DC to DC converter
Encyclopedia
Electronic circuit
An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow...
which converts a source of direct current
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
(DC) from one voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
level to another. It is a class of power converter.
Usage
DC to DC converters are important in portable electronic devices such as cellular phones and laptop computers, which are supplied with power from batteriesBattery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...
primarily. Such electronic devices often contain several sub-circuits
Electrical network
An electrical network is an interconnection of electrical elements such as resistors, inductors, capacitors, transmission lines, voltage sources, current sources and switches. An electrical circuit is a special type of network, one that has a closed loop giving a return path for the current...
, 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.
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 input voltage.
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 dissipateDissipation
In physics, dissipation embodies the concept of a dynamical system where important mechanical models, such as waves or oscillations, lose energy over time, typically from friction or turbulence. The lost energy converts into heat, which raises the temperature of the system. Such systems are called...
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 as it must be removed from the circuitry to prevent unacceptable temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
rises.
They 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 sinking is 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 FETsField-effect transistor
The field-effect transistor is a transistor that relies on an electric field to control the shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor material. FETs are sometimes called unipolar transistors to contrast their single-carrier-type operation with...
, 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.
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
Active rectification
Active rectification, or synchronous rectification, is a technique for improving the efficiency of rectification by replacing diodes with actively-controlled switches such as transistors, usually power MOSFETs or power BJTs...
. A bi-directional converter can move power in either direction, which is useful in applications requiring regenerative braking
Regenerative brake
A regenerative brake is an energy recovery mechanism which slows a vehicle or object down by converting its kinetic energy into another form, which can be either used immediately or stored until needed...
.
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 circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
s needing minimal additional components.
DC to DC converters are also available as a complete hybrid circuit
Hybrid circuit
A hybrid integrated circuit, HIC, hybrid microcircuit, or simply hybrid is a miniaturized electronic circuit constructed of individual devices, such as semiconductor devices and passive components , bonded to a substrate or printed circuit board...
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
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
in an inductor
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
or a transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
, typically in the range from 300 kHz to 10 MHz. By adjusting the duty cycle
Duty cycle
In engineering, the duty cycle of a machine or system is the time that it spends in an active state as a fraction of the total time under consideration....
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
Switched-mode power supply
A switched-mode power supply is an electronic power supply that incorporates a switching regulator in order to be highly efficient in the conversion of electrical power...
.
Many topologies exist. This table shows the most common.
Forward
|
Flyback
|
|
|---|---|---|
| No transformer
|
Step-down (Buck) Buck converter A buck converter is a step-down DC to DC converter. Its design is similar to the step-up boost converter, and like the boost converter it is a switched-mode power supply that uses two switches , an inductor and a capacitor.... - The output voltage is lower than the input voltage, and of the same polarity |
|
| True Buck-Boost - The output voltage is the same polarity as the input and can be lower or higher | ||
| Split-Pi (Boost-Buck) Split-Pi In electronics, a split-pi topology is a pattern of component interconnections used in a kind of power converter that can theoretically produce an arbitrary output voltage, either higher or lower than the input voltage. In practice the upper voltage output is limited to the voltage rating of... - Allows bidirectional voltage conversion with the output voltage the same polarity as the input and can be lower or higher |
||
| With transformer
|
H-bridge An H bridge is an electronic circuit that enables a voltage to be applied across a load in either direction. These circuits are often used in robotics and other applications to allow DC motors to run forwards and backwards... - 2 transistors drive H-bridge An H bridge is an electronic circuit that enables a voltage to be applied across a load in either direction. These circuits are often used in robotics and other applications to allow DC motors to run forwards and backwards... - 4 transistor drive |
Flyback Flyback converter The flyback converter is used in both AC/DC and DC/DC conversion with galvanic isolation between the input and any outputs. More precisely, the flyback converter is a buck-boost converter with the inductor split to form a transformer, so that the voltage ratios are multiplied with an additional... - 1 or 2 transistor drive |
In addition, each topology may be:
- Hard switched - transistors switch quickly while exposed to both full voltage and full current
- Resonant - an LC circuitLC circuitAn LC circuit, also called a resonant circuit or tuned circuit, consists of an inductor, represented by the letter L, and a capacitor, represented by the letter C...
