The circuits shown in Figures 3.21c and 3.21f
are called “switching” regulators. They use a
dynamic load that is switched ON and OFF at
very high frequencies at a varying duty cycle.
The dynamic load supplies electrical power to
an energy storage element such as a capacitor
or an inductor or a combination of both. This
energy storage element then supplies power to
the load. The percentage of time the dynamic
load is ON is varied depending on the input
voltage and load requirements. The “switching”
regulator provides the highest power efficiency
of the three circuits. However, it is the most
complex of the three regulator circuits and has
the highest potential for creating unwanted EMI
(due to the high-frequency switching).
circuit shown in Figure 3.1b or “rung” for the
circuit shown in Figure 3.1c.
With a voltage regulator, the forward voltage
applied to the LED array voltage will be
independent of supply voltage variations as long
as the voltage regulator remains in its active
region. However, ambient temperature variations
and the use of different forward voltage
categories can affect the forward current through
the LED array unless provisions are made in the
design. As mentioned earlier, current limiting
resistors, R, are needed for each string of LED
emitters. With R > y DRS, the forward current
through each string will primarily be determined
by the value of R. If the designer uses a voltage
regulator with a fixed output voltage, then the
values of these current-limiting resistors will need
to be varied for each of the different forward
voltage categories in order to compensate for the
different forward voltages at the design current.
Alternatively, the designer could use the same
value of current-limiting resistors for all forward
voltage categories. However, in this case, the
regulator output voltage would need to be varied
slightly for each different forward voltage
The performance of these different types of
regulators is compared with an example shown
in the sidebar “Comparison of Three Constant-
Current Circuits.”
The LED emitter array can be driven from either
a voltage regulator or a current regulator circuit.
With a current regulator, the total array current
will be independent of supply voltage,
temperature and forward voltage category
variations as long as the current regulator
remains in its active region. If the current
regulator is used with parallel-connected LED
emitters, such as shown in Figure 3.1b or 3.1c,
there can still be similar forward current
category to compensate for the different forward
voltages at the design current. Despite these
precautions, there will still be small variations in
the total current through the LED array due to
slightly different forward voltages of the individual
emitters. With only a small voltage drop across
the current limiting resistor, small variations in
the regulated voltage can cause large changes
in forward current through the LED emitters. In
addition, since the forward voltage of the LED
emitter varies with temperature, the forward
current through the LED array will increase at
elevated temperatures. However, it is possible
variations within the LED array as was
discussed in the section “Key Concepts for the
Electrical Design of LED Signal Lamps.” Note
that the forward current matching can be
improved with the addition of a small resistor
(ROPT > RS) in series with each string for the
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