Question about equivalent model
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Posted by on May 19, 2008, 11:37 am
Please log in for more thread options consider a pn junction, the large signal equivalent model is composed by a dc voltage (representing the cut-in voltage) and a resistor. If i use the pn junction above cut-in voltage, the space-charge-region vanishes, so it becomes a short circuit. But in a short circuit the voltage drop is ideally zero, so the voltage at anode and catode nodes are equivalent: if i consider equivalent model this is not true because there is the cut-in voltage)....what is wrong? thanks | |||||||||||||||||||
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Posted by Tim Wescott on May 19, 2008, 11:46 am
Please log in for more thread options Because it's the large-signal model. More specifically, it's the large forward current model. As you have wisely noted, you've stepped outside of the bounds where the model is valid. If you need to simulate or analyze circuit behavior that includes short circuits across the diode, you need to change to a more detailed model. -- Tim Wescott Control systems and communications consulting http://www.wescottdesign.com Need to learn how to apply control theory in your embedded system? "Applied Control Theory for Embedded Systems" by Tim Wescott Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html | |||||||||||||||||||
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Posted by John Larkin on May 19, 2008, 12:10 pm
Please log in for more thread options On Mon, 19 May 2008 08:37:12 -0700 (PDT), silusilusilu@gmail.com
wrote: >I have some doubts about equivalent model for pn junction...if i
>consider a pn junction, the large signal equivalent model is composed >by a dc voltage (representing the cut-in voltage) and a resistor. That's a terrible model. Google for a more realistic one. There is no "cut-in voltage", and current is exponential on voltage, modified by a series resistance term that starts to matter at high currents. John | |||||||||||||||||||
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> consider a pn junction, the large signal equivalent model is composed by
> a dc voltage (representing the cut-in voltage) and a resistor. If i use
> the pn junction above cut-in voltage, the space-charge-region vanishes,
> so it becomes a short circuit. But in a short circuit the voltage drop
> is ideally zero, so the voltage at anode and catode nodes are
> equivalent: if i consider equivalent model this is not true because
> there is the cut-in voltage)....what is wrong? thanks