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Fullwave rectification verses Bridge retification and VA ratings

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LAN_403

Wavetuner

Fri May 22 2009, 05:50AM

Registered Member #1500 Joined: Sat May 24 2008, 04:38PM
Location: Ojai, Ca.
Posts: 44

I have been trying to get my head around the affect of using a center tapped transformer and a classic FW rectifier (half the duty cycle) and a fullwave bridge and clearly no center tap (excepting dual bridges). A: what is the voltage gain (smoothed) of a fullwave diode verses the SQR2*V of a bridge? B: How much does halving the duty cycle increase the effective VA of a power supply of a equivilent transformer. I don't think it is 50%, does anyone know?

Myke

Fri May 22 2009, 06:16AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

I think for a two diode full wave rectifier, I think that the transformer would need twice the wire as it need to have that other secondary winding connected by a tap. I also think that in both configurations, each diode sees approximately 50% of the full waveform. In the two diode version, each diode alternates being on and off and in the bridge version, they alternating being on and off in pairs. Remember that transformers can only output AC.

EDIT: I don't think I answered you question... sorry if I didn't answer any part of it.

Sulaiman

Fri May 22 2009, 05:39PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

From (my unreliable) memory;

In times when steel and copper were relatively cheap and
rectifiers were inefficient (e.g. selenium)
or expensive (e.g. valve(s) and heater transformer windings)
the cost effective choice was full-wave (dual winding, two rectifiers)

Now that rectifiers are cheap and metals are expensive
it is more efficient to use one winding with a full bridge rectifier.

The 'ideal' ratio of primary winding area to secondary winding area is different for
full-wave (more area for the primary 'window' area than the combined secondaries)
and full-bridge (equal pri & sec 'window'area)

Today's transformers are wound as if for full-bridge
(the primary usually requires a little more area than the secondaries
due to insulation and magnetisation current)

I think that with modern transformers you would have to de-rate to 70% of nominal power
(2/sqrt(2)) if you use full-wave, compared to the same two windings in parallel with a bridge.

Modern power supplies use transformers at high frequencies (16 to 100 kHz typ.)
to minimise the ammount of metal used saving cost , weight and volume.
Commonly a Power Factor Correction switchmode front end with a pwm regulator is used

A rule-of-thumb for transformers with full-bridge rectifiers into a capacitor and load
Vdc = 1.4 x Vac, Idc = 0.62 x Iac,

so, Pdc = 0.87 x Pac . e.g. A 100 VA transformer can give 87 Wdc
for nominal specifications such as temperature rise to be maintained.

Sorry, I tend to ramble on ...and on ... ... ..

Steve Conner

Sat May 23 2009, 12:32PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

This is an example of what (I think!) they call "Transformer utilization factor". This is equal to watts of DC out of the rectifier, divided by the VA rating of the transformer.

For a full-wave rectifier with two diodes and center-tapped winding, each half of the winding is only passing current half of the time. That's actually bad, not good, because it means that the RMS-to-average ratio of the winding current is higher. The RMS is what heats the winding, and the average is what produces the DC output. So, this means more heating for a given DC output than with a bridge rectifier: and hence a worse utilization factor.

I can never remember the exact values of TUF for the bridge and full-wave circuits, but it varies depending on what size of filter capacitors you use anyway. The bigger the caps, the worse it gets.

The voltage gain of a full-wave center tap circuit is either the same as a bridge, or one-half, depending on how you look at it. If you use a 70V AC transformer with a bridge, it rectifies to 100V DC. To get 100V DC with a FWCT rectifier, you need a 70-0-70 transformer.

Imagine you had a transformer with two identical 70V windings. You can connect it as 70-0-70 and use with a FWCT, or you can parallel the two windings and use with a bridge. Both give 100V DC, but which do you think will be most efficient?

The full-wave center tap is still used in low-voltage applications where the losses due to forward drops in the diodes are greater than the copper losses in the transformer. Going from bridge to full-wave increases the copper losses, but it cuts the diode losses in half.

You can then get your copper losses back by making the transformer somewhat bigger, which is no problem in a switchmode supply, as it's usually tiny anyway.

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