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Farzana R. ZakiCSE 177/ EEE 1771 Lecture 6 & 7 Diode Models (to be continued) Zener diode Block diagram of DC power supply

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Farzana R. ZakiCSE 177/ EEE 1772 Small-Signal Model The diode is biased to operate (in this case) at 0.7V. The AC response can be modeled as a resistance equal to the inverse slope of the tangent IF it is small enough (small-signal model) This concept of restricting an AC signal to the short, linear region around some DC bias point is used throughout this course.

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Farzana R. ZakiCSE 177/ EEE 1773 Graphical Representation of Small Signal model

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Farzana R. ZakiCSE 177/ EEE 1774 Small-Signal Model Small-Signal Approximation –Valid for signals whose amplitudes are smaller than about 10mV for n=2 and 5mV for n=1

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Farzana R. ZakiCSE 177/ EEE 1775 Problem 1 Consider the same circuit for the case in which R=10k. The power supply has a DC value of 10V on which is superimposed a 60-Hz sinusoid of 1V peak amplitude. Calculate both the dc voltage of the diode and the amplitude of the sine wave that appears across it. Assume the diode to have a 0.7V drop at 1mA and an n=2.

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Farzana R. ZakiCSE 177/ EEE 1776 Problem 2 Diode Regulator –Design the following circuit to provide an output voltage of 2.4V. Assume the diodes have a current of 1 mA at a voltage of.7 V and that its voltage drop changes by.1 V for every decade of change in current.

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Farzana R. ZakiCSE 177/ EEE 1777 Problem 3 Voltage Regulation –Consider the following circuit. What is the percentage change in the regulated voltage caused by (a) a 10% change in the power-supply voltage and (b) connection of a 1k load resistance?

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Farzana R. ZakiCSE 177/ EEE 1778

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Farzana R. ZakiCSE 177/ EEE 1779 Operation in the reverse bias region-Zener diodes The very steep i-v curve that the diode exhibits in the breakdown region and the almost constant voltage drop that indicates suggest that diodes operating in the breakdown region can be used in the design of voltage regulators. Normal Si diode cant operate in breakdown region. So, special diodes are manufactured to operate specially in the breakdown region. Such diodes are called breakdown diodes or zener diodes. i-v curve for normal Si diode

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Farzana R. ZakiCSE 177/ EEE i-v characteristic curve of zener diode

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Farzana R. ZakiCSE 177/ EEE Zener diode For currents greater than knee current I zk, the i-v curve is almost constant. The manufacturer usually specifies the voltage drop across the zener diode V Z at a specifies test current I ZT. This point is labelled as Q. As the current through zener diode deviates from I ZT, the voltage across it will change slightly. For ΔI current change, voltage change, ΔV= ΔI×r Z Where r z = incremental resistance of zener diode at operating point Q = dynamic resistance of the zener r z is in the range of few ohms to a few tens of ohms. V z is in the range of few volts to a few hundred volts.

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Farzana R. ZakiCSE 177/ EEE In addition to specifying V z (at a particular I zT ), r z,V zk, the manufactures also satisfies the maximum power that the device can safely dissipate. Say, a 0.5W, 6.8V zener diode can operate safely at currents up to a maximum of about 70mA. V Z denotes the point at which the straight line of slope 1/r z intersects the voltage axis. V Z = V Z0 + I Z r Z

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Farzana R. ZakiCSE 177/ EEE Problem 4(a) A 6.8-V Zener diode in the circuit below is specified to have Vz=6.8V at Iz=5mA, rz=20 ohms, and Izk=0.2mA. The supply voltage is nominally 10V but can vary by +/- 1V.

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Farzana R. ZakiCSE 177/ EEE Problem 4b

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Farzana R. ZakiCSE 177/ EEE Problem 4c

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Farzana R. ZakiCSE 177/ EEE Problem 4d

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Farzana R. ZakiCSE 177/ EEE Problem 4e

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Farzana R. ZakiCSE 177/ EEE Problem 4f

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Farzana R. ZakiCSE 177/ EEE Diode Rectifiers power Block diagram of a DC power supply

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Farzana R. ZakiCSE 177/ EEE Components of DC power supply The power supply is fed from 120V rms 60Hz ac line, and it delivers a dc voltage (usually in the range of 5-20V) to an electronic circuit represented by the load block. Power Transformer: consists of 2 separate coils wound around an iron core that magnetically couples the 2 windings. Primary windings have N 1 turns and is connected to 120V ac power supply & secondary windings have N 2 turns and is connected to the circuit of dc power supply.

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Farzana R. ZakiCSE 177/ EEE * ac voltage, v s = 120×(N 2 /N 1 ) V(r.m.s.) develops between two terminals of secondary windings. * By choosing appropriate turns ratio ( N 2 /N 1 ) for the transformer, particular dc voltage output can be supplied. [For 8V r.m.s. in secondary winding may be appropriate for a dc output of 5V. For this, 15:1 turns ratio is required.

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Farzana R. ZakiCSE 177/ EEE Components of DC power supply (cont.) Diode Rectifier & filter: Diode rectifier converts input sine wave (v s ) to a unipolar pulsating output waveform. Pulsating nature makes it unsuitable as a dc source for electronic circuits, hence filter is required. Voltage regulator: The output of filter contains some ripple. To reduce ripple and to stabilize the magnitude of dc output supply, voltage regulator is employed. (Zener shunt regulator)

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