Jun 29, 2016

Full Bridge Rectifier



The full bridge rectifier is the most frequently used circuit for electronic DC power supplies. 

Generally we use AC device in many of the apparatus. We need AC source to operate those devices. We don’t have many devices which work on DC supplies. However with the expansion of electronics the DC source is leading its importance. Since these electronic devices gives an efficient method for AC to DC conversion. In an earlier days, we used synchronous converter and the process of conversion was ineffective. But now with the advancement of electronic devices, the diodes are used for the rectification process i.e., AC to DC to function the DC devices such as computer, battery charger, etc. All this is possible with the development of semiconductor technology. Rectifier is used, to convert the AC voltage waveform to a rectified voltage. We have other types of rectification methods that can also be used.


But the full bridge rectifier it does not need a centre tap transformer.  In the case of a full wave rectifier using a Centre tap Transformer, the centre tap may not provide an exact Centre tap and hence two input half waves may not be of equal sizes. Hence the adjacent pulses in the output waveform will be of unequal size. This is eliminated in the full bridge rectifier using four diodes.


Operation of Full Bridge Rectifier


Bridge Rectifier positive half cycles and negative half cycles
  • The circuit diagram of a bridge rectifier is shown in figure. It consists of four diodes D1, D2, D3 and D4 connected in the form of bridge ABCD.  Two leads A and C of the network are connected in the secondary coil and the other two leads D and B are connected to the load resistor RL.
  • Let V = Vm sin θ be the instantaneous sinusoidal voltage of frequency f(=50 hz) appearing across the secondary coil of transformer.
  • During the positive half - cycle the terminal A is positive with respect to C: The diode D1 and D3 are forward biased and act as shorts. At this instant the diodes D2 and D4 are reverse biased and hence acts as open. The current flows in the direction D1, B, RL, D, D3, C, and A figure (a) producing a voltage drop RL.
  • During the negative half-cycle the terminal A is negative with respect to C: The diode D2 and D4 are forward biased and the diode D1 and D3 are reverse biased. The current flows along C, D2, B, RL, D, D4, A and C (figure b) providing a voltage drop across RL.
  • Thus there is output voltage during both halves of the input cycle.  The rectifier is therefore a full-wave rectifier. The input voltage and output voltage waveforms are the same as that of full-wave rectifier. The output voltage is unidirectional, continuous but not constant.

Full Bridge Rectifier Advantages and Disadvantages

Advantages

  1. It does not require a transformer with centre tapped secondary.
  2. The output voltage is twice that of centre -tap rectifier for the same secondary voltage.
  3. The PIV rating need be only half that rating required for full- wave rectifier.
  4. It is suitable for high voltage applications.

Disadvantages

  1. It uses four diodes.
  2. Since two diodes in series are always carrying current, the voltage drop and power loss in diodes in the bridge circuit is more than that in the full - wave circuit. This factor assumes greater important in high voltage circuits.


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