Center Tapped Full Wave Rectifier

 

What is a Center Tapped Full Wave Rectifier?

A center tapped rectifier is a type of full wave rectifier circuit configuration that converts full waveform of AC signal into DC signal. This type of rectifier circuit configuration uses two power semiconductor switches and a center tap transformer, thus named as center tapped rectifier.

 

Circuit Diagram of Centre Tapped Full Wave Rectifier

The circuit diagram for the center tapped rectifier is shown in the given figure. In this figure, we see that the two semiconductor switches S₁ & S₂ are connected in such a way that the one terminal of the both switches are connected across the two terminals of the transformer’s secondary winding (A & B) and the other terminal of both the switches is tapped at the center of the secondary of the transformer via the load.

The transformation ratio of the transformer used in this circuit is 1:2. So that when the load is taped at the center of the secondary of the transformer then the transformation ratio of each half secondary winding with the primary winding is 1:1.

 

Types of Center Tapped Rectifier Full Wave Rectifier

A center tapped rectifier circuit can be designed by using any semiconductor switch that has unidirectional current conduction property. These semiconductor switches are classified as controlled switches and uncontrolled switches. So, based on the type of switch used, center tapped rectifiers can be categorized into uncontrolled center tapped rectifiers and controlled center tapped rectifiers.

 

Uncontrolled Center Tapped Full Wave Rectifier

Uncontrolled center tapped rectifiers are designed by using only uncontrolled switches such as diodes. In this type of rectifier, we can’t control the output voltage waveform, as they provide fixed DC voltage from a fixed AC source. 

The circuit diagram for uncontrolled center tapped rectifiers is shown in the given figure.

Controlled Center Tapped Rectifier Full Wave Rectifier

Controlled center tapped rectifiers are designed by using only controlled switches such as SCRs, MOSFETs etc. This type of rectifier provides variable DC voltage from a fixed AC source. In this type of rectifier, we can control the output voltage waveform by controlling the conduction of the switch used in the circuit. The conduction of these switches can be varied by their respective parameters, such as in the case of SCRs their conduction can be controlled by varying their firing angle.

 

The circuit diagram of controlled center tapped rectifiers is shown in the given figure. Here we are taking the example of a center tapped rectifier using SCRs.

How Center Tapped Full Wave Rectifier Works?

For sake of simplicity, the switches used D₁ & D₂ or SCR₁ & SCR₂ are considered to be ideal and the load is resistive. By ideal we mean that the reverse recovery time and the forward voltage of the switches are negligible.

 

Let us say the AC voltage applied to the primary winding of the transformer is V_s = V_m . sin(ωt). The given expression of the AC voltage can be illustrated as in the given figure.

The first half of the AC waveform i.e. from 0 to π, is considered the positive half because during this time the magnitude of the waveform is positive. Conversely, the second half of the waveform i.e. from π to 2π is considered the negative half due to its negative magnitude.

 

So, to simplify the explanation of the working of center tapped rectifiers, we’ll divide the discussion into two parts: during the positive half and during the negative half of the AC input supply.

 

During the positive half of the AC waveform i.e from 0 to π, the terminal A of the transformer is positive with respect to B. As a result the diode D1 in the uncontrolled rectifier or SCR1 in the controlled rectifier become forward biased, allowing them to support current conduction. Meanwhile, the diode D2 or SCR2 is reverse biased, blocking the current conduction. 

 

As diode D1 is forward biased, it will immediately start current conduction. However, in the case of SCR we have to apply the triggering method of SCR to turn on the SCR so that it will start current conduction.

 

During this time period the direction of current flows can be visualized with the help of a given figure.

In this case the reverse based switches D₂ or SCR₂ experience an inverse voltage of magnitude 2V_s. 

 

Applying KVL in the bot the loops of the rectifier circuit

 

V_a  -  V_D1  +  V_L   =   0

 

V_b  +  V_L  -  V_D2  =  0

 

V_D1  =  0 as discussed earlier the switches used in this circuit are considered ideal mean forward voltage drop V_d = 0

 

After rearranging, we get

 

V_D2   =  V_b  -  V_a   (  V_b   =  - V_m . sin(ωt),      V_a   =  V_m . sin(ωt))

 

V_D2  =  -2.V_m . sin(ωt).

 

So, by observing the above expression of the voltage across the V_D2 we can conclude that the maximum or peak voltage across the V_D2 is -2V_m , negative sign indicates the polarity of the magnitude.

 

 

Similarly, during the negative half of the AC waveform i.e. from π to 2π, the terminal B of the transformer is positive with respect to A AS a result the diode D2 in uncontrolled rectifier or SCR2 in the controlled rectifier is forward biased, therefore, they support current conduction. Meanwhile, the diode D1 or SCR1 is reverse biased hence they block the current conduction. 

During this time period the direction of current flows can be visualized with the help of a given figure.

Similarly to the previous case, in this case also the reverse based switches D1 or SCR1 experience an inverse voltage of magnitude 2Vs. 

 

Applying KVL in the outer loop of the rectifier circuit

 

V_a  +  V_L  +  V_D1  =  0  

V_b  -  V_D2  +  V_L  =  0

 

V_D2  =  0 as discussed earlier the switches used in this circuit are considered ideal mean forward voltage drop V_d = 0

 

After rearranging, we get

 

V_D1   =  V_a  -  V_b   (  V_b   =  - V_m . sin(ωt),      V_a   =  V_m . sin(ωt))

 

V_D1  =  -2.V_m . sin(ωt).

So, by observing the above expression of the voltage across the V_D2 we can conclude that the maximum or peak voltage across the V_D2 is 2V_m , negative sign indicates the polarity of the magnitude.

Output Waveform of Center Tapped Full Wave Rectifier

After the above detailed discussion, we can conclude that the output waveform of the center tapped rectifier is shown as the given figure.

Important Formulas of Center Tapped Full Wave Rectifier

The average value, RMS value and the Ripple Factor of the output voltage waveform of both the full wave rectifier circuit configuration i.e. center tapped rectifier and bridge rectifier are the same refer to full wave rectifier formulas to get more details.

Peak Inverse Voltage (PIV) of Center Tapped Rectifier

Peak inverse voltage is the maximum voltage that a switch can withstand during its blocking state. As we discussed earlier in working of center tapped rectifiers, during the both halves of the AC waveform, the reverse biased switch blocks the conduction and is subjected to a maximum voltage of -2V_m. So, the PIV of the switches used in the center tapped rectifier is 2V_m.