Voltage regulator
From HvWiki
A circuit that attempts to keep a voltage level accurate and constant
Contents |
Linear regulators
Parallel
Zener Diode Regulator
A zener diode is similar to other diodes; it conducts easily in one direction but not the other up to a certain voltage, the reverse breakdown voltage. However, while most diodes are permanently damaged when the maximum reverse voltage is exceeded, the zener diode is designed to operate at this voltage, called the zener voltage in this case.
The effect of this is that in the circuit, when the voltage rises above the zener voltage, the zener diode starts conducting and the rest of the voltage is dropped across the series resistor. In this way the zener diode keeps the voltage at the output more or less constant.
Designing Zener Diode Regulator
In use, a current flows through the series resistor given by the voltage dropped across it. Any of this current that doesn't flow through the load attached to the output must flow through the zener diode, which must be able to handle this. The zener diode needs a minimum current, IZK, through it in order to regulate properly.
Therefore, the series resistor must supply enough current to feed the load its maximum current use plus the IZK. Ie, 
.
The zener diode has to dissipate an amount of power given by its zener voltage multiplied by the maximum current flowing through it. Ie, 
. The current Iloadmin should be zero if the output is ever disconnected from any load.
Series
A series voltage regulator consists of an adjustable series resistance that is controlled by a feedback loop. This means that the regulator does not waste power when it is not loaded.
The minimum voltage drop accross the regulator ranges from 2.5 V in a standard regulator down to 0.1 V in low drop-out designs at low currents.
There is a time lag after the load changes until the feedback loop can correct for the changes and unless the regulator is properly designed the feedback loop may be unstable and cause damaging oscillations. This makes series regulators much more complex than parallel regulators.
It is rare to design voltage regulators from discrete parts except for special cases since integrated regulators today are cheap and high quality.
Switched regulators
Switched regulators are the most complex type but can achive a conversion efficiency of over 98%.
Buck
used the reduce a DC voltage to a lower DC voltage.
Boost
provides an output voltage that is higher than the input.
Buck-Boost (invert)
an output voltage is generated opposite in polarity to the input.
Flyback
an output voltage that is less than or greater than the input can be generated, as well as multiple outputs.
Push-Pull
A two-transistor converter that is especially efficient at low input voltages.
Half-Bridge
A two-transistor converter used in many off-line applications.
Full-Bridge
A four-transistor converter (usually used in off-line designs) that can generate the highest output power of all the types listed.
Primary switched
Secondary switched
IC Regulator
More sophisticated regulators have been made into discrete integrated circuits.
Calculations
To calculate the power dissipation in a voltage regulator IC, first determine the voltage drop across it. This will be the input voltage minus the output voltage. Then determine the current output either by measuring or other means. Then, plug it into the power equation:
Where "I" is the current output of the regulator IC, "V" is the volt drop across the IC, and P equals the watt dissipation. Units are in (of course) Watts, Volts and Amperes.
Common Voltage Regulation ICs
xx317 Series (KA317, LM317, etc...)
Usually adjustable from 1.2-37V by a simple outside resistor network.
78xx Series (7805,7809,7812...)
These are positive voltage regulators. The first two digits (78) indicate that it's a positive voltage regulator. The second pair indicates the output voltage. For example the 7809 would output 9V DC.
79xx Series (7905, 7909, 7912...)
These are negative voltage regulators. The "79" indicates it's a negative regulator and the second pair of numbers indicates it's voltage output. The only difference is that it regulates negative voltage.
Each of these ICs generally will output 1-1.5A maximum. However, there are more (and less) powerful voltage regulators on the market.
