Example: Junction Diode Models

Transcription

1 8/30/2005 Example Juncton ode Models empty.doc 1/7 Example: Juncton ode Models Consder the juncton dode crcut, where the juncton dode has devce parameters I S A, and n 1: I numercally solved the resultng transcendental equaton, and determned the exact soluton: ma v V Now, let s determne approxmate values usng dode models! Frst, let s try the deal dode model. Jm Stles The Unv. of Kansas ept. of EECS

2 8/30/2005 Example Juncton ode Models empty.doc 2/7 Assume IEAL dode s on. 0 Enforce v 0. Analyze the IEAL dode crcut. From KVL: 0 Check result: We therefore can approxmate the juncton dode current as the current through the deal dode model: 100 ma And approxmate the juncton dode voltage as the voltage across the deal dode model: v v 0 Compare these approxmatons to the exact solutons: ma and v V Close, but we can do better! Let s use the CV model. Jm Stles The Unv. of Kansas ept. of EECS

3 8/30/2005 Example Juncton ode Models empty.doc 3/7 07. V V Assume IEAL dode s on. Enforce v 0. Analyze the IEAL dode crcut. From KVL: 0 Check the result: Jm Stles The Unv. of Kansas ept. of EECS

4 8/30/2005 Example Juncton ode Models empty.doc 4/7 We therefore can approxmate the juncton dode current as the current through the CV model: ma And approxmate the juncton dode voltage as the voltage across the CV model: v v V Compare these approxmatons to the exact solutons: 87.4 ma and v V Much better than before, but we can do even better! Let s use the PWL model. V 0 r d Jm Stles The Unv. of Kansas ept. of EECS

5 8/30/2005 Example Juncton ode Models empty.doc 5/7 Q: But, what values should we use for model parameters V 0 and r d?? A: From the CV model, we know that s approxmately 86mA. Therefore, let s create a PWL model that s accurate n the regon between, say, 50 ma < < 125 ma. Frst, we determne at 50 ma and 125 ma. v nvt ln( IS) We now know two ponts lyng on the juncton dode curve! Let s construct a PWL model whose lne ntersects these two ponts. Recall that when the deal dode s forward based, applyng KVL to the PWL model results n: or equvalently: v V r 0 d v V 0 rd rd Insertng the juncton dode values nto ths PWL model equaton provdes: V ( 005. ) r 0 d V ( ) r 0 d Jm Stles The Unv. of Kansas ept. of EECS

6 8/30/2005 Example Juncton ode Models empty.doc 6/7 Two equatons and two unknowns!! Solvng, we get: V V and r 0.31 Ω (small!!) 0 d Therefore, the deal dode crcut s: 0.31 Ω 0.6 V Assume the IEAL dode s on. Enforce v 0. Analyze the IEAL dode crcut. From KVL: 0 Check the result: Jm Stles The Unv. of Kansas ept. of EECS

7 8/30/2005 Example Juncton ode Models empty.doc 7/7 We can therefore approxmate the juncton dode current as the current through the PWL model: ma and approxmate the juncton dode voltage as the voltage across the PWL model: v v V0 r ( ) V Now, compare these values to the exact values V and 87.4 ma. The error of the PWL model estmates s just Volts and 0.1 ma! Each model provdes better estmates than the prevous one! Jm Stles The Unv. of Kansas ept. of EECS