| In this discussion the term diode and rectifier
will be used interchangeably; however, the term diode usually implies a small signal
device with current typically in the milliamp range; and a rectifier, a power device,
conducting from1 to 1000 amps or even higher. Many diodes or rectifiers are identified as
1NXXXX . A semiconductor diode consists of a PN junction and has two(2) terminals, an
anode(+) and a cathode(-). Current flows from anode to cathode within the diode. A
diode and schematic representation are shown below.
An ideal diode is like a light switch in your home. When the
switch is closed, the circuit is completed; and the light turns on.When the switch is
open, there is no current and the light is off.
This can be shown as:
However, the diode has an additional property; it is
unidirectional, i.e.current flows in only one direction(anode to cathode internally).When
a forward voltage is applied, the diode conducts; and when a reverse voltage is applied,
there is no conduction. A mechanical analogy is a ratchat, which allows motion in one
direction only. An ideal diode characteristic would be:
However, a typical diode characteristic is more like
Forward Voltage Drop , Vf
Notice that the diode conducts a small current in the forward
direction up to a threshold voltage, 0.3 for germanium and 0.7 for silicon ; after that it
conducts as we might expect. The forward voltage drop, Vf, is specified at a forward
In the reverse direction there is a small leakage current up
until the reverse breakdown voltage is reached. This leakage is undesireable, obviously
the lower the better, and is specified at a voltage less the than breakdown; diodes are
intended to operate below their breakdown voltage.
The current rating of a diode is determined primarily by the
size of the diode chip, and both the material and configuration of the package, Average
Current is used, not RMS current. A larger chip and package of high thermal conductivity
are both conducive to a higher current rating.
The switching speed of a diode depends upon its construction
and fabrication. In general the smaller the chip the faster it switches, other things
being equal. The chip geometry, doping levels, and the temperature at nativity determine
switching speeds . The reverse recovery time, trr, is usually the limiting parameter; trr
is the time it takes a diode to switch from on to off.
The very minimal diode specifications are:
(a)Maximum reverse voltage
(b)Rated forward current
(c)Maximum forward voltage drop
(d)Maximum leakage current
(f)Maximum reverse recovery time