PROJECT REPORT SHEET
PROJECT DESCRIPTION: Diodes
NAME: Sung-Chul Yu
STUDENT ID: C61272
Diodes
The pn junction diode is a two-terminal device that acts as a one-way conductor. The schematic
symbol for the pn junction diode is shown below. The n-type material is referred to as the cathode,
and the p-type material is referred to as the anode. The component is forward biased (conducting)
when the cathode is more negative than the anode. Otherwise, the component is reverse biased
(not conducting).
FIGURE 2.1 The diode schematic symbol.
Three models are used to represent specific diode characteristics. The ideal model represents
the device as a simple switch that is either open or closed (depending on whether the device is
forward or reverse biased). The characteristics of the ideal model are summarized in Figure 2.7.
The practical model includes the diode characteristics that are considered in circuit calculations
and component substitution applications, such as forward voltage (
(
), average forward current (
), peak reverse voltage
), and forward power dissipation (
). Forward voltage
is considered in circuit calculations. (The effect of
on circuit calculations is demonstrated in
Example 2.7.) The other three ratings are considered in component substitution applications. In
terms of diode maximum ratings, it is generally safe to substitute one pn junction diode for
another when:
1. The
rating of the substitute component is at least 20% greater than the average (dc)
forward current through the circuit
2. The
rating of the substitute component is at least 20% greater than the maximum
reverse voltage that will be generated in the circuit (see Examples 2.8 through 2.11.)
The complete model is the most accurate of the diode models. It includes characteristics
that are considered under specific circumstances (such as high-frequency analysis). It is
also used to explain the differences between predicted and measured circuit values. The
complete model includes such characteristics as bulk resistance, reverse current, diode
capacitance, and diffusion current. Bulk resistance and reverse current can affect circuit
measurements. Diode capacitance can affect high-frequency operation. Diffusion current
accounts for the fact that diodes do conduct at voltages that are less than
.
A change in temperature can impact diode forward and reverse operation. An increase in
temperature causes an increase in diode forward current and a decrease in diode
forward voltage, as shown in Figure 2.23.
Diode reverse current is made up of reverse saturation current (
current (
).
) and surface-leakage
is temperature dependent and relatively independent of reverse voltage.
is affected by the magnitude of reverse voltage and is relatively independent of
temperature. Since
is typically much greater than
, diode reverse current is
affected primarily by changes in temperature. The relationship between temperature and
diode reverse current is illustrated in Figure 2.24.
The specification sheet (or spec sheet) of any component lists the device parameters and
operating characteristics. Parameters are limits, so they are always provided as
maximum values. Operating characteristics, on the other hand, may be listed as
minimum, maximum, or typical values (Figure 2.25). Diode selector guides list a variety of
components according to their average forward current (
) and peak reverse voltage
(
) ratings. As such, selector guides can be used to identify any diode(s) that can be
substituted for a faulty component.
The zener diode is a special type of diode that is designed to work in the reverse
breakdown region of its characteristic curve. (In contrast, a pn junction diode operated in
this region is normally destroyed by excessive heat.) The symbol for a zener diode is
shown below.
FIGURE 2.2 The zener diode schematic symbol.
When operated in reverse breakdown, the voltage across a zener diode remains nearly
constant over a fairly wide range of reverse current values (Figure 2.28). The voltage
across a zener diode that is operating in its reverse breakdown region is referred to as
the zener voltage (
). Typical zener voltage ratings range from around 1.8 V to
hundreds of volts. (Special-case zener diodes, called transient suppressors, can be rated
at more than 1 kV. These diodes are covered in Chapter 5.)
Several current values are used to describe the operation of a given zener diode. Zener
knee current (
) is the minimum value of zener current required to maintain regulation.
Maximum zener current (
) is the limit on the current through a zener. If the value of
is exceeded, the component will likely be destroyed by excessive heat. Zener test
current (
) is the current at which several component characteristics (including the
nominal zener voltage) are rated. These currents are identified in Figure 2.30.
Zener impedance (
) is the opposition that the component provides to a change in
current. A change in zener current (
) results in some change in zener voltage (
). Zener impedance equals the ratio of
to
(Figure 2.31).
The primary zener diode parameters are dc power dissipation and the power derating
factor. The maximum dc power dissipation rating for a zener can be used to determine
the value of
for the device (Example 2.13). The power derating factor indicates the
amount by which
decreases per 1°C rise in temperature. Maximum power
derating is demonstrated in Example 2.14.
The critical zener diode ratings (for component substitution applications) are nominal
zener voltage and maximum dc power dissipation. One zener diode can be substituted
for another when:
o
o
The two components have the same nominal zener voltage
The maximum dc power dissipation rating of the substitute component is equal to
(or greater than) that of the original
Zener diode selector guides group components by their nominal zener voltage and
maximum dc power dissipation ratings. (Examples 2.15 and 2.16, demonstrate some
zener substitution applications.)
Light-emitting diodes (LEDs) are diodes that emit light when biased properly. LEDs come
in a variety of colors. Since LED cases are clear or semiclear (transparent or translucent),
the pins are normally identified using one of the schemes shown in Figure 2.37.
LEDs have typical ranges of
to
to
, and
to
. Because of the low limit on forward current, LEDs always require
the use of a series current-limiting resistor. The process for determining the required
value for such a resistor is demonstrated in Example 2.17.
Multicolor LEDs emit one color when biased in one direction and another when biased in
the other. In essence, this type of component contains two LEDs that are connected in
reverse-parallel (Figure 2.39).
A pn junction diode can be tested using an ohmmeter. When connected to a diode, the
ohmmeter acts as a biasing source. When testing a diode, the component must be
replaced if the ohmmeter measures high forward resistance or low reverse resistance.
Diode testing with an ohmmeter is illustrated in Figure 2.40. Diodes can also be tested
using the diode check function on a digital multimeter, as shown in Figure 2.41. Zener
diodes cannot be tested in the same manner as a pn junction diode, because they are
designed to conduct in the reverse direction. The best approach to testing a zener diode
is to measure the voltage across the component in the circuit. Faulty LEDs show obvious
signs of damage and do not need to be tested (Figure 2.42)