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
H-bridge
An H bridge is an electronic circuit that enables a voltage to be applied across a load in either direction. These circuits are often used in robotics and other applications to allow DC motors to run forwards and backwards...
and Flyback
Flyback converter
The flyback converter is used in both AC/DC and DC/DC conversion with galvanic isolation between the input and any outputs. More precisely, the flyback converter is a buck-boost converter with the inductor split to form a transformer, so that the voltage ratios are multiplied with an additional...
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
Electromigration
Electromigration is the transport of material caused by the gradual movement of the ions in a conductor due to the momentum transfer between conducting electrons and diffusing metal atoms. The effect is important in applications where high direct current densities are used, such as in...
can shorten the MTBF), bipolar switches generally can't so require the use of a snubber
Snubber
A snubber is a device used to suppress voltage transients in electrical systems, pressure transients in fluid systems, or excess force or rapid movement in mechanical systems.-Electrical systems:...
(or two).
Capacitive
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
Charge pump
A charge pump is a kind of DC to DC converter that uses capacitors as energy storage elements to create either a higher or lower voltage power source. Charge pump circuits are capable of high efficiencies, sometimes as high as 90–95% while being electrically simple circuits.Charge pumps use some...
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. They are also used at extremely high voltages, as magnetics would break down at such voltages.
Electrochemical
A further means of DC to DC conversion in the kiloWatt to many MegaWatts range is presented by using redox flow batteriesFlow battery
A flow battery is a form of rechargeable battery in which electrolyte containing one or more dissolved electroactive species flows through an electrochemical cell that converts chemical energy directly to electricity...
such as the vanadium redox battery
Vanadium redox battery
The vanadium redox battery is a type of rechargeable flow battery that employs vanadium ions in different oxidation states to store chemical potential energy...
, although this technique has not been applied commercially to date.
Terminology
Step-down - A converter where output voltage is lower than the input voltage. Like a Buck converterBuck converter
A buck converter is a step-down DC to DC converter. Its design is similar to the step-up boost converter, and like the boost converter it is a switched-mode power supply that uses two switches , an inductor and a capacitor....
.
Step-up - A converter that outputs a voltage higher than the input voltage. Like a Boost converter
Boost converter
A boost 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 containing at least two semiconductor switches and at least one energy storage element...
.
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.
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.
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.
RF noise - Switching converters inherently emit radio waves
Radio waves
Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies from 300 GHz to as low as 3 kHz, and corresponding wavelengths from 1 millimeter to 100 kilometers. Like all other electromagnetic waves,...
at the switching frequency and its harmonics.
Switching converters that produce triangular switching current, such as the Split-Pi
Split-Pi
In electronics, a split-pi topology is a pattern of component interconnections used in a kind of power converter that can theoretically produce an arbitrary output voltage, either higher or lower than the input voltage. In practice the upper voltage output is limited to the voltage rating of...
or Ćuk converter
Cuk converter
The Ćuk converter is a type of DC-DC converter that has an output voltage magnitude that is either greater than or less than the input voltage magnitude....
in continuous current mode, produce less harmonic noise than other switching converters. Linear converters produce practically no RF noise. Too much RF noise causes electromagnetic interference
Electromagnetic interference
Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...
(EMI).
See also
- Switched-mode power supplySwitched-mode power supplyA switched-mode power supply is an electronic power supply that incorporates a switching regulator in order to be highly efficient in the conversion of electrical power...
- Companies that manufacture DC-DC Converters:Vicor CorporationVicor CorporationVicor Corporation designs, develops, manufactures and markets modular power components, power management and complete power systems based upon a portfolio of patented technologies...
Lambda Electronics, Texas Instruments, Tyco International, Ltd., Artesyn Technologies, Power-One, Inc., Coil Technology Corporation