75 Diodes, rectifiers, power supply

75 Diodes, rectifiers, power supply - 3/10/10 1...

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Unformatted text preview: 3/10/10 1 Semiconductor Diodes: Features and Applications Most common electronic components in many devices 2009 A. Ganago 1 2009 Diodes Diodes do NOT obey Ohms law 2009 A. Ganago 2009 Diodes 2 This is lab data: Current vs. Voltage Nearly ExponenIal Dependence 2009 A. Ganago 2009 Diodes 3 Need for Simple Circuit Models With an exponential function for voltagecurrent dependence of a diode, every equation for circuit analysis would bring a lot of pain It would be much easier to solve a linear equation, even though the results would be slightly approximate 2009 A. Ganago 2009 Diodes 4 Ofset Model for Diodes is Very Simple 2009 A. Ganago 2009 Diodes 5 Voltage V D Current I D V D0 V D I D V D0 is ~0.7 V for Si ~0.3 V for Ge ~1.7 V for LED Ofset Model for Diodes Means: 2009 A. Ganago 2009 Diodes 6 V D I D V D0 Diode has inOinite resistance at V D < V D0 Diode has zero resistance at V D > V D0 3/10/10 2 Also, Ofset Model for Diodes Means: 2009 A. Ganago 2009 Diodes 7 V D I D V D0 When diode does not conduct, voltage V D can be from to V D0 When diode conducts, voltage V D remains constant V D = V D0 Ofset Model for Diodes Also Means: 2009 A. Ganago 2009 Diodes 8 V D I D V D0 At V D < V D0 diode absorbs zero power At V D = V D0 diode absorbs power (V D0 ).(I D ) Always Add a Load Resistor To Limit the Current and Power 2009 A. Ganago 2009 Diodes 9 V S V D I D = I R V R Without a load resistor, the current and power can become dangerously high ProblemSolving Strategy 2009 A. Ganago 2009 Diodes 10 V S V D I D = I R V R Our tools are: KVL, KCL, Ohms law (for resistors only!!), Offset model for diodes KVL: V S + V D + V R = 0 Does the Diode Conduct? 2009 A. Ganago 2009 Diodes 11 V S V D I D = I R V R At irst, assume that the diode does not conduct. Find V D from KVL; look for contradictions KVL: V S + V D + V R = 0 If the Diode Does NOT Conduct 2009 A. Ganago 2009 Diodes 12 V S V D I D = I R V R KVL: V S + V D + V R = 0 Current is zero thus, from Ohms law, V R = 0; from KVL, V D = V S If V S < V D0 then all is solved. 3/10/10 3 If the Diode Conducts 2009 A. Ganago 2009 Diodes 13 V S V D I=I D =I R V R KVL: V S + V D0 + IR = 0 If V S > V D0 then the diode conducts: V D = V DO From KVL and Ohms law, IR = (V S V D0 ) Find I, etc. R What if You Skip a Load Resistor? 2009 A. Ganago 2009 Diodes 14 V S V D I D = I R V R =0 From KVL: V D = V S If V D > V D0 then the current exponentially grows, and something blows up! KVL: V S + V D + 0 = 0 The OFset Model Applies Only to Circuits with Load Resistors! 2009 A. Ganago 2009 Diodes 15 In circuits that lack load resistors, the Offset Model fails, because it does not allow V D > V D0 To explain what would happen, refer to lab data (see above)....
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This note was uploaded on 12/06/2010 for the course EECS 314 taught by Professor Ganago during the Spring '07 term at University of Michigan.

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75 Diodes, rectifiers, power supply - 3/10/10 1...

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