Question about equivalent model

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. 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

 

[Tim Wescott]

On Mon, 19 May 2008 08:37:12 -0700, silusilusilu 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. 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

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

 

[John Larkin]

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