SINGLE PHASE (1Ø ) BRIDGE RECTIFIER

WITH INDUCTIVE FILTER ON DC SIDE

By Zoran BilicNovember, 2011

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What is a rectifier ?

It is an electrical device that convertsalternate voltage and current (AC) into direct voltage and current (DC)

Difference between AC and DC Voltage

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Basic Circuit Schematic

Major circuit components:• AC Voltage Source• Diodes D1, D2, D3,

D4• Inductive Filter Ls• Load (consumer) R

Major circuit parameters• AC voltage E• AC current I• Unfiltered DC Voltage EKA

• Filtered DC Voltage Ed

• Filtered DC Current Id

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How does it work?

Diodes D1 to D4 are “semiconductor valves” that can conduct electric current in one direction only while they block it in the opposite direction

Diodes are connected in so called BRIDGE CIRCUIT

AC voltage E and rectified (unfiltered) voltage EKA waveforms

AC voltage E and DC voltage Eka.avi

Voltage EKA is not smooth Dc voltage, it contains strong “ripple”

DC voltage Eka.avi

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DC voltage EKA has an average value that is related to AC input voltage E

EKA (average) = 0.9 • E (Volts)

Also, peak value of EKA

EKA peak = √2 • E (Volts)

Example:

What would be average and peak value of EKA if AC input voltage E=120V?

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Solution:

EKA (average) = 0.9 • E = 0.9 • 120 = 108Vdc

EKA peak = √2 • E = √2 • 120 = 170Vdc

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Since the ripple (fluctuation) of the EKA is too high for many applications, there is a need for smooth ripple-less DC voltage

This can be achieved by use of inductive or capacitive filter located between EKA and load.

Without a filter DC current Id would be rippled

Current IdVoltage EKA

It also important to observe that AC current I at the input has sinusoidal shape

AC Voltage EAC Current I

DC current Id without filter.avi

AC input voltage E and Current I without filter.avi

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How does inductive filter work?

Inductor’s nature is to always oppose to any change in electrical current , including ripples of DC current Id

Installing sufficiently large inductor Ls, current ripples will disappear completely

Inductive filter LS installed on DC side of the rectifier

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Here we can observe waveforms for DC voltage EKA along with smoothed DC current Id

DC voltage EKA

DC current Id smoothed DC current Id.avi

Rippleless DC voltage Ed.avi

DC voltage EKA

DC voltage Ed

Consequently, output voltage at the load will be smooth, ripple- less equal to the average value of DC voltage EKA

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As a consequence of use of inductive filter, AC input current I will not be sinusiodail anymore.

AC Voltage EAC Current I

And that is unwanted consequence because creates another problem.

It will be rectangular shaped curve. This phenomenon is due to fact that smooth, flat DC current flows in DC side, and AC input side must provide that.

AC input voltage E and Current I with L filter.avi

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There is direct correlation between input AC current I and output DC current Id

Id(DC) = I (AC)

However input current I is still ALTERNATING (AC) current.

Example:

What would be input ac current value if load on DC demands 10 Amps of DC current?

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Solution:

I d(DC) = I (AC) = 10 Amps

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Summary

1. Rectifiers are electrical devices or circuits that convert alternating (AC) currents/voltages into direct (DC) currents/voltages

2. Diodes are semiconductor “valves” that provide rectification. They conduct current in one direction and block current flow in opposite direction

3. Rectified DC voltage and current are not smooth and without ripple unless we apply filters

4. Filters can be capacitive or inductive. Inductive filter smoothes rectified current ripple.

5. Consequence of inductive filter use is that input AC current is not sinusoidal any more. It becomes rectangular.