Handbook for Sound Engineers

320 Chapter 12

12.2.2 Diodes

The diode is a device that exhibits a low resistance to
current flow in one direction and a high resistance in the
other. Ideally, when reverse biasing the diode
(connecting the negative of the supply to the diode
anode), no current should flow regardless of the value
of voltage impressed across the diode. A forward-biased
diode presents a very low resistance to current flow.
Fig. 12-11 shows the actual diode characteristics.
Starting with the diode reverse biased, a small reverse
current does flow. The size of this reverse-leakage
current has been exaggerated for clarity and typically is
in the order of nanoamperes. The forward resistance is
not constant, and therefore it does not yield a
straight-line forward-conduction curve. Instead, it
begins high and drops rapidly at relatively low applied
voltage. Above a 0.5–1 V drop it approaches a steep
straight line slope (i.e., low resistance).

In the reverse-biased region of Fig. 12-11, when the
applied voltage (–V) becomes large enough, the leakage
current suddenly begins to increase very rapidly, and the
slope of the characteristic curves becomes very steep.
Past the knee in the characteristic, even a small increase
in reverse voltage causes a large increase in the reverse
current. This steep region is called the breakdown or
avalanche region of the diode characteristic.

The application of high reverse voltage causes the
diode to break down and stop behaving like a diode.
Peak-reverse-voltage rating, or prv is one of the two
most important diode parameters. This is also referred
to as the peak-inverse-voltage rating, or piv. This rating
indicates how high the reverse voltage can be without
approaching the knee and risking breakdown. Addi-
tional diode parameters are:

The maximum average current is limited by power
dissipation in the junction. This power dissipation is
represented by the product of forward voltage drop (VF)
and the forward current (IF):

(12-24)

Selenium Rectifiers and Diodes. A selenium rectifier
cell consists of a nickel-plated aluminum baseplate

coated with selenium, over which a low-temperature

alloy is sprayed. The aluminum base serves as a nega-

tive electrode and the alloy as the positive. Current

flows from the base plate to the alloy but encounters

high resistance in the opposite direction. The efficiency

of conversion depends to some extent on the ratio of the

resistance in the conducting direction to that of the

blocking direction. Conventional rectifiers generally

have ratios from 100:1 to 1000:1.

Selenium rectifiers may be operated over tempera-

tures of –55°C to +150°C (–67°F to +302°F). Rectifica-

tion efficiency is on the order of 90% for three-phase

bridge circuits and 70% for single-phase bridge circuits.

As a selenium cell ages, the forward and reverse resis-

tance increases for approximately one year and then

stabilizes, decreasing the output voltage by approxi-

mately 15%. The internal impedance of a selenium

rectifier is low and exhibits a nonlinear characteristic

with respect to the applied voltage, maintaining a good

voltage regulation. They are often used for battery

charging.

Selenium rectifiers, because of their construction,

have considerable internal capacitance which limits

their operating range to audio frequencies. Approximate

capacitance ranges are 0.10–0.15μF/in^2 of rectifying

surface.

The minimum voltage required for conduction in the

forward direction is termed the threshold voltage and is

about 1 V, therefore, selenium rectifiers cannot be used

successfully below that voltage.

Silicon Rectifiers and Diodes. The high forward-to-

reverse current characteristic of the silicon diode

produces an efficiency of about 99%. When properly

used, silicon diodes have long life and are not affected

by aging, moisture, or temperature when used with the

proper heat sink.

As an example, four individual diodes of 400 Vpiv

may be connected in series to withstand a piv of 1600 V.

Maximum average

current

Causes overheating of the device

Peak repetitive

current

Maximum peak value of current

on a repetitive basis

Surge current A b s o l u t e m a x i m u m a l l o w e d

current even if just momentary

PV= FIF

Figure 12-11. Actual diode characteristics.

+I

+V

Forward voltage drop

Avalanche region V

Breakdown knee

Reverse leakage

current

(exaggerated)

I