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National Institute of Technology 2005 B.Tech Production Engineering PULSE AND DIGITAL CIRCUITS - Question Paper

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PULSE AND DIGITAL ELECTRONICS – JAN 2004
SESSION one (15-1-2004 2:30 PM to 4:30 PM)

1. The diode in the figure has 300ns mean lifetime of minority carriers and 100pF transition capacitance. The input source is a symmetric square wave of 1000kHz frequency and 10V peak. Plot the output waveform and voltage across the diode showing voltage and time values properly. The diode inductance and bulk resistance maybe ignored and it maybe presumed that continuity formula defines the diode behavior completely in forward and reverse directions. Also obtain the frequency of input at which the entire waveform (minus diode drop) starts to appear across the 10 Ohms resistance.

2. The diode in the figure has a storage time constant of 1.5?s and transition capacitance of 20pF.The switch S1 was kept closed for a long time and is opened at t=0. compute and plot the minority charge in the diode , diode current , voltage across diode and voltage across resistor if the switch S2 is closed at (i) t = 2?s (ii) 4?s and (iii) 15?s.

3. The figure on the left indicates a situation that comes up frequently in Power Electronic Circuits. presume that the 50A has been flowing through D and L2 in steady state prior to t=0.The switch S is closed at t=0. presume that L2 contains the inductance of the diode and D represents the junction alone. (I) What is the voltage that appears at the current source junction immediately after t=0 ? Diode mean lifetime is 1microsecond. obtain and plot the 2 inductor currents, voltage across the inductors, diode minority carrier stored charge by solving the diode minority carrier continuity formula up to the instant the charge storage goes to zero. (iii) What is the equivalent circuit after that instant? Solve the equivalent circuit using Laplace Transform and suitable initial conditions and find expressions for the voltage across the junction and the voltage at the current source node and plot the waveforms. What is the maximum reverse current in the diode and maximum reverse biasing voltage that appears across the diode ? presume that the transition capacitance of the diode 250pF. (iv) Look up ‘ferrite beads’ in the Internet and obtain out what they are and what their applications are. Do you think ferrite beads can be of any help in this power switching situation ? explain.

4. A capacitor C is shunted across the resistor in the circuit for issue 1. compute the optimum value of C such that the diode absorbs the reverse voltage without any transient. compute and plot the voltage across diode and resistance if (i) half the above computed value is truly used and (ii) twice the computed value is used.

5. A Transistor has Ft = 75MHz, Cte = four pF , Cc = four pF and operates with Vcc = 15V and Rc = 1k?. The base-emitter potential is initially kept at cut-in voltage value. obtain out the base current that must be applied to drive the transistor to saturation in 0.3?s.Calculate the storage time of the transistor with this value of base current if the base of transistor is shorted to emitter subsequently.

6. The input wave in the figure on the right is a 500kHz square wave with 6V high level and 0 V low level. The transistor parameters are Bmin = 50, Cte = four pF, Cc = five pF, rbb' = 50 ohms, Ts = 300ns and Ft = 10MHz. obtain all the components of switch-off delay and switch-on delay of the transistor and draw the output voltage waveform showing the different switching delay components.

7. In the circuit in Figure on the right Vdc = 100V, R = 10 Ohms , Cc = 100pF , Ft = 10 MHz , Bmin = 20 , rbb’ = 0 , Ts = 1?s. Vb is a square wave of +5V and –5V and frequency 20kHz.Find Rb such that the switch-off time will be less than 0.5?s and compute the switch-on time with this value of base resistance. Also compute the switching power loss in the transistor.

8. If the transistor used in the above circuit truly had a ? of 100 compute the switch-on and switch-off times and the switching power loss in the transistor.

9. Calculate the saturation charge in issue 7. compute the value of capacitor that should be shunted across Rb such that the storage time of the transistor will be decreased to near zero.How will this capacitor affect the switch-on time of the circuit ?

10. The transistor in the figure on the right has CC = 50pF , Ft = 75 MHz , Bmin = 50 , rbb’ = 0 , Ts = 1.2?s. The diodes D1 and D2 have negligible switching times compared to the transistor and they have forward drops of 0.6 volt every. (i) Will the transistor saturate? (ii) How much current will flow through every diode (take Vbe of transistor at 0.8V) ? The base voltage is a square wave of +5V and –5V and frequency 20kHz. (iii) What is the function of D2 ? (iv) compute the switching times of the transistor. (v) Do the diodes affect the switching power loss in the transistor ? discuss. (vi) Do the diodes affect the conduction loss in the transistor ? discuss. (vii) Will there be any advantage in using Schottky diodes (read up on them if you do not know what they are) for D1,D2 and D3 ? discuss.

100V

SI

X 400V

S2

1 10 Ohms

S dosed at