Thursday, February 7, 2013

Power Diode





 
Figure 1. Diode



The power diode (or mosly known as diode) is the simplest but most important of the power semiconductor devices. Diode (see Figure 1) is a two terminal semikonductor devices (Anode-A and Catode-K), comprising a P-N junction. Compare it to ideal switch.

Diode Applications
In power applications diodes are used to rectify, that is, to convert alternating current (ac) to direct current (dc). It is used also to allow current freewheeling (Figure 2). That is, if the supply to an inductive load is interrupted, a diode across the load provides a path for the inductive current and keep the sensitive components of the circuit in safe by a presence of high voltages spikes (L.di/dt).




Figure 2. Freewheeling[1]


How can diode do its job? To do its job, diode role as a switch. Let’s we take a look Figure 3 which clearly shown the V-I characteristics of a diode.










Figure 3. V-I Characteristics of Typical Diode





Forward Biases (ON-State):


In the forward direction (VA > VK), diode conducts with a small voltage drop (typically 1.0 V). This condition makes diode becoe conduct (say ON). See Figure 4, below.







Figure 4. Diode Biassed



Reverse Biases (OFF-State):


In the reverse direction (VA < VK), it blocks voltage with a small leakage current. This condition makes diode turn-off (See Figure 4)





Do not do this for real practice!


With a high enough reverse voltage are being applied to diode, it goes through avalanche breakdown (VBD) and is destroyed (Booom!). We can see Figure 4.





 
Blown up


Figure 4. Diode blown up!







The most important DC Diode parameters are the following :


  • Forward diode voltage VF is mostly known as the voltage drop of a diode across A and K at a defined current level when it is forward biased.
  • Diode Breakdown voltage VBD is the voltage drop across the diode at a defined current level when it is beyond reverse-biased level (also known as avalanche).
  • Reverse current IR is the current at a particular voltage, and which is below the breakdown voltage.[See more... read Rashid (2001)]
Dynamic characteristics (usefull to find the AC parameters):


A practical switching characteristics (turn-off) of the diode are shown in Figure 4. As reverse voltage is applied in the forward biased, the current decreases linearly (due to leakage inductance), reverses, and then turns off with a snap. Then, a high rate of change of current (di/dt) at turn-off causes a reverse voltage (Vrrm) which higher than the applied voltage (VAK). Others parameters, say recovery charge Qrr and the corresponding recovery time trr are also important diode parameters. [See more...Bose (2006).





Power Diode Classification:


There are 3 type of power diodes: slow-recovery diodes, fast-recovery diodes, and Schottky diodes. Slow-recovery diodes are used in rectifier. Fast-recovery types are used in feedback or freewheeling and snubbers. Schottky diodes, the fastest one, are used in many topology of high-frequency converters.

How to choose Diode? See my next article.




References:


[1] Aripriharta, 2005-2012. Lecture Note. Power Electronics & Drives. Unpublished-for Education Only. State University of Malang, Faculty of Engineering, Departement of Electrical Engineering. (The original note, written in Bahasa Indonesia)


[2] Bose, Bimal K. (2006). Power Electronics and Motor Drives Advances and Trends. Elseiver. Mostly cited from this books!


[3] Rashid (2001). “Power Electronics Handbook”. ACADEMIC PRESS


[4] Kularatna, N. 1998. “Power Electronics Design Handbook Low-Power Components and Applications”.