Power
Supply
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There are several articles out there regarding the "Current Doubler" topology and describing the advantages of using it. Unfortunately, none of them (or at least none of those published in reputable magazines, proceedings, or written by well-known designers) are telling the whole story. This article is an attempt to give readers a much more accurate image of what actually happens when you decide to use a "Current Doubler" topology instead of a "Center Tap".
Myth: " ... operation on the primary side, including duty-cycle is unchanged ... diode and output capacitor stresses are identical to full-wave technique ... "
Reality:
-Identical operation on the primary side and identical stress on IDENTICAL output capacitors can not be achieved simultaneously. From the table and waveforms shown above it becomes clear that you have two major options (among many other), that you can choose from.
-One major option would be to preserve, relative to the "Center Tap", the same operation in the primary side (peak and RMS primary current). By doing that, you need the output inductors to have, each of them, an inductance 11 times higher than the "Center Tap" design! You have a benefit by doing this, the output capacitor can have 5 time lower capacitance than the "Center Tap", for the same OUTPUT VOLTAGE RIPPLE.
-Second option would be to preserve the SAME OUTPUT VOLTAGE RIPPLE, using the SAME OUTPUT CAPACITANCE as the "Center Tap". This would allow you to use a value for the output inductors twice the value used in the Center Tap topology, value that is suggested in most articles describing the Current Doubler topology. However, this approach will change drastically the current shape in the primary of the power transformer. In our example, the peak current changed by almost 50%. Someone may argue that this is not that bad. This is absolutely true, and actually may be beneficial in achieving soft switching. However, this kind of reasoning logically is fundamentally flawed, as one would not compare apple with apple. If you decide that a higher peak current in the primary side is acceptable (or desirable), you should go back, and analyze also a center tap topology with a higher ripple current. Therefore, we consider a MUST to compare Center Tap topology with the equivalent Current Doubler having the SAME primary currents. Otherwise we would compare apple with pears and not apple with apple.
Myth: " ... the current doubler working under same conditions as a full wave rectifier will reduce the copper loss in the transformer secondary by approximately 50% ... "
Reality:
-First, the 50% claim is for secondary copper loss, not for total transformer loss. In our example, if you maintained the same currents in the primary, the secondary copper loss decreased by approximately 35%. If you considered using the output inductors twice the value of the center tap configuration, the secondary loss decreased only by 26%, while the primary loss INCREASED by 14%! This is because, not preserving same input current waveshape, the rms value of the primary and secondary will increase.
-Second, the TOTAL MAGNETICS LOSSES may increase when you change the output stage topology from center tap to current doubler! In our example, the total magnetics losses increased by 27%, or 31.6% if you have not used the correct current doubler equivalent circuit.
-In our personal opinion, the secondary copper loss can realistically be reduced by approximately 35%, if you are using an "optimum design", using methodologies like the ones described in papers written by Dowell, Jongsma, Carsten, etc. This reduced secondary copper loss would result in a typical reduction of the total power transformer loss of 25%. This could mean A LOT, or NOTHING for a power supply efficiency, depending on the application!!!
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