DC-DC Converter
DC-DC Converter
DC-DC Converter
The inefficiency wastes power and requires higher-rated and consequently more
expensive and larger components. The heat dissipated by high-power supplies is a
problem in itself and it must be removed from the circuitry to prevent unacceptable
temperature rises.
Linear regulators are practical if the current is low, the power dissipated being
small, although it may still be a large fraction of the total power consumed. They
are often used as part of a simple regulated power supply for higher currents: a
transformer generates a voltage which, when rectified, is a little higher than that
needed to bias the linear regulator. The linear regulator drops the excess voltage,
reducing hum-generating ripple current and providing a constant output voltage
independent of normal fluctuations of the unregulated input voltage from the
transformer/bridge rectifier circuit and of the load current.
Linear regulators are inexpensive, reliable if good heat sinks are used and much
simpler than switching regulators. As part of a power supply they may require a
transformer, which is larger for a given power level than that required by a switchmode power supply. Linear regulators can provide a very low-noise output voltage,
and are very suitable for powering noise-sensitive low-power analog and radio
frequency circuits. A popular design approach is to use an LDO, Low Drop-out
Regulator, that provides a local "point of load" DC supply to a low power circuit.
Switched-mode conversion
Electronic switch-mode DC to DC converters convert one DC voltage level to
another, by storing the input energy temporarily and then releasing that energy to
the output at a different voltage. The storage may be in either magnetic field
storage components (inductors, transformers) or electric field storage components
(capacitors). This conversion method is more power efficient (often 75% to 98%)
than linear voltage regulation (which dissipates unwanted power as heat). This
efficiency is beneficial to increasing the running time of battery operated devices.
The efficiency has increased since the late 1980s due to the use of power FETs,
which are able to switch at high frequency more efficiently than power bipolar
transistors, which incur more switching losses and require a more complicated
drive circuit. Another important innovation in DC-DC converters is the use of
synchronous rectification replacing the flywheel diode with a power FET with low
"on resistance", thereby reducing switching losses. Before the wide availability of
power semiconductors, low power DC to DC converters of this family consisted of
an electro-mechanical vibrator followed by a voltage step-up transformer and a
vacuum tube or semiconductor rectifier.
Most DC-to-DC converters are designed to move power in only one direction,
from the input to the output. However, all switching regulator topologies can be
made bi-directional by replacing all diodes with independently controlled active
rectification. A bi-directional converter can move power in either direction, which
is useful in applications requiring regenerative braking.
Drawbacks of switching converters include complexity, electronic noise (EMI /
RFI) and to some extent cost, although this has come down with advances in chip
design.
DC-to-DC converters are now available as integrated circuits needing minimal
additional components. They are also available as a complete hybrid circuit
component, ready for use within an electronic assembly.
Magnetic
In these DC-to-DC converters, energy is periodically stored into and released from
a magnetic field in an inductor or a transformer, typically in the range from
300 kHz to 10 MHz. By adjusting the duty cycle of the charging voltage (that is,
the ratio of on/off time), the amount of power transferred can be controlled.
Usually, this is applied to control the output voltage, though it could be applied to
control the input current, the output current, or maintain a constant power.
Transformer-based converters may provide isolation between the input and the
output. In general, the term "DC-to-DC converter" refers to one of these switching
converters. These circuits are the heart of a switched-mode power supply. Many
topologies exist. This table shows the most common.
Flyback
Forward
Energy goes from the
Energy goes from the
input,
through
the
No
is
the
same
input voltage
o SEPIC - The output
voltage can be lower
or higher than the
input
Inverting:
the
output
(Buck-
Boost)
o uk - Output current
is continuous
True Buck-Boost - The output voltage is the same polarity as the
input and can be lower or higher
Split-Pi (Boost-Buck) - Allows bidirectional voltage conversion
with the output voltage the same polarity as the input and can be
lower or higher.
Cuk (Cuk) - Allows bidirectional voltage
conversion with the output voltage of inverted
polarity.
