hello everyone..i am doing my final year project, and my project title is semicoductor tap changer for power transformer.as shown in the picture, the output from the full wave reticfier is distorted.what action i should take to solve this problem?thank you very much....from: ching shi

 

hello everyone..i am doing my final year project, and my project title is semicoductor tap changer for power transformer.as shown in the picture, the output from the full wave reticfier is distorted.what action i should take to solve this problem?thank you very much....from: ching shi

Looks like a perfectly good full wave rectifier output to me....what's the problem?

 

I think he's talking about the "flat spot" around the zero crossing caused by the Vf of the diodes in the bridge rectifier.

If you want to get away from that "flat spot", you could either use a synchronous rectifier circuit (uses power MOSFETs as "ideal diodes") or a precision rectification circuit.

Some ideas: http://en.wikipedia.org/wiki/Precision_rectifier
Precision FW rectification circuit on EDN: http://www.edn.com/contents/images/6250014.pdf
Another article on precision rectification: http://www.arrl.org/tis/info/pdf/0707053.pdf
(but it uses 741 opamps - I'd use something better, like an LF353, TL072 or perhaps an LT1057)
A paper on synchronous rectification: http://scholar.lib.vt.edu/theses/available/etd-173510281975580/unrestricted/chapter2.pdf
Google search: http://www.google.com/search?hl=en&q=precision+rectifier+circuit (84,000 hits)

 

It still won't be perfect, bu their is also Schottkey Diodes. They drop a less forward voltage.

 

BAT54's have among the lowest Vf, if you don't pass much current through them. 100uA will give a max Vf of around 240mV. Basically, the less current through, the lower the Vf. If our OP substituted BAT54's for his bridge, the "flat spot" would narrow considerably, but wouldn't go away completely.