DC-to-DC converter
From Blogger From: Sunday Emmanuel E.
A
DC-to-DC converter is an electronic circuit which converts a source of direct current
(DC) from one voltage level to another. It is a class of power
converter.
Contents
Usage
DC
to DC converters are important in portable electronic devices such as cellular phones
and laptop computers, which are supplied with power from batteries primarily. Such electronic devices often contain several
sub-circuits, each with its own voltage level requirement different from
that supplied by the battery or an external supply (sometimes higher or lower
than the supply voltage). Additionally, the battery voltage declines as its
stored power is drained. Switched DC to DC converters offer a method to
increase voltage from a partially lowered battery voltage thereby saving space
instead of using multiple batteries to accomplish the same thing.
Most
DC to DC converters also regulate the output voltage. Some exceptions include
high-efficiency LED power sources, which are a kind of DC to DC converter that regulates the
current through the LEDs, and simple charge pumps which double or triple the
output voltage.
Conversion methods
Electronic
Linear
Linear
regulators can only output at lower voltages from the input. They are very inefficient when the voltage drop is large and the current is high as
they dissipate heat equal to the product of the output current and the
voltage drop; consequently they are not normally used for large-drop
high-current applications.
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
switch-mode 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 or synchronous rectifier contacts on the vibrator.
Most
AC-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.
True Buck-Boost - The output voltage is the same polarity as the input
and can be lower or higher
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Split-Pi (Boost-Buck) - Allows bidirectional voltage conversion with the output
voltage the same polarity as the input and can be lower or higher.
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Cuk (Cuk) - Allows bidirectional voltage conversion with the output
voltage of inverted polarity.
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With
transformer
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Flyback - 1 or 2 transistor drive
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In
addition, each topology may be:
- Hard switched - transistors switch quickly while exposed to both full voltage and full current
- Resonant - an LC circuit shapes the voltage across the transistor and current through it so that the transistor switches when either the voltage or the current is zero
Magnetic
DC-to-DC converters may be operated in two modes, according to the current in
its main magnetic component (inductor or transformer):
- Continuous - the current fluctuates but never goes down to zero
- Discontinuous - the current fluctuates during the cycle, going down to zero at or before the end of each cycle
A
converter may be designed to operate in continuous mode at high power, and in
discontinuous mode at low power.
The
Half bridge
and Flyback topologies are similar in that energy stored in the
magnetic core needs to be dissipated so that the core does not saturate. Power
transmission in a flyback circuit is limited by the amount of energy that can
be stored in the core, while forward circuits are usually limited by the I/V
characteristics of the switches.
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
witched
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
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
Step-down
Step-up
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 Ćuk 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.
This 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
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