Bridge Rectifiers all Concepts

Rectifiers are the electronics circuits or devices that convert AC voltage into DC voltage and the process of conversion of AC voltage into DC voltage is known as rectification. These are the most important circuits in modern electronics because DC power is the fundamental need of almost every electronic device or circuit. Although there are various DC power sources available, these are limited and expensive.

Majorly, there are two types of rectifiers that are half wave rectifiers and full wave rectifiers. Half wave rectifiers offer half wave rectification, which means the conversion of half of the AC waveform into DC signal, whereas full wave rectifiers offer full wave rectification which means the conversion of full waveform of the AC signal into DC signal. For the conversion of full waveform of AC signal into DC signal a full wave rectifier uses two types of circuit configurations that are center tapped rectifier and bridge rectifier.

Among all of these rectifiers bridge rectifiers are the most popularly used rectifiers, that’s why they need separate attention to be discussed. 

In this article, we will cover all the essential concepts of bridge rectifiers. If something is missed there, please do let us know in the comment section.

 

Bridge Rectifier

A Bridge Rectifier is a type of full wave rectifier circuit configuration in which four power semiconductor switches are connected in a bridge-like arrangement. These rectifiers are also known as full wave bridge rectifiers and full bridge rectifiers. 

A full bridge rectifier converts the full waveform i.e. both positive half and negative half of the AC waveform into a DC signal. As this circuit utilizes both halves of the AC supply, it provides higher average and less ripple output voltage waveform compared to half wave rectifiers. 

Bridger rectifiers are the most efficient rectifiers among all the rectifier circuits and are most popularly used once.

Bridge Rectifier’s Circuit Diagram

In the bridge rectifier circuit four power semiconductor switches are connected in a bridge-like arrangement in the same orientation as shown in a given figure. 

In the given figure, you can see four switches S₁, S₂, S₃ and S₄ are connected in a bridge-like arrangement. The AC voltage source V_s =V_m sinωt is connected between two diagonally opposite terminals a & b of this bridge arrangement and the load is connected between the other two diagonally opposite terminals c & d.

The above given schematic of the bridge rectifier can be redrawn as shown in the given figure.

Bridge Rectifier Working

In the bridge rectifier circuit, two diagonally opposite switches conduct current during the positive half of the AC supply, while the other two diagonally opposite switches conduct current during the negative half of the AC supply.

Let us understand this in detail with the help of the circuit diagram. To make it easy to understand we will refer to the bridge rectifier circuit with a resistive load and consider that the components used in the circuit are ideal.

As we know, unlike a DC signal, the AC voltage waveform is sinusoidal in nature, as shown in the given figure, and can be expressed as V_s =V_m sinωt. The magnitude of this waveform changes continuously with time, it is positive for the 0 to π, called positive half and it is negative for 0 to 2π called negative half. After 2π, this waveform repeats itself, means the fundamental time period of this waveform is 2π.

Where,

• V_s        =         Instantaneous Value of the applied voltage waveform
• V_m       =         Peak Value of the applied voltage waveform
• ω        =        Angular frequency
• t          =         time
• V_sr      =          RMS Value of the applied voltage waveform

During the positive half of the AC supply, the anodes of the switches S₁ and S₃ are positive with respect to their cathodes, therefore, these switches become forward biased and start current conduction. Meanwhile, the anodes of the switches S₂ and S₄ are negative with respect to their cathodes therefore, these switches become reversed biased and do not conduct the current. During this period, the direction of the current flow is as shown in the given figure, and the voltage across the load is the same as the source voltage.

During the negative half of the AC supply, the anodes of the switches S₂ and S₄ are positive with respect to their cathodes, therefore these switches become forward biased and start current conduction. Whereas, the anodes of the switches S₁ and S₃ are negative with respect to their cathodes therefore, these switches become reversed biased and do not conduct the current. During this period, the direction of the current flow is shown as the given figure, and the voltage across the load is the negative of the source voltage.

After the above discussion we can successfully conclude that the output waveform for one complete cycle of the input is as shown in the given figure.

The output waveform of the bridge rectifier given above is valid only if the bridge rectifier is designed by using only uncontrolled switches such as diodes. If a bridge rectifier circuit employs controlled switches or the combination of both controlled and uncontrolled switches then these output waveforms will differ (discussed further). However, the basic principle of working is the same as discussed above.

Types of Bridge Rectifier

A bridge rectifier circuit can be designed by using only un-controlled switches, only controlled switches or the combination of both. Based on the type of switches used, a bridge rectifier circuit can be classified as controlled bridge rectifier and uncontrolled bridge rectifier and half controlled or semi controlled bridge rectifier.

