Page 31 - Rashid, Power Electronics Handbook

2

The Power Diode

Dr. Ali I. Maswood, 2.1 Diode as a Switch................................................................................. 15
Associate Professor 2.2 Some Properties of PN Junction............................................................. 15
School of EEE, Nanyang 2.3 Common Diode Types .......................................................................... 17
Technological University,
Nanyang Avenue, Singapore- 2.4 Typical Diode Ratings ........................................................................... 17
639798 2.4.1 Voltage Ratings 2.4.2 Current Ratings
2.5 Snubber Circuits for Diode.................................................................... 19
2.6 Standard Datasheet for Diode Selection ................................................... 19
Tutorial 2.1. Reverse recovery and overvoltages Tutorial 2.2. Ideal diode operation,
mathematical analysis and PSPICE simulation
2.7 Typical Applications of Diodes ............................................................... 23
2.8 Standard Datasheet for Diode Selection ................................................... 23
2.8.1 General-Use Recti®er Diodes
References ........................................................................................... 25

2.1 Diode as a Switch sented as shown in Fig. 2.2b. In Fig. 2.2b the forward
characteristic is expressed as a threshold voltage V with a
O
Among all the static switching devices used in power elect- linear incremental or slope resistance r. The reverse character-
ronics (PE), the power diode is perhaps the simplest. Its circuit istic remains the same over the range of possible leakage
symbol, shown in Fig. 2.1, is a two terminal device, and with currents irrespective of voltage within the normal working
terminal A known as the anode and terminal K known as the range.
cathode. If terminal A experiences a higher potential
compared to terminal K, the device is said to be forward 2.2 Some Properties of PN Junction
biased and a forward current (I ) will ¯ow through the device
F
in the direction as shown. This causes a small voltage drop
From the forward and reverse-biased condition characteristics,
across the device (<1 V), which under ideal conditions is
one notices that when the diode is forward biased, current
usually ignored. By contrast, when a diode is reverse biased, it
rises rapidly as the voltage is increased. Current in the reverse-
does not conduct and the diode then experiences a small
biased region is signi®cantly small until the breakdown voltage
current ¯owing in the reverse direction called the leakage
of the diode is reached. Once the applied voltage is over this
current. Both forward voltage drop and leakage current are
limit, the current will increase rapidly to a very high value
ignored in an ideal diode. In PE applications a diode is usually
limited only by an external resistance.
considered to be an ideal static switch.
DC Diode parameters. The most important are the follow-
The characteristics of a practical diode depart from the
ing:
ideals of zero forward and in®nite reverse impedance, as
shown in Fig. 2.2a. In the forward direction, a potential Forward voltage V is the voltage drop of a diode across
F
barrier associated with the distribution of charges in the A and K at a de®ned current level when it is forward
vicinity of the junction, together with other effects, leads to biased.
a voltage drop. In the case of silicon this is in the range of 1 V Breakdown voltage V is the voltage drop across the
B
for currents in the normal range. In the reverse direction, diode at a de®ned current level when it is beyond
within the normal voltage operating range, a very small reverse-biased level. This is known as avalanche.
current ¯ows that is largely independent of the voltage. For Reverse current I is the current at a particular voltage,
R
practical purposes the static characteristics are often repre- and which is below the breakdown voltage.
15
Copyright # 2001 by Academic Press.
All rights of reproduction in any form reserved.