Diode Models

Hello, fellows, I hope all of you are enjoying your life. In today’s tutorial, we will have a look at Diode Models. A diode is an electronic device that has 2 terminals positive terminals called an anode and a negative terminal called a cathode. Currently, almost every electronic devices use a diode. The main function of a diode is to convert alternating current into DC current. There are such appliances that do not operate on AC supply as we know in our power system AC is generated and used. So in these devices rectifiers circuits are used to convert AC into DC.

If the rectifier converts full wave of ac into dc than it is known as a full-wave rectifier and if convert half wave it called a half-wave rectifier. In today’s post, we will have a detailed look at the different diode models and compare them with the biasing conditions and find their parameters.  In the previous tutorial, I write a complete post on V-I characteristics of a diode. It will help you to understand the modeling of a diode. So let’s get started with Diode Models.

Diode Models

• For circuit analysis and calculation of different parameters of diodes, mathematic expressions are formed called diode models in electronics.
• As we know V-I curve of a diode is not linear for the understanding of practical behavior of diode simple circuits model of diode needed.

• In today’s post, we will discuss the diode models for ideal and practical diode and compare them with the detailed.

Diode Bias Connections

Diode Forward-Bias

• In the given figure, the circuit denoted as (a) shown the forward biasing of a diode.

• In this circuit, you can see that anode of the diode is connected with the positive terminal of the battery and negative terminal of a battery with the cathode of the diode.
• The resistance between anode and battery is used to limit the current to save value.
• (IF) is the forward current flowing through cathode to anode and VF is the forward voltage drop across the diode is equal to the potential barrier of a diode.

Diode Reverse-Bias Connection

• In the above figure, the circuit denoted as (b) shows the reverse-biased connection of diode.
• In this circuit positive terminal of a battery is connected with the cathode of diode and negative terminal is connected with the anode.
• The reverse-biased current is very less and can be neglected and there is no need of limiting resistance in reverse biasing.

• In-circuit you can see that all Vbise or applied voltage is shown across the diode.

Ideal Diode Model

• In the given figure, the two circuits are shown first discuss the forward biasing ideal diode model and second in reverse biasing ideal diode model.

• You can see in forward biased condition diode is behave like a closed switch and reveres biased like an open switch.
• In ideal condition the potential barrier, dynamic resistance in forward biasing and reverse current is ignored.
• This model is inadequate for troubleshooting when you are trying to define if the diode is functioning correctly.
• in given figure V-I characteristics of an ideal diode is shown.

• As the dynamic resistance and potential barrier are ignored so the voltage across the diode is zero in forward biasing.
• In forward bias, the curve of an ideal diode is shown on the vertical Y-axis in blue color.

V_F =0

• The value of forward current (I_F) across the diode can be found by using the value of limiting resistance and biased voltage using Ohm’s Law.

I_F =V_BIAS /R_LIMIT

• As reverses current is ignored in an ideal diode and its value supposed to be ‘0’. It is shown in the V-I curve on negative axis.

I_R=0A

• The value of reverse voltage is equal to the biased voltage.

V_R =V_BIAS

Practical Diode Model

• In the given figure there are two circuits shown for practical diode first is denoted as (a) it is forward bias and second denoted as (b) is reverse bias.

 

• in a practical model of diode potential barrier is included. In a forward-biased circuit, you can see that diode is like a closed switch having a series voltage source (VF) equal to the (0.7) volts its positive side attached with the anode.
• This source is a potential barrier voltage, not a practical voltage source. The value of a biased voltage should be larger to operate the diode.
• While in reverse bias the circuit of a diode is like an open switch and there is no effect in reverse biasing.
• in a given figure, the V-I curve for a practical model of a diode is shown.

• At a positive X-axis of diode forward biased condition of the diode is shown in this biasing potential barrier is included so it shown in the graph.

V_F = 0.7 V

• The forward current is determined as follows by first applying Kirchhoff’s voltage law to Figure 2–16(a):
• If we apply KVL on the forward-biased circuit denoted as (a) then the value of forwarding current is given as.

V_BIAS – V_F – VR_LIMIT = 0

VR_LIMIT = I_FR_LIMIT

• Now we have.

I_F= (V_BIAS-V_F)/ R_LIMIT

• As the reverse current is zero as shown on the negative X-axis of the curve.

I_R = 0 A

V_R = V_BIAS

• For troubleshooting of a less voltage circuit’s practical model of a diode is the best option.
• In this situation, 0.7 volts around diode can be beneficial and should be taken into consideration.
• Friends if you have to design a basic circuit of diode than you should prefer a practical diode model.

 Complete Diode Model

• Now we discuss the complete model of diode that has potential barriers and dynamic forward resistance.
• With these 2 parameters, this complete model also has high value interior reverse resistance.
• In this model, the reason of reverse resistance inclusion is that this resistance provides a path for the reverse current movement.

• In the given figure, two circuits are shown for a complete model of a diode. The circuit denoted as (a) shown the complete model for forward bias and circuit denoted as (b) describe the reverse-biased complete model.

• In forward bias, condition diode is like a closed switch having a potential barrier (VB) and less value forward dynamic resistance.
• While in reverse biased condition diode is operating like open circuit switch and has reverse interior resistance in parallel.
• In reverse biasing potential barrier is not included as it does not have any effect in this biasing.
• The V-I curve for a complete model of a diode is shown in a given figure.

• During forward biasing as a potential barrier and dynamic resistance is included so there is (VF) forward voltage across diode exits.
• The voltage (VF) will be equal to the sum of potential barrier voltage and the voltage drop across dynamic resistance. This voltage is shown on the positive X-axis in the curve.
• The graph in forward bias is like a slop due to increment in a voltage drop across the dynamic resistance with the increase in current.
• The given below mathematical expression is for a complete model of a silicon diode.

V_F = 0.7 V + I_Fr’_d

I_F = (V_BIAS – 0.7 V)/(R_LIMIT + r’_d)

• During reverse biasing reverse current IR taken into consideration and shown on the negative X-axis of the curve.
• On this curve breakdown region of the diode is not shown as it does not save the operation region for some diodes.
• Due to the complex calculations completer model is not suitable for troubleshooting of diode.
• The complete model can be used when the computer is used to solve complicated calculations.

So friends that is the detailed post on the diode models if you have any queries about this post ask in comments. See you in the next interesting tutorial. Thanks for reading. Have a good day.