Analysis of Circuits Containing Diodes

Definition of Diode Current and Voltage

• Forward Bias– When ID > 0mA

and VD > 0V

• Reverse Bias– When ID < 0mA

and VD < 0V

Three Techniques

• Calculations Using Ideal Diode Equation• Load Line Analysis• Apply Piecewise Model

Example

Approach 1:Ideal Diode Equation

1. Define the diode voltage and current.2. Write the appropriate set of equations.3. Determine (usually given)

a) ambient temperatureb) reverse saturation current and c) ideality factor

4. Add in the ideal diode equation.5. Solve for the diode current and voltage.

+

VD

-

ID

mAIVV

VenAkV

enAI

nnAI

eII

VIRV

DD

DmV

V

mV

V

D

S

nkT

qV

SD

DD

D

D

D

35.4 and 648.0

011.1415

11.14

specified.not if (300K) ure temperatroom Assume

D1N4002 afor 98.1 and 1.14

1

015

3.51

3.51

Approach 2:Load Line Analysis

1. Define the diode voltage and current.2. Obtain the I-V characteristic for the diode.

3. Determine the open circuit voltage, VOC. This is the maximum voltage that could possibly be dropped across the diode.

Find it by replacing the diode with an open and calculate the voltage drop, VD.

4. Determine the short circuit current, ISC. This is the maximum current that could possibly flow through the diode.

Find it by replacing the diode with a short and calculate the current, ID.

5. Plot VOC and ISC on the graph.Draw a line between them. The intersection of the load line with the I-V of the diode is the operating point of the diode.

+

VD

-

ID

Open Circuit Voltage

+

VD

-

ID

+

VOC

-

ID

VVOC 5

Short Circuit Current

+

VD

-

ID

+

VD

-

ISC

mAk

VISC 5

1

5

VD

ID

I-V Curve

• Note that if you calculated the open circuit voltage and short circuit current before measuring the I-V characteristic– you could have determined whether you

should:• collect the forward bias or reverse bias I-V• set the maximum current and voltage range.

Approach 3:Piecewise Models

OFF ON

BREAKDOWN

“ON” Model

• VD = Von

• ID ≥ 0 mA

ID

+

VD

_

Von

_

ID

“OFF” Model

VBR <VD < Von

ID = 0 mA

ID

+

VD

-

+

VD

-

ID

“BREAKDOWN” Model

VD ≤ VBR, VD ≤ -Vz

ID < 0 mA, IZ > 0 mA

IDID

IZ

+

VD

-

+

VD

-

1. First, determine which model should substituted for the diode.

2. Calculate the diode voltage and current using the model.

3. Verify that the diode voltage and current are acceptable for the model chosen.

If not, chose another model and try again.

What I do to quickly select models is to try the OFF model first and calculate the diode voltage (which is equal to VOC).

From the load line analysis, we know that VOC =

5V.

This is much greater than the built-in voltage so this is the wrong model and the correct one is probably the ON model.

+

VD

-

ID

Vg = 0.4V

+

VD

-

ID

Vg = 0.4V

Von

= 0.4V

mAI

VIkV

VV

VIkV

D

D

D

6.4

04.015

4.0 :Given

015

on

on

The calculated diode current is positive when the turn-on voltage is 0.4V. This is acceptable for the ON model so this model is the correct one to use.

Schottky Diodes

– Designed to have very low turn-on voltages and can operate at extremely high frequencies

Zener Diodes

– Designed to handle large currents during breakdown

– When RZ = 0 W