With
transformer
May be
isolated
Although MOSFET switches can tolerate simultaneous full current and voltage
(although thermal stress and electromigration can shorten the MTBF), bipolar
switches generally can't so require the use of a snubber (or two).
Capacitive
Main article: Charge pump
Switched capacitor converters rely on alternately connecting capacitors to the input
and output in differing topologies. For example, a switched-capacitor reducing
converter might charge two capacitors in series and then discharge them in parallel.
This would produce an output voltage of half the input voltage, but at twice the
current (minus various inefficiencies). Because they operate on discrete quantities
of charge, these are also sometimes referred to as charge pump converters. They
are typically used in applications requiring relatively small amounts of current, as
at higher current loads the increased efficiency and smaller size of switch-mode
converters makes them a better choice.[citation needed] They are also used at extremely
high voltages, as magnetics would break down at such voltages.
Electromechanical
Main article: Motor-generator
A motor-generator or dynamotor set may consist either of distinct motor and
generator machines coupled together or of a single unit motor-generator. A single
unit motor-generator has both rotor coils of the motor and the generator wound
around a single rotor, and both coils share the same outer field coils or magnets.
Typically the motor coils are driven from a commutator on one end of the shaft,
when the generator coils output to another commutator on the other end of the
shaft. The entire rotor and shaft assembly is smaller in size than a pair of machines,
and may not have any exposed drive shafts.
Motor-generators can convert between any combination of DC and AC voltage and
phase standards. Large motor-generator sets were widely used to convert industrial
amounts of power while smaller motor-generators were used to convert battery
power (6, 12 or 24 V DC) to a high DC voltage, which was required to operate
vacuum tube (thermionic valve) equipment.
Electrochemical
A further means of DC to DC conversion in the kilowatts to megawatts range is
presented by using redox flow batteries such as the vanadium redox battery,
although this technique has not been applied commercially to date.
Terminology
The simplest way to reduce a DC voltage is to use a voltage divider circuit, but
voltage dividers waste energy, since they operate by bleeding off excess power as
heat; also, output voltage isn't regulated (varies with input voltage). Buck
converters, on the other hand, can be remarkably efficient (easily up to 95% for
integrated circuits) and self-regulating, making them useful for tasks such as
converting the 1224 V typical battery voltage in a laptop down to the few volts
needed by the processor.
BOOST CONVERTER
A boost converter (step-up converter) is a power converter with an output DC
voltage greater than its input DC voltage. It is a class of switching-mode power
supply (SMPS) containing at least two semiconductor switches (a diode and a
transistor) and at least one energy storage element. Filters made of capacitors
(sometimes in combination with inductors) are normally added to the output of the
converter to reduce output voltage ripple.
Continuous Current Mode - Current and thus the magnetic field in the inductive
energy storage never reach zero.
Discontinuous Current Mode - Current and thus the magnetic field in the inductive
energy storage may reach or cross zero.
Noise - Since all properly designed DC-to-DC converters are completely inaudible,
"noise" in discussing them always refers to unwanted electrical and
electromagnetic signal noise.
RF noise - Switching converters inherently emit radio waves at the switching
frequency and its harmonics. Switching converters that produce triangular
switching current, such as the Split-Pi or uk converter in continuous current
mode, produce less harmonic noise than other switching converters.[1] Linear
converters produce practically no RF noise. Too much RF noise causes
electromagnetic interference (EMI).
Input noise - If the converter loads the input with sharp load edges. Electrical noise
can be emitted from the supplying power lines as RF noise. Which should be
prevented with proper filtering in the input stage of the converter.
Output noise - The output of a DC-to-DC converter is designed to have a flat,
constant output voltage. Unfortunately, all real DC-to-DC converters produce an
output that constantly varies up and down from the nominal designed output
voltage. This varying voltage on the output is the output noise. All DC-to-DC
converters, including linear regulators, have some thermal output noise. Switching
converters have, in addition, switching noise at the switching frequency and its
harmonics. Some sensitive radio frequency and analog circuits require a power
supply with so little noise that it can only be provided by a linear regulator. Many
analog circuits require a power supply with relatively low noise, but can tolerate
some of the less-noisy switching converters.[1]