DC to DC converter
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In electronic engineering, a DC to DC converter is a circuit which converts a source of direct current (DC) from one voltage level to another. It is a class of power converter.
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[edit] 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 than that supplied by the battery or an external supply (sometimes higher or lower than the supply voltage, and possibly even negative 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.
[edit] Conversion methods
[edit] Electronic
[edit] Linear
Linear regulators can output a lower, but not a higher, voltage from the input. They are very inefficient if the voltage drop is large and the current high as they dissipate as heat power 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 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.
[edit] 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 in since the late 1980's due to the use of power FETs, which are able to switch at high frequency more efficiently than power bipolar transistors, which have more switching losses and require a more complex drive circuit. Another important innovation in DC-DC converters is the use of synchronous switching which replaces the flywheel diode with a power FET with low "On" resistance, thereby reducing switching losses.
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. DC to DC converters are also available as a complete hybrid circuit component, ready for use within an electronic assembly.
[edit] 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 done to control the output voltage, though it could be done to control the input current, the output current, or maintaining 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.
Forward
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Flyback
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|---|---|---|
No transformer
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Step-down (Buck) - The output voltage is lower than the input voltage, and of the same polarity |
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| 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 | ||
With transformer
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Flyback - 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 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.
Mosfet switches can tolerate simultaneous full current and voltage (although thermal stress and electromigration can shorten the MTBF), bipolar switches generally don't so require the use of a snubber (or two).
[edit] 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 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.
[edit] Electrochemical
A further means of DC to DC conversion in the kW to many MW range is presented by using redox flow batteries such as the vanadium redox battery, although this technique has not been applied commercially to date.
[edit] Terminology
Step up - Also known as a Boost Converter, this is a converter that outputs a voltage higher than the input voltage.
Step down - A converter where output voltage is lower than the input voltage. ie. Buck Converter
Continuous Current Mode - Current and thus the magnetic field in the energy storage never reach zero.
Discontinuous Current Mode - Current and thus the magnetic field in the energy storage may reach or cross zero.
[edit] See also
[edit] References
- Rudy P. Severns, G. Ed Bloom (1985). Modern DC-DC Switchmode Power Conversion Circuits. Van Nostrand Reinhold. Out of Print.
- George C. Chryssis (1989). High Frequency Switching Power Supplies: Theory and Design. McGraw-Hill. ISBN 0070109516.
- Andre S. Kislovski, Richard Redl, Nathan O. Sokal (1991). Dynamic Analysis of Switching-Mode DC/DC Converters. Van Nostrand Reinhold. ISBN 0442239165.
- Yim-Shu Lee (1993). Computer-Aided Analysis and Design of Switch-Mode Power Supplies. Marcel Dekker. ISBN 0824788036.
- Abraham I. Pressman (1997). Switching Power Supply Design. McGraw-Hill. ISBN 0-07-052236-7.
- Philip T. Krein (1997). Elements of Power Electronics. Oxford University Press. ISBN 0195117018.
- Robert W. Erickson, Dragan Maksimovic (2001). Fundamentals of Power Electronics. Kluwer Academic Publishers. ISBN 9780792372707.
- Ned Mohan, Tore M. Undeland, William P. Robbins (2002). Power Electronics : Converters, Applications, and Design. Wiley. ISBN 0-471-22693-9.
- Chi Kong Tse (2003). Complex Behavior of Switching Power Converter. CRC Press. ISBN 0849318629.
- Christophe Basso, Switch-Mode Power Supplies: SPICE Simulations and Practical Designs. McGraw-Hill. ISBN 0071508589
