# Lecture 6 & 7 Diode Models (to be continued) Zener diode

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

Farzana Rahmat zaki 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. Farzana R. Zaki CSE 177/ EEE 177

Graphical Representation of Small Signal model
Farzana R. Zaki CSE 177/ EEE 177

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

Farzana Rahmat zaki 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. Farzana R. Zaki CSE 177/ EEE 177

Problem 2 Diode Regulator
Farzana Rahmat zaki 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. Farzana R. Zaki CSE 177/ EEE 177

Problem 3 Voltage Regulation
Farzana Rahmat zaki 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? Problem 3 Farzana R. Zaki CSE 177/ EEE 177

Farzana R. Zaki CSE 177/ EEE 177

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 Farzana R. Zaki CSE 177/ EEE 177

i-v characteristic curve of zener diode
Farzana Rahmat zaki i-v characteristic curve of zener diode Farzana R. Zaki CSE 177/ EEE 177

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

In addition to specifying Vz (at a particular IzT), rz,Vzk, 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. VZ denotes the point at which the straight line of slope 1/rz intersects the voltage axis. VZ = VZ0 + IZrZ Farzana R. Zaki CSE 177/ EEE 177

Farzana Rahmat zaki 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. Farzana R. Zaki CSE 177/ EEE 177

Farzana Rahmat zaki Problem 4b Farzana R. Zaki CSE 177/ EEE 177

Farzana Rahmat zaki Problem 4c Farzana R. Zaki CSE 177/ EEE 177

Farzana Rahmat zaki Problem 4d Farzana R. Zaki CSE 177/ EEE 177

Farzana Rahmat zaki Problem 4e Farzana R. Zaki CSE 177/ EEE 177

Farzana Rahmat zaki Problem 4f Farzana R. Zaki CSE 177/ EEE 177

Block diagram of a DC power supply
Farzana Rahmat zaki Diode Rectifiers power Block diagram of a DC power supply Farzana R. Zaki CSE 177/ EEE 177

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 N1 turns and is connected to 120V ac power supply & secondary windings have N2 turns and is connected to the circuit of dc power supply. Farzana R. Zaki CSE 177/ EEE 177

ac voltage, vs = 120×(N2/N1) V(r. m. s
* ac voltage, vs = 120×(N2/N1) V(r.m.s.) develops between two terminals of secondary windings. * By choosing appropriate turns ratio ( N2/N1) 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. Farzana R. Zaki CSE 177/ EEE 177

Components of DC power supply (cont.)
Diode Rectifier & filter: Diode rectifier converts input sine wave (vs) 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) Farzana R. Zaki CSE 177/ EEE 177

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