knowledge, this is the first article (and the only one to
date), to realistically compare the Center Tap power supply topology
with the Current Doubler equivalent using waveforms and
numbers, and not "feelings" or "qualitative"
explanation of what is happening. In this article, the
reader will find actual calculations and simulation
waveforms of main parameters.
It is unfortunate that far too many
technical articles are concentrating on an extremely
narrow aspect of a particular problem, ignoring that the
problem is part of a much wider picture. It is true that,
in order to analyze a phenomenon, an engineer, a
scientist, or a doctor must break a problem in smaller
pieces, being easier to handle and analyze. But ignoring
the bigger picture is misleading the reader.
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".
We will consider, for our analysis, the DC/DC section of the following switch mode power supply:
- fsw=200kHz (we will
use as a definition of the switching frequency, the frequency "seen" by the power/isolation
- Duty Cycle=0.793
Three options will be
- "Center Tap"
Doubler", with the output inductors value
chosen to preserve the same primary currents as
Doubler 2", with the output inductors value
chosen twice the value used in the "Center
Following is a table
summarizing the results for the three power supply topologies. You can see also the
corresponding waveforms or calculated values, by clicking
on the highlighted parameter.
And now the comments regarding this power supply topology:
Myth: " ... operation on
the primary side, including duty-cycle is unchanged ... diode and
output capacitor stresses are identical to full-wave technique
- 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
- 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.
" ... the current
doubler working under same conditions as a full wave rectifier
will reduce the copper loss in the transformer secondary by
approximately 50% ... "
- 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
- 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
Other Power Supply design considerations:
- First of all, we compared
the two topologies using an example, because it was
easier to see the advantages and disadvantages compared
with a theoretical approach. We did not develop specific
generalized formulae to compare the topologies. However,
do not be mistaken in believing that we reached the above
conclusions based on a particular example. Similar
results were obtained for power supplies ranging in power
from 500W to 10kW, and output voltages of 3.3V, 5V, 12V,
- In one article I found the
"... the current doubler
output stage needs twice the number of turns on the
secondary side 'then its full wave counter part, but only
one winding is needed ...". Let's not play with words. A current
doubler power transformer has the SAME total number of
turns on the secondary as its center tap equivalent.
- Core loss, in the output
inductors incorrectly scaled to twice the value for the
center tap, is much higher, and may require only ferrite
material to be used.
- If you correctly design the
current doubler topology to preserve the same currents in
the primary side, you end up with a much higher output
inductor compared with the center tap. However, the
required output capacitance for the same output voltage
ripple will be significantly lower. The result is that
the total energy stored in the output stage is
significantly lower, because, usually, most of
the energy is stored in output capacitors and not in the
- The voltage loop of the
output stage is different in center tap then in current
double, and the compensation network has to be changed.
- For the current doubler
topology to work properly, the currents in the two output
inductors have to be balanced. In order to have them
balanced, you can not use voltage mode control or average
current mode control for the output stage. You MUST use
peak current mode control or a similar control method to
force equal average currents in the two output inductors.
- Some of the current doubler
advantages: lower loss in the power transformer, simpler
mechanical structure for the output transformer, and
lower output capacitance required for the same voltage
- When the current doubler
should be consider as an alternative to center tap:
output currents over 100A for fan cooled, and over 25A
for natural convection cooled. For currents below 100A in
fan cooled units or below 25A for natural convection
cooled units, we consider that the center tap topology is
still a better alternative.
- Do not rush to conclusions
such as which topology is better after just reading
technical articles. Use your own judgment for your
particular application, considering: maintaining same
currents in the primary side, calculating ALL
losses in ALL the magnetics components,
calculating ALL costs (parts and labor),
Note: For information
regarding formula and Spice simulation files used in the article
click on: SMPS Design
and SMPS PSpice Simulation .
Current Doubler topology,
- 1930 - 1940 ? Topology used
with vacuum tubes.
- 1987 July
22, Denmark patent DK1987000003826, Ole S. Seiersen.
- 1988 July 21, US patent
filed, Ole S. Seiersen.
- 1990 Feb 6,
US patent 4,899,271, Ole S. Seiersen.
- 1991 Jun, HFPC Proceedings,
"A New Efficient High Frequency Rectifier Circuit"
by C. Peng, M. Hannigan and O. Seiersen. Topology name: "Hybridge".
- 1991 Jun, HFPC Proceedings,
"A New Output Rectifier Configuration Optimized For
High Frequency Operation" by Kevan O'Meara. The
topology is not explicitly named "Current Doubler
topology", but it is emphasized the "... output
current doubling property ...".
- 1994 Dec, Unitrode Design
Note DN-63, "The Current-Doubler Rectifier: An
Alternative Rectification Technique For Push-Pull And
Bridge Converters", by Laszlo Balogh.
- This article contains information for
which SMPS Power Supplies and its partners may claim
Copyright and/or Trademark rights and may be subject of a
Patent application. Also SMPS Power Supplies and its
partners may claim the status of "First to be
published", relative to ideas published in this
article. Reasonable parts of this article may be quoted
by any third parties, without contacting us, assuming
that the source is clearly identified and a link to the
full article is included. If you wish to incorporate
information from this article within a commercial
product, you should contact us for permission.
- First Revision: 14 Feb
- Web first published: 17
- Last Revision: 28 Aug 2002