 

Uncontrolled Bridge Rectifier

A bridge rectifier circuit that employs only uncontrolled switches such as diodes, is known as uncontrolled bridge rectifier. This type of rectifier provides fixed DC output voltage from a fixed AC source. In these rectifier circuits we can’t control the DC output voltage waveform.

Here, we are taking the example of a bridge rectifier using diodes.  A diode is an uncontrolled switch in which we cannot control its conduction state or blocking state. The circuit diagram for the bridge rectifier circuit using diodes is shown in the given figure.

The working and the output waveform of these rectifiers are exactly the same as discussed above.

Full Controlled Bridge Rectifier

Bridge rectifier circuits that use only controlled switches are known as controlled bridge rectifiers. These rectifiers are commonly known as full converters. In these circuits, we have full control over the DC output waveform therefore, we can get variable DC output voltage from a fixed AC source. 

By “full control” we mean that we can get both positive or negative polarity DC output voltage from a fixed AC source, while the direction of current flow remains the same due to the unidirectional conduction nature of switches used. This property makes these rectifiers operate in two quadrants, one is where both voltage and current are positive and other is where voltage is negative and the current is positive. This two-quadrant operation allows these rectifiers to act as an inverter, while charging the battery.

Here we are taking the example of a bridge rectifier circuit using SCR. A SCR (Silicon Controlled Rectifier) is a controlled switch in which we can control its conduction state by controlling the gate current or firing angle. 

The circuit diagram for bridge rectifiers using SCR is shown in the given figure.

As we discussed above, the basic working of bridge rectifiers is the same for all types of bridge rectifier circuit. The only thing to consider here is that SCRs do not conduct even when they are forward biased, they only start conduction after applying the gate current or any other triggering methods.

So, during the positive half of the AC supply, SCR₁ & SCR₃ are forward biased, but they do not start conducting the current because they remain in forward blocking mode (refer to the working of SCR). To start the current conduction, we need to apply a triggering method of the SCR.  Once, the triggering method is apple to SCRs, they will immediately start conducting the current. During this period, the voltage across the load follows the input voltage after SCRs start conduction.

Similarly, during the negative half of the AC supply SCR₂ & SCR₄ are forward biased and they start conducting the current immediately after applying any triggering method. During this time period the voltage across the load is the negative of the voltage applied.

The output waveform of these rectifier circuits is shown in the given figure.

In the above figure, we clearly see that by varying the firing angle α of SCR, we can vary the output voltage waveform consequently, its average and rms values.

 

Half Controlled Bridge Rectifier

A bridge rectifier circuit that is designed by using the combination of both controlled switches and uncontrolled switches is called half controlled bridge rectifier. These are also known as semi controlled bridge rectifiers or semi converters.  

As the name implies, in this type of rectifier we have limited control on the DC output voltage waveform. These rectifiers provide only the positive polarity DC output voltage from a fixed AC source, although it is variable.  

A half-controlled bridge rectifier offers one quadrant operation, in which the polarity of voltage and current is positive. Therefore, these rectifiers cannot act as inverters. 

Here we are taking the example of a bridge rectifier circuit using the combination of the SCRs and diodes.

The working and the output waveform of these rectifiers for resistive load is similar to the controlled bridge rectifiers.

 

Output Waveforms of Bridge Rectifier 

After the detailed discussion on the working of bridge rectifiers, we can conclude that the output waveforms of the bridge rectifiers are shown in the given figure.

Important Formulas of Bridge Rectifier

Average or DC Output Voltage Formula of the Bridge Rectifiers

• For Uncontrolled Bridge Rectifier or Diode Bridge Rectifier

• For Full Controlled Bridge Rectifier or Bridge rectifier using SCRs

• For Half Controlled Bridge Rectifier or Bridge rectifier using the combination of both diodes and SCRs

 

RMS value of the Output Voltage of the Bridge Rectifiers

• For Uncontrolled Bridge Rectifier or Diode Bridge Rectifier

• For Full Controlled Bridge Rectifier or Bridge rectifier using SCRs

• For Half Controlled Bridge Rectifier or Bridge rectifier using the combination of both diodes and SCRs

Advantages of using Bridge Rectifier

• Bridge Rectifiers are the most preferred rectifiers among all the rectifiers due the advantages listed below.

• Bridge rectifiers utilize both halves of the AC input waveform. Therefore, these rectifiers provide higher average output voltage consequently, have higher efficiency. 

• In bridge rectifiers ripples in the output waveform are less as compared to half wave rectifiers resulting in less filtering requirements. 

• Unlike a center tapped rectifier circuit, a bridge rectifier does not require a center tapped transformer, which is bulkier and costly.

• The switches used in the bridge rectifier circuit have half PIV (Peak Inverse Voltage) as compared to center tapped rectifiers. High PIV switches are expensive therefore, bridge rectifier circuits are more cost effective than center tapped rectifiers.

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