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Lecture 5&6_ Jan 24&26 2011-1

Lecture 5&6_ Jan 24&26 2011-1 - Chapter 3...

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Chapter 3 Solid-State Diodes and Diode Circuits Lecture 5&6 Cover sections: 3.12-3.16, 3.18

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Analysis of Diodes in Reverse Breakdown Operation Choose 2 points (0V, -4 mA) and (-5 V, -3 mA) to draw the load line. It intersects the i-v characteristic at the Q-point: (-2.9 mA, -5.2 V). - 20 = V D + 5000 I D Using load-line analysis: Chap 3 .12
Analysis of Diodes in Reverse Breakdown Operation Using the piecewise linear model : I Z =- I D 0 20 - 5100 I Z - 5 = 0 I Z = (20 - 5) V 5100 = 2.94 mA Since I Z > 0 ( I D < 0), the solution is consistent with Zener breakdown. Chap 3 .12

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Voltage Regulator Using the Zener Diode The Zener diode keeps the voltage across load resistor R L constant. For Zener breakdown operation, I Z > 0. I S = V S - V Z R = (20 - 5) V 5 k = 3 mA I L = V Z R L = 5 V 5 k = 1 mA | I Z = I S - I L = 2 mA I Z = V S R - V Z 1 R + 1 R L 0 | R L R V S V Z - 1 = R min For proper regulation, Zener current must be positive. If the Zener current < 0, the Zener diode no longer controls the voltage across the load resistor and the voltage regulator is said to have “dropped out of regulation”. Chap 3 .12
Voltage Regulator Using a Zener Diode: Example Including Zener Resistance Problem: Find the output voltage and Zener diode current for a Zener diode regulator. Given data: V S = 20 V, R = 5 k , R Z = 0.1 k Ω, V Z = 5 V Analysis: The output voltage is a function of the current through the Zener diode. 0 mA 9 . 1 100 V 5 V 19 . 5 100 V 5 V 19 . 5 0 5000 100 V 5 5000 V 20 = - = - = = = + - + - L Z L L L L V I V V V V Chap 3.12

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Line and Load Regulation Line regulation characterizes how sensitive the output voltage is to input voltage changes. Line Regulation = dV L dV S mV/V For a fixed load current, Line Regulation = R Z R + R Z Load regulation characterizes how sensitive the output voltage is to changes in load current withdrawn from regulator. Load regulation is the Thévenin equivalent resistance looking back into the regulator from the load terminals. Chap 3.12 Load Regulation = dV L dI L For changes in load current, Load Regulation =- R Z R
Rectifier Circuits A basic rectifier converts an ac voltage to a pulsating dc voltage.

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