Thyristors Characteristics

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1 Electrical Engineering Division Page 1 of 15 A thyristor is the most important type of power semiconductor devices. They are extensively used in power electronic circuits. They are operated as bi-stable switches 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 anode, cathode and the gate. The word thyristor is coined from thyratron and transistor. It was invented in the year 1957 at Bell Labs. The Different types of Thyristors are SCR o SCR: silicon-controlled rectifiers o GTO: Gate Turnoff Thyristor o TRIAC: Triode on AC 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 frequencies ranging from 1 KHz to 20 KHz. Used as a latching switch that can be turned on by the control terminal but cannot be turned off by the gate. The structure of the Silicon Controlled Rectifier (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 diodes: A K G

2 Electrical Engineering Division Page 2 of 15 The construction of SCR shows that the gate terminal is kept nearer the cathode. The approximate thickness of each layer and doping 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. Qualitative Analysis Fig.2 Structure of a generic thyristor 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 voltage V AK being small, only leakage current flows through the device. The SCR is then said to be in the forward blocking state. If V AK 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 breakdown voltage V BO. 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 V BO, in practice, the forward voltage is maintained well below V BO 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 electron 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 electrons 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 V BO. With increase in gate current, breakdown occurs earlier.

3 Electrical Engineering Division Page 3 of 15 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 (onstate) mode. In the reverse direction, the thyristor appears similar to a reverse biased diode, 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 negative resistance 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.

4 Electrical Engineering Division Page 4 of 15 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 I H 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 Fig.4 Effects on gate current on forward blocking voltage 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 I L 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:

5 Electrical Engineering Division Page 5 of 15 V = i(t)r L di(t) dt By applying Laplace transform, V = I R LsI Then, I = V R Ls = V R (1 L R s) By applying inverse Laplace transform, i(t) can be obtained as: i(t) = V R (1 e R L ) 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, Hence the SCE will not be triggered since: i(t) = e. = 10mA i(t) = 10mA < I = 20mA Example 2: A SCR is connected in series with a 0.5H inductor 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: I = 4mA i(t) = V R (1 e R L ) When i(t) is equal to latching current I, SCR must be turned ON.Hence, if i(t) = I I = V R (1 e R L ) 4 10 = (1 e )

6 Electrical Engineering Division Page 6 of 15 Solving above equation by taking the ln of two sides, the required width to trigger the SCR is equal to:t = 20μsec 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 turnon the thyristor successfully the gate current and voltage should be Fig.5 Ig(min) < Ig < Ig(max) Vg (min) < Vg < Vg (max) The characteristic of Fig. 5 also shows the curve for constant gate power (P g ). 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 frequency 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 applied for following reasons (advantages): 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. ii) Once thyristor turns-on, there is no need of gate drive. Hence, gate drive in the form of pulses is suitable. a Fig.6 b c

Electrical Engineering Division Page 7 of 15 iii) The DC gate voltage and current increases losses in the thyristor. Pulsed gate drive has reduced losses.

7 Electrical Engineering Division Page 7 of 15 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 transformers 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 (P gmax ) x pulse width (T p )) x (Pulse frequency (f)) Allowable average gate power (P av ), iv) The width of the pulse should be sufficient to turn-on the thyristor successfully T P >>t ON. 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.

8 Electrical Engineering Division Page 8 of 15 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 V cc = 15V and Ic = 1.5A, and insert the P G(max av) line. With the gate firing circuit as shown in Fig. aside, a 1:1 isolating transformer, V p amplitude of 20V, and R l = R 2 = 20Ω, determine the possible range of V GC and I G. 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 V GC and A for I G

9 Electrical Engineering Division Page 9 of 15 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 t gd. It is also defined as the duration between 90% of the gate trigger pulse and the instant at which the anode current rises to 10% of its peak value. t gd is usually in the range of 1µsec. Fig.8 Once t gd 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 t gd and t r gives the turn on time t on of the thyristor. Variations of i A and V AK with time are shown in Fig.9, where V o is the initial voltage before triggering and V f is the steady voltage drop of SCR. Practically t d depends on the amplitude and the slope of the trigger pulse I g. Higher and steep trigger pulse I g, reduces t d. (t d depend on the parameters of the trigger circuit). t s 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 di A /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.

10 Electrical Engineering Division Page 10 of 15 A small external L ext in connected in series with SCR to reduce di A /dt. If I G is suddenly increased, the anode current I A will immediately increase which resulting to undesirable turn-on of thyristor. During spread time t on, the conduction spread over the complete cross-section area of SCR. The I A reach to its maximum value. And the V AK 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 t rr 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 t g r. Thus, the turn-off time t q 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 t c be greater than SCR turn off time t q. 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.

11 Electrical Engineering Division Page 11 of 15 Light: If light be allowed to fall on the junctions of a thyristor, charge carrier 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 anodecathode such that it is greater than the forward breakdown voltage V BO. 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 i j2 is the current through the junction j 2 and C j2 is the junction capacitance and V j2 is the voltage across j 2, then From the above equation, we see that if dv/dt is large, i j2 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.

12 Electrical Engineering Division Page 12 of 15

This feature lead to use it of more compact inverter and chopper circuits since no commutation circuits are required.

13 Electrical Engineering Division Page 13 of 15 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 cathodes. 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.

14 Electrical Engineering Division Page 14 of 15 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 voltages 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-toback) 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 Triacs I- V characteristics curves. Ι + Mode = MT2 current positive (+ve), Gate current positive (+ve)

Electrical Engineering Division Page 15 of 15 Ι Mode = MT2 current positive (+ve), Gate current negative (-ve) ΙΙΙ + Mode = MT2 current negative (-ve), Gate current positive (+ve) ΙΙΙ Mode = MT2

15 Electrical Engineering Division Page 15 of 15 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 resistor, 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 diac is a bidirectional semiconductor device that helps provide a sharp trigger current pulse to fully turn-on the triac). 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 capacitor, 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.

Lecture 2. Power semiconductor devices (Power switches)

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