During the switching-off of a forward conducting diode, the diode won’t stop conducting immediately after its forward current decays to zero, but continues to conduct in reverse direction for a certain period of time called reverse recovery time trr. This is due to the presence of minority charges stored in the diode. This stored charge is present both at the depletion region and in the semiconductor layers. Actually an ideal diode should always prevent the flow of current in reverse direction, but due to the presence of these charges the diode continues to conduct in reverse direction until these charges are removed. This current is called as reverse recovery current as shown in the below figure.
Showing posts with label power electronics. Show all posts
Showing posts with label power electronics. Show all posts
V - I characteristics of power diode
A power diode is a two terminal power semiconductor device which conducts and allows the flow of current under forward biased condition and blocks the current under reverse biased condition.
Forward biased condition of power diode:
A diode is said to be forward biased when its anode is more positive with respect to cathode. The below circuit shows a forward biased diode connected to a battery of voltage V and the below figure shows the V - I characteristics of a power diode. Initially when the applied voltage V is zero (V=0) the diode doesn't conduct, and hence
Structure of A Power Diode
Power Diode
A power diode is a high power capability power semiconductor device used in power electronic circuits. A power diode is also a PN junction device and its functionality is similar to that of a normal low-power PN junction diode. But the construction of power diode is different from its counterpart and it is designed to carry high power.
Class A Commutation - Load Commutation
Class A commutation is also called as load commutation or resonant commutation or self-commutation. This type of commutation is prevalent in thyristor circuits supplied from DC source. This load commutation is possible only in DC circuits and not in AC circuits.
Description of Class A Commutation
For load commutation of a thyristor, the commutating components i.e. Inductance L and capacitance C are connected
Holding Current of a Thyristor
Holding current
Holding current is defined as the minimum value of anode current of a thyristor below which it must fall for turning off of a thyristor.
The value of holding current will always be lesser than that of latching current. A conducting SCR can’t be controlled with a gate signal and to turn off a thyristor the forward anode current should made less than that of the holding current.
Latching Current of a Thyristor
Latching Current
Latching current is defined as the minimum value of the anode current required during turn on process of a thyristor to remain in conduction even after the gate signal is removed.
After the thyristor is turned on and is conducting in forward conduction mode the reverse biased
Thyristor Commutation Techniques
Commutation
Commutation is defined as the process of turning off a thyristor. A thyristor can be turned on by applying a gate pulse but the gate has no control over turning off a thyristor. The turning-off of a thyristor requires the below two conditions;- The cathode should be at higher potential than
Programmable Unijuction transistor – PUT
A programmable unijuction transistor is a pnpn device like an SCR. The difference between the PUT and SCR is that the gate is connected to N type material near to the anode instead of P type material near to cathode as in case of an SCR. The below figure shows the diagram of a PUT along with its symbol.
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| Programmable Unijuction Transistor |
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| symbol of PUT |
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| V - I characteristics of PUT |
PUTs are used in SCR triggering circuits, time delay and logic circuits. These are available with voltage and current ratings of upto 200V and 1A respectively.
Types of Power Diodes
The power diodes are classified into three types based on their reverse recovery characteristics. They are:
- General purpose diodes
- Fast recovery diodes
- Schottky diodes
General purpose diodes
The general purpose diodes have high reverse recovery time typical in the order of 25μs. Hence these diodes cannot be used for fast switching applications. They are available with voltage ratings ranging from 50V to 5000V with current ratings ranging from 1 A to several thousand amperes. These diodes are used in battery charging, traction drives and uninterruptible power supplies (UPS) etc.
Fast-recovery diodes
These diodes will have less reverse recovery time of about 5μs or less. These diodes are available with their voltage and current ratings ranging from 50V to 3kV and 1 A to several thousand amperes respectively. These diodes are used in choppers, SMPS – switched mode power supplies, and commutation circuits etc.
Schottky Diodes
These types of diodes have no reverse recovery time and can be turned off much faster than a normal p-n junction diode. These diodes use metal to semiconductor junction instead of p-n junction. The forward current in schottky diode is only due to electrons and there are no holes. Hence as there are no holes there is no storage charge and no reverse recovery time.
The schottky diodes are used in high frequency instrumentation and switching power supplies and are available with current rating of about 1 to 300 A.
Power Electronics
Power Electronics is a technology associated with flexible and efficient way of conversion and control of electric power. It is the application of electronic principles at the power level instead of at the signal level as used in electronic circuits. There are various power semiconductor
DC Chopper
Chopper is a power electronic circuit that converts fixed direct current input voltage to variable DC output voltage. Chopper can increase (with a step up chopper) or decrease (with step down chopper) the level of the applied voltage. Similar to that of a transformer
Drive | Electric Drive
Electric Drive
An electric drive is an electric motor either DC or AC along with its control equipment and energy transmitting device. For example an electric pump, the combination of electric motor and pumping system is called an electric drive though there is no fluid is being pumped. |
| electric drive |
Turn-ON methods of a Thyristor / SCR
The thyristor or a silicon controlled converter (SCR) is the most widely used power semiconductor device in power electronics. It can be turned ON by the following methods with maintaining positive voltage across the thyristor from
Types of power electronic converters
Rectifiers
A rectifier conv
Advantages of Power Electronics
Power electronics is the application of electronics principles at the higher power level instead of at the signal level as in electronic circuits. Power electronic systems offer wide range of advantages over the conventional systems. The following the lists the various advantages of power electronics.
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