Thursday , October 18 2018

Full Wave Rectifier And Its Types AC/DC Converter

The DC level obtained from a sinusoidal input can be improved 100% using a process called full wave rectification. Although half wave rectifiers have some applications, the full wave rectifier is the most commonly used type in DC power supplies.

Full Wave Rectifier

A full wave rectifier allows unidirectional (One-way) current through the load during the entire 360° of the input cycle, whereas a half wave rectifier allows current through the load only during one half of the cycle. The result of full wave rectification is an output voltage with a frequency twice the input frequency that pulsates every half cycle of the input.

Full wave rectifier

The number of positive alternations that make up the full wave rectified voltage is twice that of the half wave rectifier voltage for the same interval. The average value, which is the value measured on a DC voltmeter for a full wave rectified sinusoidal voltage is twice that of the half-wave rectifier.

VAVG = 2Vp

VAVG is approx 63.7% of Vp for a full wave rectified voltage.

Full wave rectifier are further classified into two types

  1. Center-Tapped Full Wave Rectifier
  2. Bridge Full Wave Rectifier

Center-Tapped Full Wave Rectifier

A center tapped rectifier is a type of full wave rectifier that uses two diodes connected to the secondary of a center tapped transformer. The input voltage is coupled through the transformer to the center tapped secondary. Half of the total secondary voltage appears between the center tap and each of the secondary winding.

For a positive half-cycle of the input voltage, the polarities of the secondary voltages as below:

Full Wave Center Tapped positive cycle

In this condition D1 is forward-biased and D2 is reversed biased. The current path is through D1 and the load resistor RL as indicated.

For a negative half cycle of the input voltage, the voltage polarities on the secondary are as below:

Full Wave Center Tapped negative cycle

In this condition D2 is forward-biased and D1 is reversed biased. The current path is through D2 and the load resistor RL as indicated.

As the output current is during both the positive and negative portion of the input cycle in the same direction through the load. The output voltage developed across the load resistor is a full wave rectified DC voltage.

Effect Of The Turns Ratio On Output Voltage

If the transformer’s turn ratio is 1, the peak value of the rectified output voltage equals half the peak value of the primary input voltage less the barrier potential. Half of the primary voltage appears across each half of the secondary winding (Vp(sec) = Vp(pri)). In order to obtain an output voltage with a peak equal to the input peak, a step-up transformer with a turns ratio of n=2 must be used. The total  voltage across transformer secondary coil is twice the primary voltage (2Vpri), so the voltage across each half of the secondary is equal to Vpri.

In any case, the output voltage of a center-tapped full wave rectifier is always one half of the total secondary voltage less the diode drop, no matter what the turn ratio.

Vout = (Vsec/2) -0.7 V

Peak Inverse Voltage (PIV)

Each diode in the full wave rectifier is alternately forward-biased  and then reverse-biased. The maximum reverse voltage that each diode must withstand is the peak secondary voltage Vp(sec) .

Full Wave Bridge Rectifier

The bridge rectifier uses four diode connected as shown below:

Full Wave Bridge Rectifier

When the input cycle is positive D2 and D4 are forward biased and conduct current. A voltage is developed across RL that looks like the positive half of the input cycle. During this time D1 and D3 are reverse biased.

When the input cycle is negative, diode D1 and D3 are forward biased and conduct current. A voltage is developed across RL that looks like the positive half cycle. During the negative half cycle D2 and D4 are reversed biased.

A full wave rectified output voltage appears across RL as a result of this action.

Bridge Output Voltage

During the positive half cycle of the total secondary voltage, diode D2 and D4 are forward biased. Neglecting the diode drops, the secondary voltage appears across the load resistor. The same is true when D1 and D3 are forward-biased during the negative half cycle.

Vp(out) = Vp(sec) 

As, two diodes are always in series with the load resistor during both the positive and negative half cycles. If these diode drops are considered, the output voltage is

Vp(out) = Vp(sec) – 1.4 V

Peak Inverse Voltage (PIV)

The PIV of each diode (ideal) is obtained at the peak of the positive region of the input signal. For the indicated loop the maximum voltage across R is Vm and the PIV rating is defined as:

PIV = Vp(out)

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