Power Diode

Power Electronics

Power Electronics can be defined as that technology which is associated with efficient conversion and control of electric power by the use of power semiconductor devices. It combines power, electronics, and control. So,then what is the goal of power electronics ? In simple word, it is used to control the flow of energy from the electric source to the electric load.

Application of the power electronics

1. They are highly used for heat control, light controls, motor controls, power supplies,vehicle propulsion system.
2. It is highly used in HVDC.

Power Diode

The power semiconductor diode, known simply as thePower Diode [1], has a much larger PN junction area compared to its smaller signal diode cousin, resulting in a high forward current capability of up to several hundred amps (KA) and a reverse blocking voltage of up to several thousand volts (KV). Power diodes are designed to have a forward “ON” resistance of fractions of an Ohm while their reverse blocking resistance is in the mega-Ohms range.

Depletion layer

The term “depletion” refers to the fact that the region near the junction is depleted of their respective majority charge carriers.When physical contact is made between p and n junction, free electrons from n-type region diffuse across the junction, into the p-type region. Diffusion of each electron from n region to p region leaves a positive charge behind in the n-region near the junction. Similarly, the diffusion of holes from p-type region to n-type region leaves negative charges behind in the p-region near the junction. These two layers of positive and negative charges form the depletion region.

V-I Characteristics And Switching

A power diode is a two terminal pn-junction device. When the anode potential is positive with respect to the cathode, the diode is said to be forward biased and the diode conducts. When the cathode potential is positive with respect to the anode, the diode is said to be reversed biased.

The V-I characteristics show in figure below can be expressed by an equation known as Schockley diode equation, and it is given under dc steady state operation by:

$$Id = Is({e^{Vd/nVt}} - 1)$$

where Id = Diode Current, Is = saturation current, Vd = Diode Voltage in Forward-Biased, n = Emission Coefficient, Vt = KT/q where k = Boltzmann Constant, T = Absolute temperature in kelvin, Q= charges in columbos

Forward Biased Region

In the forward-biased region, Vd > 0. The diode current Id is very small if the diode voltage is less than the terminal voltage. When diode voltage is greater than terminal voltage, the diode conducts fully. Thus, that voltage is also referred as the threshold voltage,cut-in voltage or turn-on voltage.

Reversed Biased Region

In the reversed biased region, due to the higher resistance of the depletion layer as the result of applying negative diode voltage, a negligible diode current flows through which is nearly equal to Saturation current.

BreakDown Region

As we go on increasing the negative diode voltage across the diode, at a specified voltage known as the breakdown voltage with the small change in reverse voltage the reverse current increases rapidly. And as the result, the diode might get damaged.

Reverse Recovery Time

The current in the forward-biased junction diode is due to the net effect of the majority and minority carriers. Once the diode is in a forward conduction mode and then it’s forward current is reduced to zero, the diode continues to conduct due to minority carriers that remain stored in the p-n junction and the bulk semiconductor material. The minority carriers require a certain time to recombine with opposite charges and to be neutralized. This time is called as the reverse recovery time of the diode. It is also defined as the interval between zero-crossing of current and 25 % of peak reverse current while coming back to zero.

Here, T_rr = t_a + t_b where t_a = time due to storage of minority charge in the p-n junction, t_b = time due to minority charge in the bulk semi-conductor material.If t_b > t_a, then such characteristics is known as soft recovery and if t_b /t_a < 1 as shown in fig, then such characteristics are known as abrupt recovery characteristics

Derivation

The derivation of the reverse current is provided below:

$$\eqalign{
& \quad Qrr \cong {1 \over 2}Irr*Ta + {1 \over 2}Irr*Tb \cr
& \quad \quad {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} {\kern 1pt} \cong {1 \over 2}Irr*Trr \cr
& or,{\kern 1pt} {\kern 1pt} Irr{\kern 1pt} \; \cong {\kern 1pt} \,\,2*{{Qrr} \over {trr}} - - - - - - - - - - - - \; \to \;(1) \cr
& Also, \cr
& \;\;\,\,\,Irr\,\, \cong {\kern 1pt} Ta*{{di} \over {dt}}\;\; - - - - - - - - - - - - \; \to \;(2)\;\;\;\;\; \cr
& where\,\,\,\,{{di} \over {dt}}i{\mathop{\rm s}\nolimits} {\kern 1pt} \,in\,reverse\,\,direction. \cr
& From\;equation\;(1)\;and\;equation\left( 2 \right),we\;get \cr
& {{2Qrr} \over {trr}}\;\; = \;\;Ta*{{di} \over {dt}}\; \cr
& if\;Ta\; = \;trr,\;then \cr
& trr = \sqrt {{\textstyle{{2*Qrr} \over {{\textstyle{{di} \over {dt}}}}}}} \;\;\,\, \cr
& Now,from\;equation\;(1),\;we\;get \cr
& Irr = \;{{2*Qrr} \over {\sqrt {{\textstyle{{2*Qrr} \over {{\textstyle{{di} \over {dt}}}}}}} }} \cr
& Therefore,\;Irr = \;\sqrt {2*Qrr*{\textstyle{{di} \over {dt}}}} \cr} $$

Types of Diode

Depending on the recovery characteristics and manufacturing techniques, the power diodes can be classified into the following three categories:

• Standard or general purpose diodes
• Fast-recovery diodes
• Schottky diodes

Standard or General Purpose Diode : The general[2] purpose diodes have relatively high reverse recovery time about 25 us, and are used in low-speed applications. Their rating cover a wide range of current[ 1 A to 2000 A] and voltage[50 v to 5 kv]. These type of diodes are usually used in battery charging, electroplating, wielding,etc.

Fast Recovery Diode: These are used in high-frequency circuits in combination with controllable switches where a small reverse recovery time is needed. At power levels of several hundred volts and several hundred amperes, these diodes have trr ratings of less than a few microsecond.

Schottky Diode: These diodes are used where a low forward voltage drop (usually 0.3V) is needed in low output voltage circuits. These diodes are limited in their blocking voltage capabilities to 50 – 100V.

Power Diode Applications:

As a rectifier Diode
For Voltage Clamping
As a Voltage Multiplier
As a freewheeling Diode

References

[1] Electronics Tutorial. Power Diodes And Rectifier. n.d. <http://www.electronics-tutorials.ws/diode/diode_5.html>.

[2] H.Rashid, Muhammad. POWER DIODE TYPES. India: PEARSON PUBLICATION, 2013.