Power Semiconductor Devices:
- Diode or PN Junction is the Basic Building Block of all Power Semiconductor Devices.
- By Making little Modifications to the Low power devices we can make those devices to handle Larger Power.
Power Diode:
- Two Terminal Device and Uni directional Device
- Uncontrollable
Structure of Power Diode:
Fig.1.Crossectional view of Power Diode
Monolithically, the base is heavily doped n+ Substrate on the top of which Lightly Doped n- substrate has to be grown. To form the PN Junction we are diffusing the P+ Region into that, which serves as Anode.Obvious, the n+ substrate serves as Cathode.
But,Cross sectional area of Power Diode depends on the total amount of current to be carried in that device.
What is the Difference between Low Power Diode and High Power Diode?
- The n- layer (epitaxial) only appears at High Power Diodes.
- Function of that Layer is to absorb the Depletion Layer formed during the Reverse bias of PN Junction.
- The n-layer decides the Reverse Breakdown Voltage of the Device.
Symbol of Power Diode:
Fig.2.Symbol of Power Diode
Operation of Power Diode:
- When Anode is Positive with respect to the Cathode, then the Diode is said to be Forward Biased. It has small forward voltage drop.The conduction of current happens obviously due to PN Junction action.Hence,this is the Turn on mode of the Diode Switch.
- When Cathode is Positive with respect to Anode, then the Diode is said to be Reverse biased. There is no conduction, if it is an Ideal Switch.But, practically speaking the diode will conduct at Small Reverse leakage current due to Minority carrier action present in the diode. It increases slowly and at a certain point the Avalanche Breakdown will occur. This leads to large Reverse current.
Steady State V-I Characteristics of the Diode:
Fig 3.V-I Characteristics of Diode
- For Most of the Purposes we consider Diode as an Ideal Switch. The characteristics have been shown in fig.3. (b).
- V-I Characteristics of Practical Diode also can be interpreted from the Shockley Diode Equation, under steady state operation,
ID=IS(eVd/nVt -1),
Where,
ID=Current through the Diode, in Amperes (A),
IS=Leakage current through the Diode, (Usually fro 10-6 to 10-15 A),
VD=Voltage across Diode when Anode is Positive with respect to Cathode,
n=Emission coefficient (For Ge (n=1), Si (n=1.1 to 1.8),
VT=Thermal Voltage=kT/q,
Where,
K-Boltzmann Constant=1.3806×10-23J/K,
T-Absolute Temperature in Kelvin (273+0C),
Q-Charge of an electron-1.6022×10-19C.
Diode operation Consisting of Three Regions:
Reverse Recovery characteristics of Power Diode:
Fig 4.Recovery time of Diode
Reverse Recovery Time:
Even when Reverse voltage is applied to the diode, it continues to conduct due to the minority carries that stored in the PN Junction and Bulk semiconductor material . These minority carriers require some time to Recombine and to Neutralize. This time is called Reverse Recovery Time.
The Following Image File will Clearly Derive and explain you the:
1.Types of Recovery
2.Reverse Recovery Time Derivation
3.Reverse Recovery Charge Derivation
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| Types of Recovery and Derivation for Recovery Time and Charge |
By depending on the Switching Recovery Time and On state Drop, the Power Diode are,
General Purpose Diode:
- High Reverse recovery time [25µs]
- Used in low speed application[Converter,Rectifier]
- It covers 1Ato 1000A,50v to5kv
- Manufactured by Diffusion Process.
Fast Recovery Diode:
- Low Recovery Time [0.5µs]
- Used in DC-DC,DC-AC converters
- Epitaxial Diodes have a Narrow Base width, resulting in fast recovery time of as low as 50ns.
Schottky Diodes:
- Charge storage at PN junction can be eliminated in this type of diode.
- Accomplished by setting a ‘barrier potential’ with a contact between a metal and semiconductor.Here, there is no recovery time at all, If exists it will be 10ps.
Advancements in Power Electronics:
Silicon Carbide(Sic) Diodes:
- No Reverse Recovery Time.
- No Temperature Influence on Switching.
Applications of Power Diodes:
- Almost All Power converters.
- Un-controlled Rectifiers.
- Feedback and Freewheeling operations in Choppers, Inverters.
Note:
Requesting the Learners to Leave the Queries in the Comment Section.Because, you can expect more information from me in my reply also. Sharing is the real knowledge.So, let we Share it. Power Converter Related questions are Expected.
Reg,
S.Karthik Kumaran,
powerelectronicsresearch@gmail.com
