JNTU
JNTU B.Tech III POWER ELECTRONICS Previous Question Paper (February/March - 2022 - R16)
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Discuss about the theory of operation of SCR.
Answer
A Silicon Controlled Rectifier (SCR) is a four-layer, three-junction (p-n-p-n) semiconductor device with three terminals: Anode (A), Cathode (K), and Gate (G). Its operation is governed by the state of its three junctions (J1, J2, and J3).
1. Forward Blocking Mode: When the anode is positive with respect to the cathode and the gate is open, junctions J1 and J3 are forward-biased, while J2 is reverse-biased. Only a small leakage current flows, and the device acts as an open switch.
2. Forward Conduction Mode: When a positive gate current is applied, charge carriers are injected into the p-layer near the gate. This increases the leakage current, which triggers a regenerative process. The device enters a low-impedance state, and the anode current increases rapidly until it is limited only by the external circuit. Once conducting, the gate loses control, and the SCR remains ON as long as the anode current exceeds the holding current.
3. Reverse Blocking Mode: When the anode is negative with respect to the cathode, junctions J1 and J3 are reverse-biased, and J2 is forward-biased. The device blocks current flow, allowing only a small reverse leakage current, similar to a standard diode.
A 400-V, 50-Hz ac source is connected with an SCR and a resistance load circuit. During the forward blocking condition, the blocking junction capacitance is 1.6 nF. The parameters and rating of the SCR at 250C are: IT(rms) = 110 A, VDRM = VRRM = 800 V, VRSM = 900 V, VF = 2.0 V, VGT = 2.5, IGT = 50 mA, Tjmax = 1200C, dv/dt = 100 V/μs and di/dt = 30 A/μs. Show that the snubber circuit is not used, even a spike of 250 V and 2-μs duration will be sufficient to trigger the device undesirably.
Answer
To determine if a voltage spike triggers the SCR, we calculate the capacitive current generated by the spike. The current through the junction capacitance C is given by:
Given that the spike is 250 V over a duration of 2 μs, the rate of rise of voltage (dv/dt) is:
The junction capacitance is C = 1.6 nF. The charging current produced by this spike is:
The gate trigger current required to turn on the SCR is IGT = 50 mA. Since the current produced by the voltage spike (200 mA) is significantly greater than the required gate trigger current (50 mA), the spike will provide sufficient charge to trigger the SCR into conduction undesirably. This confirms that the device is susceptible to false triggering without a snubber circuit.
Explain about MOSFET with the help of its characteristics.
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Design a suitable gate trigger circuit for an 800 V, 110 A SCR connected with a 6-V dc power supply. The maximum permissible current and the short circuit current of the dc source are 200 mA and 500 mA, respectively. The SCR has the following gate parameters: vG = 2.5 V, IG = 50 mA, VGM = 3 V, IGM = 100 mA, PGM = 0.5 W.
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Derive the expression for source current in single phase full wave controlled converter with R load.
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An armature (RLE load) of a separately excited dc motor is fed from three phase, six pulse, fully controlled converter. The supply ac voltage V = 150 V, Ra = 0.06 Ω, La = 0.85 mH, Ia(rated) = 150 A, speed (rated) = 875 rpm, Va(rated) = 200 V. Find the output voltage and α for a rated load torque when E = 160 V.
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Derive the expression for output current in six pulse converter with R load.
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A three phase six full converter is connected with a 400 V, three phase input voltage supply. A highly inductive RL load (10Ω) is connected at the output. Find the power factor of the converter for α = 600.
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Explain the working of anti parallel connection of thyristors with R load.
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A single phase, 220 V, 1-kW electric room heater is connected across 220 V supply through a triac. For a delay angle of 900, calculate the power dissipated by the heater element.
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Discuss the principle of operation of three phase cycloconverters.
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The resistance of a heating element of a 230 V, 5.3 kW resistance furnace which is supplied through a full wave AC voltage controller is 10 Ω. Find the power factor for 50% of the input power.
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Explain with waveforms, the principle of operation of Morgan’s chopper.
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A battery powered electric vehicle uses a dc motor drive, controlled by a dc chopper working at 20 kHz. The battery voltage is 220 V. The minimum turn on and off time of the chopper switch are 4 μsec and 20 μsec respectively. Determine the minimum and maximum dc voltage that the chopper can deliver to the motor.
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With the help of neat sketch, explain the working of parallel capacitor inverter.
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A single phase half bridge inverter feeds a resistive load (R = 10 Ω). If the source voltage is 240 V, determine the rms value of fundamental component of output voltage and the output power.
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