Power Supply Circuits Primer & Tutorial Includes:
Power supply electronics overview Linear power supply Switch mode power supply Over-voltage protection PSU specs Digital Power Power management bus: PMbus
Power supplies are an important element in many items of electronics equipment. While some are battery driven, others need mains power supplies, and the power supply electronics circuitry and design is of paramount importance to the successful operation of the whole equipment.
Power supply electronics circuits can be split into a number of sections or building blocks. Each is important to the operation of the power supply as a whole, but each section of the power supply electronics is required to perform its function satisfactorily for the successful operation of the whole unit.
WARNING!: Many power supplies will contain mains or line voltages which can be hazardous. Extreme care must be taken when dealing with these circuits, as electric shocks could be fatal. Only qualified personnel should deal with the internal circuitry of power supply electronics circuits.
Types of electronics power supply
There are three main types of power supply that can be used. Each ash their own advantages and disadvantages and as a result each is used under slightly different circumstances.
The three major types of electronics power supply are:
- Rectified and smoothed power supply: These electronics power supplies are the simplest types, and are generally used for non-critical applications where performance is not a major issue. This type of power supply was widely used in thermionic valve or vacuum tube equipment as it was not so easy to regulate supplies, and often the requirements were not so critical.
- Linear regulated power supply: This form of electronics power supply is able to provide a very high level of performance. However the fact that it uses a series regulator element means that it can be comparatively inefficient, dissipating a significant proportion of the input power as heat. Nevertheless these power supplies can offer very high levels of regulation with low values of ripple, etc. . . . . Read more about Linear Power Supplies.
- Switch mode power supply: In this form of power supply, electronics circuits use switching technology to regulate the output. Although spikes are present on the output, they offer very high levels of efficiency and in view of this they can be contained in much smaller packages than their linear equivalents. . . . . Read more about Switch Mode Power Supplies, SMPS.
The different types of power supply are each used for different types of application according to their advantages. As such they are all widely used, but in different areas of electronics.
Each type of building block and power supply is covered in greater detail on other pages on this website. Links to these pages can be found on the left hand side of the page below the main menu in the "Related Articles" section.
Major power supply electronics blocks
A power supply can be split into a number of elements, each providing a function within the overall power supply. Naturally these areas can be rather arbitrary, and may vary slightly dependent upon the actual power supply design, but they can be sued as a rough overall guide.
- Power input filtering: It some instances it is necessary to ensure that spikes from the power line do not enter the power supply, and that noise that might be generated by the power supply does not enter the power lines. To achieve this circuitry to remove noise and limit the effects of incoming spikes is placed at the input to the power supply. In many cases any filtering at this point is quite minimal, although for specialist supplies more complicated circuits may be used.
- Input transformer: If a power supply using mains / line supplies of 110 or 240 volts AC is used, then the input usually has a transformer to transform the incoming line voltage to required level for the power supply design.
- Rectifier: It is necessary to change the incoming AC waveform to a DC waveform. This is achieved using an AC rectifier circuit. Two types of rectifier circuit may be used - full wave and half wave rectifiers. These effectively block the part of the waveform in one sense and allow through the part of the waveform in the other sense.
Note on Diode Rectifier Circuits:
Diode rectifier circuits are used in many areas from mains power supplies to radio frequency demodulation. The diode rectifier circuits use the capability of the diode to only pass current in one direction. There are several varieties from half wave to full wave, bridge rectifiers, peak detectors and more.
Read more about Diode Rectifier Circuits
- Rectifier smoothing: The output from the AC rectifier circuit consists of a waveform varying from zero volts to 1.414 times the RMS input voltage (less any losses introduced by the rectifier). In order that this can be used by electronics circuits, it needs to be smoothed. This is achieved using a capacitor. It will charge up over part of the cycle and then as the voltage falls it will supply the current to the circuit, charging up again as the voltage rises. . . . . . Read more about Capacitor Smoothing Circuits.
- Regulation: Even after the rectified voltage has been smoothed, there may still be significant levels of residual hum. Also the voltage will vary as different levels of current are drawn. To provide a stable voltage output from the power supply with little residual hum and noise a voltage regulator circuit is required. Regulators are able to provide a stable voltage at a set or variable level dependent upon the requirement. This may use either linear or switching mode techniques to bring the output voltage to the required level.
- Over voltage protection: In the event of the failure of the regulator it is possible under some circumstances that the output voltage from the power supply could rise to a level that could damage the circuitry being powered. To prevent this occurrence over-voltage protection circuitry can be used. This circuit element detects the level of the output voltage and if it starts to rise above its acceptable limits it will trip, removing the supply from the regulator and usually clamping the output from the regulator to zero volts, thereby protecting the remaining circuitry from damage.
Not all of these power supply electronics building blocks are used in every power supply. Most will have a transformer, smoothing and a regulator, but the other elements may or may not be included dependent upon the specification.
Power supply specifications
When buying or selecting a power supply it is necessary to check the specifications and understand what they mean. Everything from the volatge and current ratings, to ripply, load regulation, input voltage regulation and the like.
. . . . Read more about Power Supply Specifications & Parameters
Power supplies, whether just smoothed, regulated using a linear or switch mode regulator are widely used. The switching regulator approach is used most widely, especially in computers, and very many other items of electronic equipment. Many ICs are available for the function and they are light, efficient and very cost effective.
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