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| Linear regulators are generally not very efficient, since they use a series control element which is really nothing more than a controlled series ]. This "resistance" will dissipate the unwanted portion of the input voltage as heat. A much more efficient regulator can be realised using a switching design. Instead of controlling a variable resistance, the output is controlled by rapidly switching a series device on and off. The ] of the switching sets how much ] is transferred to the load. This is controlled by a similar feedback mechanism to a linear regulator. Because the series element is either fully conducting, or switched off, it dissipates almost no power, and this is what gives the switching design its efficiency. Switching regulators are also able to generate output voltages which are higher than the input, or of opposite polarity - something not possible with a linear design. Switching designs have largely superseded linear circuits where high levels of power are involved (above a few watts). The main disadvantages of the switching regulator is greater complexity and hence cost, and high frequency noise at the output, which must be filtered out. | Linear regulators are generally not very efficient, since they use a series control element which is really nothing more than a controlled series ]. This "resistance" will dissipate the unwanted portion of the input voltage as heat. A much more efficient regulator can be realised using a switching design. Instead of controlling a variable resistance, the output is controlled by rapidly switching a series device on and off. The ] of the switching sets how much ] is transferred to the load. This is controlled by a similar feedback mechanism to a linear regulator. Because the series element is either fully conducting, or switched off, it dissipates almost no power, and this is what gives the switching design its efficiency. Switching regulators are also able to generate output voltages which are higher than the input, or of opposite polarity - something not possible with a linear design. Switching designs have largely superseded linear circuits where high levels of power are involved (above a few watts). The main disadvantages of the switching regulator is greater complexity and hence cost, and high frequency noise at the output, which must be filtered out. | ||
| == combination == | |||
| Many power supplies use both. | |||
| The switching regulator accepts a wide range of input voltages and efficiently generates a (somewhat noisy) voltage slightly above the desired output. | |||
| That is followed by a linear regulator that generates exactly the desired voltage, and eliminate nearly all the ] generated by the switching regulator. | |||
| == See also == | == See also == | ||
| * ] |
* ] | ||
| * ] | |||
| ] | ] | ||
| ] | |||
| ] | |||
Revision as of 23:39, 3 February 2005
A voltage regulator is a mechanical, solid state or vacuum tube device designed to regulate DC voltage levels.
Mechanical regulators
Early automobile generators and alternators had a mechanical voltage regulator using three or two relays and ballast resistors to stabilize the generator's output on 6 or 12 V, independent of the engine's rpm. Later designs used 'solid state' technology to do the same.
These regulators fall into the class of 'shunt' regulators, which work by diverting ('shunting') unwanted current from the load, through a ballast resistor, to ground. The diverted current causes the ballast resistor to get hot, and therefore wastes power.
Mains regulators
In mains-powered devices a transformer is used to bring the mains AC power down to a lower AC voltage, then a rectifier (diode bridge) to convert AC to DC, followed by a linear regulator or Zener diode to stabilize it to the required voltage.
Solid-state linear regulators
(main article: Linear regulator)
Linear regulators exist in shunt (see above) and series types. Shunt types, because of their inefficiency, are used only to supply very small currents, of the order of microamperes or a few milliamperes. With a series regulator, the current through the regulator is approximately equal to the current through the load, and there is no ballast resistor. The regulator itself dissipates heat, causing some power to be wasted.
Voltage regulators operate by comparing the actual output voltage to some internal fixed reference voltage. Any difference is amplified and used to control the series regulation element, which is usually one or more heavy-duty transistors. This forms a negative feedback control loop. If the load demand changes, a rise in current will generally cause a small drop in the output voltage, which will cause the circuit to turn on the series element more strongly, restoring the voltage to its desired level.
Solid state regulator semiconductor chips come in either fixed or variable types. Common solid-state series voltage regulators are the LM78xx (for positive voltages) and LM79xx (for negative voltages), and common fixed voltages are 5 V (for transistor-transistor logic circuits) and 12 V, e.g. in personal computers. In fixed voltage regulators the reference pin is tied to ground, whereas in variable regulators the reference pin is connected to the centre point of a fixed or variable voltage divider fed by the regulator's output. A variable voltage divider (such as a potentiometer) allows the user to adjust the regulated voltage.
Switching regulators
(main article: Switched-mode power supply)
Linear regulators are generally not very efficient, since they use a series control element which is really nothing more than a controlled series resistance. This "resistance" will dissipate the unwanted portion of the input voltage as heat. A much more efficient regulator can be realised using a switching design. Instead of controlling a variable resistance, the output is controlled by rapidly switching a series device on and off. The duty cycle of the switching sets how much charge is transferred to the load. This is controlled by a similar feedback mechanism to a linear regulator. Because the series element is either fully conducting, or switched off, it dissipates almost no power, and this is what gives the switching design its efficiency. Switching regulators are also able to generate output voltages which are higher than the input, or of opposite polarity - something not possible with a linear design. Switching designs have largely superseded linear circuits where high levels of power are involved (above a few watts). The main disadvantages of the switching regulator is greater complexity and hence cost, and high frequency noise at the output, which must be filtered out.
combination
Many power supplies use both. The switching regulator accepts a wide range of input voltages and efficiently generates a (somewhat noisy) voltage slightly above the desired output. That is followed by a linear regulator that generates exactly the desired voltage, and eliminate nearly all the noise generated by the switching regulator.