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Introduction to Power Supply and Rectifier Circuits !

The ac power supply coming from the wall outlet is not suitable for every electronic device. For devices which work with dc current, a suitable voltage must be extracted from the available source. Besides "converting" an ac voltage into dc, power supplies must maintain voltage at a constant value independently of the load to which it is applied. In the picture shown below, a dc power supply which takes its input from the wall outlet but provides 5 volts dc at the output is shown.

dc-analog-power-supply

 

Power supply for a CD/DVD player:

In a CD or DVD player you'll find various noise sources that reduce the performance of the player. Noise in its circuitry is caused by the switching of logic circuitry, mechanical components like motors or transformers,  servo and switching logic, environmental noise coming from outside sources or from unstable and noisy operating voltages provided by the power supply. All these sources influence the performance of the player, i. e. they contribute to jitter on the master clock oscillator, which is the component responsible for generating precise clock signals needed for the proper functioning of the digital devices. As a consequence, the demands on a power supply for a digital player are various. It must keep up with the fast switching speeds (around 16 million times per second!) needed by these circuits, otherwise, the performance will be compromised. Any fluctuations in the power supply will cause variations in logic level switch points and thus variations in switch timing, and the circuit components might lose synchronization.

A solution against this problem could be a separate, highly stable, high-speed and quiet power supply which isolates any noise from the player and provides stable operating voltages to the different analog and digital sections of the player.

  • High-current, quiet transformer with low mechanical hum and low stray magnetic fields

  • High-power, ultra-fast diodes resulting in a rectifier with lower noise and lower switching losses

  • 10,000 uF filter capacitors with very low series resistance for reducing ac ripple (noise) prior to the regulation stage

  • 12 VDC regulated output with noise cancelation which uses a precision ultra-low noise voltage reference

Diode Rectifiers: 

Rectifiers built with diodes are an inexpensive and simple way to convert an ac input into dc in an uncontrolled manner (as opposed to controlled power electronic systems operated without a line-frequency transformer but with a high-frequency one). In most applications, the rectifiers are supplied directly from the utility source without a 50 - 60 Hz transformer. The avoidance of the usually costly and bulky transformer is important in most modern power electronics systems. Among some other applications of line-frequency diode rectifiers are:

  • switching dc power supplies
  • ac motor drives
  • dc servo drives

The dc output voltage of a rectifier should be as ripple free as possible. Therefore, a large capacitor is connected as a filter on the dc side. The capacitor gets charged to a value close to the peak of the ac input voltage. As a consequence, the current through the rectifier is very large near the peak of the input voltage sine wave and it does not flow continuously. So even though diode rectifier circuits are simple and often applied, their main disadvantage is that they draw high current from the utility. For applications requiring very stable input currents, simple diode rectifiers should not be used. This is due to the fact that they produce too high current distortions.

 

ac-dc-diode-rectifier-circuit-with-capacitor-filter

Rectifiers can be classified according to the waveform they rectify into:

  • half-wave rectifiers
  • full-wave rectifiers

and also according to the type of input they rectify into:  

  • single-phase rectifiers
  • three-phase rectifiers

  

Voltage Regulators:

Voltage regulators provide a constant dc output voltage that is practically independent of the input voltage, output load current, and temperature. The voltage regulator is one part of a power supply, as you can see on the diagram at the top of this page. The input voltage at the regulator comes from the filtered output of a rectifier derived from an ac voltage or from a battery in case of portable systems. Most voltage regulators fall into two categories:

  • linear regulators (containing the subcategories linear series and linear shunt regulators)
  • switching regulators (containing the subcategories step-down, step-up, and inverting)

ac-dc-diode-rectifier-circuit-with-volrage-regulator

An example of a positive linear regulator, namely LM7812CT, is shown in the diagram above. Linear regulators are available either for positive or for negative output voltages or even available in dual form, which means they provide both positive and negative outputs. In this course we will work with a positive, linear, series regulator.

 

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