Full Wave Bridge Rectifier vs.centre tap rectifier

Question

Why does the DC output in bridge rectifier higher than centre tap rectifier.

Answer 1

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. (a) A single winding with full-wave bridge rectifier. (b) A centre-tapped transformer with rectifier diodes.

• I am assuming that the centre-tapped transformer has twice the number of turns as the single-winding transformer. e.g., If (a) is a 0 - 12 V transformer then (b) will be a 12 - 0 - 12 V transformer.
• If you trace the current flow in (a) when the dot end is positive you will see that the current will flow through D2, out to the load via V+, back on V- and through D3 back to the secondary. Each diode will create a voltage drop of 0.7 V so the voltage out will be down about 1.4 V.
• You can see in (b) that when the dot end is positive that D5 will conduct and the return will be straight to the centre-tap. The voltage drop will be 0.7 V approx.

The centre-tapped version is more efficient but probably more expensive to make.

Note that the XFMR2 wire gauge can be lighter than XFMR1 as it only carries current on alternate half-cycles. Total weight of copper will be about the same.

Answer 2

You can get more current from a full-wave rectifier/capacitor than a CT transformer rectifier because each half of the transformer secondary is used 100% of the time rather than 50%. You can generally get about 25% more DC current from the same transformer-rectifier-capacitor circuit by using 4 diodes and ignoring the CT rather than two diodes.

In the below circuit XFMR1 is rated for the same VA as XMFR2 but double the voltage CT.

^{simulate this circuit – Schematic created using CircuitLab}

You do lose one more diode drop in voltage with the full wave bridge.

Answer 3

The full bridge is a voltage doubler circuit with half the current for the same VA rating as it uses the full output winding in each charge pump.

The centre tap XFMR uses a half bridge which does not double voltage but doubles current, so this is used in PC ATX forward converter outputs for more current.

If a transformer has a 10:1 turns ratio with 324Vp input or 230V rms and only some VA power rating. Then with that tapped transformer you can get twice the voltage or half the current for the same VI product.

Basically the half bridge only sees half the secondary windings

But the transformer flux in VA limited in rating, so you can get twice the output current at half the voltage or visa versa.

Scope shows peak max and min. Here I changed the transformer ratio to 5:1