Understanding How Voltage Regulators Work

Understanding How Voltage Regulators Work
Understanding How a
Voltage Regulator Works
General Design Fundamentals
What are some of the switching regulator topologies?
A voltage regulator generates a fixed output voltage of
a preset magnitude that remains constant regardless of
changes to its input voltage or load conditions. There are two
types of voltage regulators: linear and switching.
There are three common topologies: buck (step-down), boost
(step-up) and buck-boost(step-up/stepdown). Other topologies
include the flyback, SEPIC, Cuk, push-pull, forward, full-bridge,
and half-bridge topologies.
A linear regulator employs an active (BJT or MOSFET) pass
device (series or shunt) controlled by a high gain differential
amplifier. It compares the output voltage with a precise
reference voltage and adjusts the pass device to maintain a
constant output voltage.
How does switching frequency impact regulator designs?
Higher switching frequencies mean the voltage regulator can
use smaller inductors and capacitors. It also means higher
switching losses and greater noise in the circuit.
What losses occur with the switching regulator?
A switching regulator converts the dc input voltage to a
switched voltage applied to a power MOSFET or BJT switch.
The filtered power switch output voltage is fed back to a
circuit that controls the power switch on and off times so
that the output voltage remains constant regardless of input
voltage or load current changes.
Losses occur as a result of the power needed to turn the
MOSFET on and off, which are associated with the MOSFET’s
gate driver. Also, MOSFET power losses occur because it
takes a finite time to switch to/from the conduction to nonconduction states. Losses are also due to the energy needed
to charge and discharge the capacitance of the MOSFET gate
between the threshold voltage and gate voltage.
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What are the usual applications for linear and switching
regulators?
What design specifications are important for a voltage
regulator IC?
The linear regulator’s power dissipation is directly proportional
to its output current for a given input and output voltage,
so typical efficiencies can be 50% or even lower. Using the
optimum components, a switching regulator can achieve
efficiencies in the 90% range. However, the noise output from a
linear regulator is much lower than a switching regulator with
the same output voltage and current requirements. Typically,
the switching regulator can drive higher current loads than a
linear regulator.
Among the basic parameters are input voltage, output voltage,
and output current. Depending on the application, other
parameters may be important, such as output ripple voltage,
load transient response, output noise, and efficiency. Important
parameters for the linear regulator are dropout voltage, PSRR
(power supply rejection ratio), and output noise.
How does a switching regulator control its output?
Switching regulators require a means to vary their output
voltage in response to input and output voltage changes.
One approach is to use PWM that controls the input to the
associated power switch, which controls its on and off time
(duty cycle). In operation, the regulator’s filtered output voltage
is fed back to the PWM controller to control the duty cycle. If
the filtered output tends to change, the feedback applied to the
PWM controller varies the duty cycle to maintain a constant
output voltage.
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