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electronic engineering, a DC to DC converter is a circuit which converts a source of direct current (DC) from one voltage to another. It is a class of power converter.
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[] 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. Such electronic devices often contain several sub-circuits with each sub-circuit requiring a unique voltage level different than that supplied by the battery (sometimes higher or lower than the battery voltage, and possibly even negative voltage). Additionally, the battery voltage declines as its stored power is drained. DC to DC converters offer a method of generating multiple controlled voltages from a single variable battery voltage, thereby saving space instead of using multiple batteries to supply different parts of the device.
[] Conversion methods
[] Electronic
[] 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 regulators are much simpler than switching DC-DC converters. However, unlike switching DC-DC converters, linear regulators cannot generate:
- higher voltages
- higher currents
- voltages of the opposite polarity
An even simpler approach to use a resistors in series with the voltage supply. That resistor and the load form a voltage divider resulting is a lower voltage. However, this method offers no regulation.
[] 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
| Flyback
| |
|---|---|---|
No transformer
| Step-down (Buck) - 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 | ||
With transformer
|
| Flyback - 1 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.,
[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. 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 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 that outputs a voltage lower than the input.
[] Step up
A converter that outputs a voltage higher than input.
[] 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.
