But it can also be triggered by a negative gate current, mode Ι–. Similarly, in Quadrant ΙΙΙ, triggering with a negative gate current, –ΙG is also common, mode ΙΙΙ– along with mode ΙΙΙ+. Modes Ι– and ΙΙΙ+ are, however, less sensitive configurations requiring a greater gate current to cause triggering than the more common Triac triggering modes of Ι+ and ΙΙΙ–. Triac Applications

A common type of Triac switching circuit uses phase control to vary the amount of voltage, and therefore power applied to a load, in this case a motor, for both the positive and negative halves of the input waveform. This type of AC motor speed control gives a fully variable and linear control because the voltage can be adjusted from zero to the full applied voltage as shown.

This basic phase triggering circuit uses the Triac in series with the motor across an AC sinusoidal supply. The variable , VR1 is used to control the amount of phase shift on the gate of the Triac which in turn controls the amount of voltage applied to the motor by turning it ON at different times during the AC cycle. The Triac’s triggering voltage is derived from the VR1 – C1combination via the Diac (The is a bidirectional that helps provide a sharp trigger current pulse to fully turn-ON the ). At the start of each cycle, C1 charges up via the variable resistor, VR1. This continues until the voltage across C1 is sufficient to trigger the diac into conduction which in turn allows , C1 to discharge into the gate of the triac turning it “ON”. Once the triac is triggered into conduction and saturates, it effectively shorts out the gate triggering phase control circuit connected in parallel across it and the triac takes control for the remainder of the half-cycle. As we have seen above, the triac turns-OFF automatically at the end of the half-cycle and the VR1 – C1 triggering process starts again on the next half cycle.