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LT4320 working with Center-tapped Transformer for Split-rail PSU?
LT4320 working with Center-tapped Transformer for Split-rail PSU?
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Old 2nd November 2019, 07:18 PM   #1
nattawa is offline nattawa  Canada
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Join Date: Apr 2010
Location: Mississauga, Ontario
Default LT4320 working with Center-tapped Transformer for Split-rail PSU?

I searched and didn't come up with much more than "it doesn't work". So I did some home work.

LT4320 is called something like an "ideal rectifier bridge controller". But the Figure 2 waveform in the datasheet shows it is a bit less than what you would call "ideal", in that the bottom FETs conduct over most of the input AC voltage cycle, much longer than the duration the top FETs conduct to charge up the output capacitor for.

Apparently such behavior results in the negative node of DC output gets tied to the bottom AC input node most of an AC cycle, and if we were to hook up a center-tapped transformer as well as the split-rail output capacitors, C1, C2, as we usually would to construct a full-wave split-rail PSU, the bottom capacitor C2 would be shorted out by the transformer windings through the bottom FETs during their extra conducting time period (hatched). This appears to be why "it doesn't work".

However, if we could somehow gate the bottom FET drives in such a way that they conduct and cutoff in sync with that of the top FETs', therefore, eliminating the extra conducting time, we probably could make it to work with a center-tapped transformer and be able to make a split-rail PSU with one LT4320.

I came up with a circuit as attached, for a 2 x 24 Vrms input. LTspice simulation indicates it would work just fine.

The 72V maximum voltage rating of LT4320 limits the use of the circuit to up to about +/-35V output. For higher output voltages we are out of luck but have to use two bridges and a transformer with fully separate output winding leads. Nevertheless for preamps and class A power amps that most likely operate under +/-35V, and especiall when having a center-tapped transformer flying around, this circuit perhaps is useful in either saving an expensive LT4320 and the four extra MOSFETs going with it or saving buying a expensive power transformer.
Attached Images
File Type: png LT4320-WaveForm.png (88.7 KB, 61 views)
File Type: png LT4320-split-rail.png (48.6 KB, 63 views)
Attached Files
File Type: asc LT4320-SPLIT-RAIL.asc (5.0 KB, 2 views)
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Old 2nd November 2019, 10:44 PM   #2
Elvee is online now Elvee  Belgium
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Join Date: Sep 2006
If you need such discrete complication around the 4320 to end up being limited by voltage, why don't you take the plunge into full discrete: no more limitations, and you are not married to a specialty IC.

One or two sources for ideas, to brew your own, dedicated controller:
6.3 filament from 5vac winding
6.3vdc form 6.3v ac PSU
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