Full Wave Bridge Rectifier operation with Capacitor Filter

In the previous article, we have discussed a center tapped full wave rectifier, which requires a center tapped transformer and the peak output of the rectifier is always half of the transformer secondary voltage. Where the Bridge rectifier is the full wave rectifier with no such requirement and restriction. The average output of bridge rectifier is about 64% of the input voltage. The Bridge type full wave rectifier can convert an AC to DC by the mean of four diodes in such configuration that the output peak voltage remains equal to secondary of transformer peak. In each half cycle, a set of two diodes conduct and block the current alternately. In contrast to the center tapped rectifier the Bridge rectifier need four diodes instead of two diodes, which become expensive.

full bridge rectifier block diagram

bridge rectifier schematic :

As the name suggests, the configuration of four diodes connection forms a bridge. At the two corners of the bridge, the input AC voltage is applied and at the other two corners of the bridge, the output DC voltage is collected.

full wave rectifier circuit diagram

Operation of Fullwave Bridge Rectifier :

Positive Half Cycle:

During the positive cycle of the AC input, the upper corner of the bridge is comparatively positive where diode D1 and D2 are connected. In addition, the lower corner of the bridge is comparatively negative where diode D3 and D4 are connected.

In this situation, the diode D2 is forward biased as its anode is connected to comparatively higher potential and diode D1 is reversed biased as its cathode is connected to the comparatively higher voltage. Similarly, at the lower corner, the diode D3 is forward biased as its cathode is connected comparatively lower voltage and diode D4 is reversed biased as its anode is connected to the comparatively higher voltage.

For the positive cycle, the current flows from the upper corner of the bridge through diode D2, then through the load resistor from point a towards point b and diode D3, completing its path to the lower corner.

full bridge rectifier current flow during positive half cycle

Negative Cycle:

During the negative cycle of the AC input, the upper corner of the bridge is comparatively negative where diode D1 and D2 are connected. In addition, the lower corner of the bridge is comparatively positive where diode D3 and D4 are connected.

In this situation, the diode D1 is forward biased as its cathode is connected to comparatively lower potential and diode D2 is reversed biased as its anode is connected to the comparatively lower voltage. Similarly, at the lower corner, the diode D4 is forward biased as its anode is connected to a comparatively higher voltage and diode D3 is reversed biased as its cathode is connected to the comparatively higher voltage.

For the negative cycle, the current flows from the lower corner of the bridge through diode D4, then through the load resistor from point a towards point b and diode D1, completing its path to the higher corner.

full bridge rectifier current flow during negative half cycle

Note that during both cycles, the current flow in the load is from point a towards point b and the current is unidirectional like DC rather than AC.

Average Output of the Bridge rectifier:

For a complete input sinusoidal cycle, the output of the center tapped rectifier repeat twice. In other words, the cycle the time-period of output is $\pi $ instead of $2\pi $. So, the average of the output waveform will be

Full wave bridge rectifier average output waveform

$v_{avg}=\frac{V_{p}}{\pi }(\int_{0}^{\pi }{\sin t dt} )$

$v_{avg}=\frac{V_{p}}{\pi }(2)$

$v_{avg}=\frac{2V_{p}}{\pi }$

$v_{avg}=0.637 V_{p}$

Peak Inverse Voltage of the Bridge rectifier :

Consider a positive half cycle, where D2 and D3 are forward biased and D1 and D4 are reversed biased. Peak inverse voltage appears across the diode D1 and D4. The inverse voltage across the diode D4 can be determined by applying KVL at the loop

$v_{p}-PIV_{D4}-0.7v=0$

$PIV_{D4}=v_{p}-0.7v$

Ripple factor of the rectifier:

Ripple factor shows the effectiveness of the filter and defined as

$r=\frac{v_{r(pp)}}{v_{dc}}$

Filter Output of Full wave bridge rectifier and ripple factor calculation

Where v$_{r(pp)}$ is the ripple voltage (peak-peak) and v$_{dc}$ value of the filtered output. The formulas for v$_{dc}$ and v$_{r(pp)}$ is given below

$v_{r(pp)}=(\frac{1}{fR_{L}C})(\frac{v_{p(s)}}{2}-0.7)$

$v_{dc}=(1-\frac{1}{2fR_{L}C})(\frac{v_{p(s)}}{2}-0.7)$

Notice the output waveform of the rectifier that the frequency the output voltage is twice of the input voltage.

Conclusion :

  • Bridge rectifier converts both halves of the AC input cycle into DC output
  • The rectifier uses four diodes that’s why it is considered expensive
  • The average output of the Bridge rectifier is twice that of half wave rectifier
  • The ripple voltage is less than that of half wave rectifier
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