Power Supplies
From MARS Wiki
Contents |
The Basics
Sources of Power
In short, you have two choices AC or DC. AC can come from the outlet in your house or from a generator connected to your bicycle tire; DC can come from a battery or some other storage device. In either case; there exist methods in which starting from either source type one can manipulate/transform the power into any number combination of V's and I's and v(t)'s and i(t)'s. That is what this tutorial is all about.
One thing to keep in mind: Pout is always less then Pin -> Pout < Pin Pout is related to Pin by the efficiency of the supply -> Pout = Pin * Efficiency
AC/AC Conversion
AC to AC conversion can be done most efficiently with a Voltage Transformer. In summary A transformer is two inductor that are placed close enough to each other that they can inductively couple energy through a magnetic field. The transformer usually lists its ratio of turns and thus one can figure out the voltage out given the voltage in or vise versa. A transformer can step up, step down, offset (e.g provide negative voltages),provide no change (circuit isolation), and provide multiple voltages levels via taps. The output current will always be set by the load. Transformers do have current ratings set by their internal resistance. Exceeding their limit will cause excessively I2R heating and eventually failure if not a few sparks.
Example to put a transformer into perspective
In case of a 1:2 ratio transformer, for every 1 volt I put into the primary I get 2V out. For every 1 amp that I pull out of the secondary I have to put in 2A assuming the transformer is 100% efficient (in reality there about 97%).
There are other ways of manipulating AC voltages like transistors used as amplifiers, but these methods are uncommonly used in power supplies and usually require some sort of power supply themselves.
AC/DC Conversion
Often refereed to as Rectification . And the following is pulled directly from the wiki site and as I feel can be stated no better.
Half-wave rectification
In half wave rectification, either the positive or negative half of the AC wave is passed, while the other half is blocked. Because only one half of the input waveform reaches the output, it is very inefficient if used for power transfer. Half-wave rectification can be achieved with a single diode in a one phase supply, or with three diodes in a three-phase supply.
Image:Halfwave.rectifier.en.png
Full-wave rectification
A full-wave rectifier converts the whole of the input waveform to one of constant polarity (positive or negative) at its output. Full-wave rectification converts both polarities of the input waveform to DC (direct current), and is more efficient. However, in a circuit with a non-center tapped transformer, four diodes are required instead of the one needed for half-wave rectification. (See semiconductors, diode). Four rectifiers arranged this way are called a diode bridge or bridge rectifier:
For single-phase AC, if the transformer is center-tapped, then two diodes back-to-back (i.e. anodes-to-anode or cathode-to-cathode) form a full-wave rectifier (in this case, the voltage is half of that for the non-tapped bridge circuit above, and the diagram voltages are not to scale).
Image:Fullwave.rectifier.en.png
A LP filter usually just a basic capacitor is added to the output of the rectifier to flatten the output closer to a DC level. There is still an associated ripple and usually a DC/DC converter or rectifier is added on to the end of this.
DC/DC Conversion
Sometimes referred to as a Regulator.
Linear Regulator
An inefficient method as simple as a resistor divider network or a zener diode in reverse bias. In which the unused power is dissipated over the regulator itself. Linear regulators being inherently inefficient in nature have low component counts and are usually utilized to save space and cut cost. They can be used efficiently if the supply power is close to the output. A nice trick is using a switcher to bring the supply down closer to the output and then slapping on a linear regulator to remove the ripple voltage from the charge/discharging switching reg.
Switching Regulator
A highly efficient method (>80%) of converting DC to another DC level that uses a switch(s) a.k transistors and energy storage devices (L's - C's). Again as in any case the LOAD always sets the current out of the supply unless you designed a smart power supply with current limiting capabilities.
Topologies Types
Buck - Step Down
Boost - Step UP
Buck-Boost - Step up and Down
SEPIC -
Flyback -
Push-Pull -
Etc etc
DC/AC Conversion
Commonly referred to as an Inverter. Basically an Inverter is an H-Bridge at heart with some LP filter to shape the waveform.
The Fun Begins
The 7805 Linear Regulator
Once we get the problem fixed with uploading I will upload the schematics to build.
