Essay Example on Introduction To Diodes









Diodes are the simplest form of semiconductors They consist of two layers a single junction and two terminals The junction is composed from the p type and n type semiconductor and there are also two leads Anode which joins the p type and Cathode which joins the n type The structure of a common diode and its symbol is shown in the Fig below Power diodes are like signal diodes but have some differences in its construction Elbuluk 2010 While the doping level of both P and N sides is same in signal diodes and results in a PN junction power diodes junction is formed between a heavily doped P and a lightly doped N layer which is epitaxial grown on a heavily doped N layer resulting in a structure that looks as shown in the figure below Muller 2003 1 1 2V I Characteristics of Power Diodes Looking at the figure below we can see that the V I characteristics of a power diode is very similar to that of a signal diode For forward biased region in signal diodes the current increases exponentially whereas high forward current in power diodes leads to high ohmic drop which dominates the exponential growth and the curve increases nearly linearly The ultimate reverse voltage that the diode can hold is symbolized by VRRM i e peak reverse repetitive voltage 

Above this voltage the reverse current will suddenly be very high and since the diode is not designed to dissipate such high amount of heat it may get broken This voltage may also be called as peak inverse voltage PIV Elbuluk 2010 1 1 3Reverse Recovery Characteristics of Power Diode The figure above shows the reverse recovery characteristic of a power diode The current decays from IF to zero whenever the diode is switched off and further continues in reverse way due to the charges stored in the space charge region and the semiconductor region This reverse current gets to a peak IRR and again start approaching zero value and eventually the diode turns off after time trr This duration is known as reverse recovery time and is defined as time between the instant forward current reaches zero and the instant the reverse current decays to 25 of IRR The diode is said to obtain its reverse blocking capability after this time Grove 2005 1 1 4Categories of Diodes 1 2 Power Transistors 1 2 1Introduction to Transistors Power Bipolar Junction Transistor BJT is the first semiconductor device to give full control over its Turn on and Turn off functionality The BJT was the first semiconductor man to closely approximate an exemplary completely controlled Power switch

A Power transistor is conveniently used for large current switching at low to moderate a few kHZ frequency Elbuluk 2010 Basic construction of bipolar junction transistors is shown below 1 2 2 Switching characteristics of a Power Transistor Power transistors are usually utilized as switches in a power electronic circuit i e they work in either cut off switch OFF or saturation switch ON regions Nonetheless the operating properties of a power transistor largely varies from an ideal controlled switch in the following respects It can conduct only limited amount of current in a single direction when ON It can prevent only a finite voltage in one direction It has a voltage drop during ON case It drives a small leakage current during OFF case Switching process is not immediate It necessitates non zero control power for switching Switching of Power transistors from ON saturation to OFF cut off state requires plentiful redistribution of minority carriers Hence the operation of switching is not instantaneous Switching features of a power transistor is significantly influenced by the outer load circuit and the base drive circuit Elbuluk 2010 1 2 3 Energy Loss in Power Transistors 

There are some wastages in power Transistors upon switching from cut off region to saturation region Energy loss happens over each switching operation of a power transistor because of the simultaneous presence of collector current and voltage This condition is called switching loss There is also the conduction loss which is energy loss that takes place during ON state of the transistor Conduction loss during the OFF condition of a Power transistor is trivial and cab be neglected Switching power loss is in proportion with the switching frequency whereas conduction power loss is proportional to the duly cycle BJT as a minority carrier device has higher switching loss and lower conduction loss compared to other majority carrier transistors due to the fact that it has lower ON state voltage drop and longer switching delay times 1 3 Power Thyristors 1 3 1 Introduction to Thyristors Thyristor consists of four layer three junction p n p n semiconductor switching device It contains 3 terminals that are called anode cathode and gate SCRs being solid state devices are tightly pact have low loss and possess high reliability SCR is fabricated from silicon it works as a rectifier It has very weak resistance in the forward direction and large value resistance in the reverse direction Thus SCRs are unidirectional devices Shur 1995

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