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.

Contents

1 Usage
2 Conversion methods

2.1 Electronic

2.1.1 Linear
2.1.2 Switched-mode conversion

2.1.2.1 Magnetic
2.1.2.2 Capacitive

2.2 Electrochemical

3 Terminology

3.1 Step down
3.2 Step up
3.3 Continuous Current Mode
3.4 Discontinuous Current Mode

4 See also
5 References
6 External links

[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 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 drop the input voltage to a lower output voltage. They are inefficient, as they convert the dropped voltage into heat dissipation.

Linear regulator are inexpensive, reliable, and much simpler than switching regulators. Linear regulators also can provide a very low-noise output voltage, and are much more suited to powering sensitive analog circuits.

However, unlike switching DC-DC converters, linear regulators cannot generate:

Higher voltages than their input
Large currents (due to excess heat)

[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 components (inductors, transformers) or capacitors. This conversion method is more power efficient (often 80% to 98%) than linear voltage regulation (which dissipates unwanted voltage as heat). This efficiency is beneficial to increasing the running time of battery operated devices. Drawbacks of switching converters include cost, complexity and electronic noise (EMI / RFI).

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

These DC to DC converters convert one DC voltage to another by storing energy into a magnetic component (an inductor or a transformer) for a period of time (usually in the 30 kHz to 5 MHz range). By adjusting the PWM Duty Cycle (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

Energy goes from the input, through the the magnetics and to the load, simultaneously

Flyback

Energy goes from the input and stored in the magnetics
Later, it is released from the magnetics to the load

No transformer

Non-isolated

Step-down (Buck) - The output voltage is lower than the input voltage, and of the same polarity

Non-inverting: The output voltage is the same polarity as the input

Step-up (Boost) - The output voltage is higher than the input voltage
SEPIC - The output voltage can be lower or higher than the input

Inverting: the output voltage is of the opposite polarity as the input

Inverting (Buck-Boost)
Ćuk - Output current is continuous

True Buck-Boost - The output voltage is the same polarity as the input and can be lower or higher

With transformer

May be isolated

Half bridge - 2 transistors drive
Full bridge - 4 transistor drive

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 goes down to zero at 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

[edit] Step down

A converter where output voltage is lower than the input voltage.

[edit] Step up

Also known as a Boost Converter, this is a converter that outputs a voltage higher than the input voltage.

[edit] Continuous Current Mode

Current and thus the magnetic field in the energy storage never reach zero.

[edit] Discontinuous Current Mode

Current and thus the magnetic field in the energy storage may reach or cross zero.

[edit] See also

Switched-mode power supply

[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

[edit] External links

A general description of DC-DC converters.
A beginner's guide to switching regulators.
Power Electronics Books
Switching regulator application note for LCD Power supply
Switching Regulator Simulator

Retrieved from "http://en.wikipedia.org/wiki/DC_to_DC_converter"

Categories: Power supplies | Power electronics | Battery electric vehicle components | Electrical power conversion

Hidden categories: All articles with unsourced statements | Articles with unsourced statements since January 2008

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This page was last modified on 5 May 2008, at 14:33.
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