Diode is a semiconductor device with P-N junction. So, when positive voltage is applied to P side of diode, it is said to be forward biased. When this voltage exceeds 0.7V, the diode is set to breakdown and the diode starts conducting and allows current to flow. When forward voltage is below 0.7V, diode doesn't conduct. There are diodes with forward breakdown up to 1.1V. The breakdown depends on the type of semiconductor material used for diode construction. There are When the same voltage is applied to N side of diode, the diode is to be reverse biased and never conducts. but when this reverse voltage is above some threshold, it starts conducting.
The voltage characteristics of a normal diode and it's symbol are shown below:
The diode symbol is as shown in the figure and you can see a strip around the cylindrical diode which actually represents the cathode. Diodes are generally available in cylindrical or glass packages. Some of the applications of diodes include power conversion, over voltage protection, reverse current protection and in other logic circuits. The general application where the normal diode is used can be seen from the below figure.
The forward drop of D1 is 1.1V in the above figure. So, the voltage seen at the resistor R1 is (12 - 1.1) = 10.9 V. If you check the data sheet of D1, the reverse voltage specification is 50 V. If the current has to flow from R1 through D1, the voltage at the load has to exceed by 50 V. This is the normal functionality of diode and it's application. Anyways in the above simple circuit we are not much bothered about the reverse voltage.
Various diode symbols can be seen in the below figure:
What happens if we replace D1 in the above with Schottky diode?
Schottky diode best suits this application. Schottky diode has low voltage power drop and not more than 0.5 V. Schottky diodes have a voltage drop in the range 0.2 - 0.45 V. So, the voltage at the first node of R1 will be measured as 12 - 0.2 = 11.8 V. This is with reference to forward drop. The main advantage we are getting here is less voltage drop and hence less power consumption.
So, in applications where reverse current need to be gated and forward drop must be very less schottky diode is preferred. Let us consider a case, where from the other end of D1 we have a voltage greater than breakdown voltage. In this case, the diode is reverse biased and current flows in other direction. In this case, let us assume the reverse voltage goes below supply voltage. In this case, the diode has to switch back to forward biased condition. This happens pretty quickly in schottky diode. A normal diode may take time to switch back to forward biased mode. So, important point here is Schottky diode has rapid turn on and turn off times. One has to consider the fact that the reverse voltage drop of Schottky diode is less than normal diode. But in this application, reverse voltage doesn't matter much.
Can we use a zener diode in the above application?
Definitely, a zener diode can be used if we consider a forward biased condition. The forward drop is same as normal diode. But it has voltage drop greater then Schottky diode. So, Schottky is always preferred in this case compared to zener.
What is the case of reverse breakdown when we consider Zener, Schottky and normal diode?
Zener - Reverse voltage varies from 1.8 V to 200 V as per the diode selected
Normal diode: Available with forward drops in the range 50-1000V
Schottky: In the range 20-45 V
So, considering above cases, when is a zener diode used?
Consider, the previous circuit and let us assume that we want a specific voltage of 3.6V at the load even though the supply is +12V. We can connect a zener diode at the load end to clamp the voltage or in other words to regulate the voltage to desired level. the same circuit with zener diode as per requirement gets modified as follows:
So, when there is a need to regulate the voltage a zener diode is used. Here, even a schottky diode can be used but the problem is in schottky diodes the breakdown voltage is less than the reverse voltage at which current flows. Where as in a Zener diode, the breakdown voltage is little greater than actual reverse voltage.
The voltage characteristics of a normal diode and it's symbol are shown below:
The diode symbol is as shown in the figure and you can see a strip around the cylindrical diode which actually represents the cathode. Diodes are generally available in cylindrical or glass packages. Some of the applications of diodes include power conversion, over voltage protection, reverse current protection and in other logic circuits. The general application where the normal diode is used can be seen from the below figure.
The forward drop of D1 is 1.1V in the above figure. So, the voltage seen at the resistor R1 is (12 - 1.1) = 10.9 V. If you check the data sheet of D1, the reverse voltage specification is 50 V. If the current has to flow from R1 through D1, the voltage at the load has to exceed by 50 V. This is the normal functionality of diode and it's application. Anyways in the above simple circuit we are not much bothered about the reverse voltage.
Various diode symbols can be seen in the below figure:
What happens if we replace D1 in the above with Schottky diode?
Schottky diode best suits this application. Schottky diode has low voltage power drop and not more than 0.5 V. Schottky diodes have a voltage drop in the range 0.2 - 0.45 V. So, the voltage at the first node of R1 will be measured as 12 - 0.2 = 11.8 V. This is with reference to forward drop. The main advantage we are getting here is less voltage drop and hence less power consumption.
So, in applications where reverse current need to be gated and forward drop must be very less schottky diode is preferred. Let us consider a case, where from the other end of D1 we have a voltage greater than breakdown voltage. In this case, the diode is reverse biased and current flows in other direction. In this case, let us assume the reverse voltage goes below supply voltage. In this case, the diode has to switch back to forward biased condition. This happens pretty quickly in schottky diode. A normal diode may take time to switch back to forward biased mode. So, important point here is Schottky diode has rapid turn on and turn off times. One has to consider the fact that the reverse voltage drop of Schottky diode is less than normal diode. But in this application, reverse voltage doesn't matter much.
Can we use a zener diode in the above application?
Definitely, a zener diode can be used if we consider a forward biased condition. The forward drop is same as normal diode. But it has voltage drop greater then Schottky diode. So, Schottky is always preferred in this case compared to zener.
What is the case of reverse breakdown when we consider Zener, Schottky and normal diode?
Zener - Reverse voltage varies from 1.8 V to 200 V as per the diode selected
Normal diode: Available with forward drops in the range 50-1000V
Schottky: In the range 20-45 V
So, considering above cases, when is a zener diode used?
Consider, the previous circuit and let us assume that we want a specific voltage of 3.6V at the load even though the supply is +12V. We can connect a zener diode at the load end to clamp the voltage or in other words to regulate the voltage to desired level. the same circuit with zener diode as per requirement gets modified as follows:
So, when there is a need to regulate the voltage a zener diode is used. Here, even a schottky diode can be used but the problem is in schottky diodes the breakdown voltage is less than the reverse voltage at which current flows. Where as in a Zener diode, the breakdown voltage is little greater than actual reverse voltage.
No comments:
Post a Comment