Diode Equivalent Circuits.ppt

Diode Equivalent Circuit

1. Diode Equivalent Circuits/
Diode Models
Dr.G.Puvaneswari
Assistant Professor(SG)
ECE, Coimbatore Institute of
Technology, Coimbatore

2. Diode Equivalent Circuits/Diode
Models
•Itisacombinationofelementsthat
representstheactualterminalcharacteristics
ofadeviceinitsoperatingregion
•Diodemodelsarethemathematicalmodels
usedtoapproximatetheactualbehaviorof
realdiodesforanalysispurpose.
•Thediodecircuitsaresolvedbyusingdiode
modelsintheplaceofdiodeswithout
affectingthesystembehavior

3. Why diode models
•Diode’s V-I characteristics is nonlinear
•nonlinearity complicates the calculations in
circuits involving diodes
•Use of diode models or equivalent circuits
enables the use of conventional circuit
analysis techniques

4. Diode Models
•Three models are used
–Ideal Diode Model
–Simplified
–Piecewise linear model

5. Ideal diode model
•It is the least accurate approximation
•It is represented by a simple switch
•An ideal forward-biaseddiode acts like a closed (on)
switch
•An ideal reversed-biaseddiode acts like an open (off)
switch
•The barrier potential, the forward dynamic resistance,
and the reverse current are neglected
•Diode is replaced by simple switch
•Thesemodelsareusedintroubleshooting,to
determinewhetherthediodeisworkingproperlyand
wheretheexactvaluesofvoltageorcurrentarenot
needed

7. Simplified diode model/Practical
diode model
•The practical model includes the barrier
potential
•Itstatesthataforward-biasedsilicondiode
underdcconditionhasavoltagedrop(cut-in
voltage)of0.7V(forSidiodes)acrossitatany
levelofdiodecurrentwithintheratedvalues
•Itconsistsofavoltagesourcerepresentingthe
cut-involtageofthediodeandanidealdiode
actingasaswitch

8. Simplified equivalent circuit

9. Circuit with ideal diode

10. Forward bias
•forward current is determined as follows by
applying Kirchhoff’s voltage law

11. Reverse bias
•diode is assumed to have zero reverse current

12. Piecewise model
•This model approximates the characteristics of the
device by straight-line segments
•But straight-line segments do not result in an exact
duplication of the actual characteristics, especially in
the knee region
•This model approximates the diode characteristic
curve as a series of linear segments
•In mathematics, piecewise means taking a function
and breaking it down into several linear segments

13. Piecewise Approximation

14. •This model contains a voltage source representing
the cut-in voltage and diode resistance (rav)
•Battery (0.7V) specifies the voltage across the device
must be greater than the threshold battery voltage
before conduction through the device in the
direction
•When conduction is established the resistance of the
diode is the specified value of rav.

15. •approximate level of ravcan be determined
from a specified operating point on the
specification sheet
•For a silicon semiconductor diode, if IF=10 mA
at VD=0.8 V, for silicon that a shift of 0.7 V is
required before the characteristics rise

16. Summary

17. References
•Robert L. Boylesteadand Louis Nasheresky
"Electron Devices and Circuits: Theory and
Practice", Prentice Hall of India, 11th Edition,
2013.
•“Fundamentals of Analog Circuits” by Thomas
Floyd