Which full-wave rectifier scheme should I prefer for obtaining symmetric DC?

[dannyf]

Worth noting that, a pulse should be defined as 1/4 up, 1/2 down, 1/4 up.  Not 1/2 up, 1/2 down.

Is it worth noting that the two sequences are the same?

"1/4 up, 1/2 down, 1/4 up" + "1/4 up, 1/2 down, 1/4 up" + "1/4 up, 1/2 down, 1/4 up" + ...

= "1/4 up, " + "1/2 down, 1/4 up + 1/4 up" + "1/2 down, 1/4 up + 1/4up" + "1/2 down, 1/4 up + 1/4up" + ...
= "1/4 up, " + "1/2 down, 1/2 up" + "1/2 down, 1/2 up" + "1/2 down, 1/2 up" + ...

Also, for what the OP is trying to do, a half bridge would work.

[IanB]

Is it worth noting that the two sequences are the same?

Not the same. Look at the sequence of values.

In the first case the values are: 1/4, -1/4, 1/4, -1/4, ...

In the second case the values are 1/2, 0, 1/2, 0, 1/2, ...

In the first case the average (DC) current is 0.

In the second case the average current is 1/4.

A non-zero DC average current through a transformer core will magnetize it towards saturation.

[dannyf]

You need an L-input filter

It depends on what kind of load it is powering.

[David Hess]

I'm guess he's trying to build an inverter that outputs a nice sine wave. Though I can't immediately see how the negative going part of the 50Hz cycle will be achieved... Depending on the voltage and current levels required, it might be worth investigating class D amplifiers for this sort of thing.

I'm changing the output polarity to achieve the negative part with this method.

Page 11 of Linear Technology application note 18 shows this kind of output stage used to make an amplifier with a bipolar +/- 100 volt output.  A pair of SCRs are used after the 4 diode bridge rectifier so only 2 diodes are active at a time making it effectively a pair of switched full wave center-tap rectifiers.

[hkBattousai]

Signal timing diagrams for the PWM are as shown in this post. The circuit structure has greatly changed since I started this project though, the PWM signal format is still the same.

Sorry, I wrongly stated this detail. The signals are not the same. At the moment, the PWM signal is continuously generated by TL494 (TL494 is never turned off or disabled unlike in the block diagram I previously gave a link of), and the transformer is driven with that by using an H-bridge. The change in PWM duty cycle (50Hz) is much slower then the change in PWM frequency (50kHz), so the transformer is driven equal positive and negative pulses, so there is no DC flux inside the transformer core. The transformer output is rectified by a bridge rectifier, then filtered by an inductor and alternating polarities are formed by the polarity changer circuit. That's how I get the sine wave.

[T3sl4co1l]

Alright...

If that's how it is, then it's a proper forward converter, and as long as the PWM doesn't ramp up too quickly, you don't have to worry about transient imbalance.  You do still have to worry about systematic / DC offset (pulse width imbalance), which can be helped with a nice beefy coupling capacitor (half bridge is usually done by splitting the bypass caps, which also serves as coupling, so you don't need much extra).

Tim