Analysis of Circuits Containing Diodes

Definition of Diode Current and Voltage Forward Bias –When I D > 0mA and V D > 0V Reverse Bias –When I D < 0mA and V D < 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 temperature b) reverse saturation current and c) ideality factor 4.Add in the ideal diode equation. 5.Solve for the diode current and voltage.

+ V D - IDID

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, V OC. 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, V D. 4.Determine the short circuit current, I SC. 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, I D. 5.Plot V OC and I SC 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.

+ V D - IDID

Open Circuit Voltage + V D - IDID + V OC - IDID

Short Circuit Current + V D - IDID + V D - I SC

VDVD IDID

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 V D = V on I D ≥ 0 mA IDID + V D _ V on _ IDID

“OFF” Model V BR <V D < V on I D = 0 mA IDID +VD-+VD- +VD-+VD- IDID

“BREAKDOWN” Model V D ≤ V BR, V D ≤ -V z I D 0 mA IDID IDID IZIZ +VD-+VD- + V D -

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 V OC ). From the load line analysis, we know that V OC = 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-+VD- IDID V  = 0.4V

+VD-+VD- IDID V on = 0.4V 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 R Z = 0 