## Wednesday, June 10, 2009

### power electronics units I - IV

1.Explain the transfer and output characteristics of MOSFETs.
2.Why does the concept of saturation differ in BJTs and Power MOSFETs.
3.What are the differences between enhancement type MOSFETs and depletion type MOSFETs.
4.Explain the SCR firing circuits in detail with the help of circuits and waveforms
(a) Resistance firing circuits (b) RC firing circuit
5.Ten thyristors are used in a string to withstand a d.c. voltage of 12KV. Themax. leakage current and recovery charge difference of SCRs is 10mA and 50μCrespectively. The values of R for steady state equalizing circuit is 40k ohms and value of capacitance C of dynamic qualizing circuit is 0.2μF. Find the steady state and transient derating factor.
6.The latching current of a thyristor with d.c. voltage source of 100V is 50mA.Calculate the value of minimum width of the gate pulse current when connected to a pure inductive load of 1H. Compute the effect, if a resistance of 10 ohms is connected in series with the load.
7.The specifications of a Thyristor operating from a peak supply of 400V is as follows:
Repetitive Peak current Ipk = 200A (di/dt)max = 15A/us , (dv/dt)max =108V/us.Choosing a factor of safety of 2 for Ipk, (di/dt)max and(dv/dt)max, design a suitable snubber circuit, if the minimum loads (RLmin) be 10 ohms resistive.
8.Explain in detail various voltage ratings and current ratings of a thyristor.
9.A string of Thyristor connected in series to withstand a d.c. voltage of Vs= 15KV.The max. leakage current and recovery charge difference of thyristor are 10mA and 150 μC respectively. A derating factor of 20% is applied for the steady state and transient voltage sharing is 1000V. Determine
(a) Steady state voltage sharing resistance R for each thyristor
(b) The transient voltage capacitance.
10.Explain the operation of a singe phase half wave converter for R-load with neat circuit diagram and necessary waveforms. Also derive the output average voltage and current for
α = 300.
11.Explain the operation of a single phase full wave mid-point converter with R-load with the help of circuit and output waveforms with respect to supply voltages.Derive the output voltage for α= 450.
12.Explain the operation of single phase dual converter for RL loads with non-circulating current modes with circuit and necessary voltage and current waveforms.
13.Explain the operation of single phase half-controlled bridge converter with RL loads with and without free wheeling diode. Also sketch circuit and waveforms for α = 600.
14.Explain the working of single phase half-controlled bridge converter with RL loads for discontinuous and continuous current mode of operations with circuit and waveforms for α = 45 degrees.
15.Discuss the effect of source-inductance on the performance of a single phase fully controlled converter, indicating clearly the conduction of various thyristors during one cycle. Derive an expression for its output voltage in terms of Vm, α and μ