herunterladen
Application Note
SLUA323 − September 2004
1
Current Doubler Rectifier Offers Ripple Current
Cancellation
Steve Mappus System Pow
er
ABSTRACT
The current doubler rectifier is a popular alternate choice for the output stage of a buck
derived power converter, which would otherwise use a conventional center-tapped
transformer with secondary-side, full-wave rectification. Power topologies within this class
would include the push pull, half bridge and full bridge. There are many advantages that can
be gained by using a current doubler rectifier but by far the least mentioned is the ability to
cancel ripple current seen at the output capacitor. Output ripple current cancellation reduces
the required output capacitance producing less noise at the power supply output. The degree
of ripple current cancellation is duty cycle dependent, so design specifications such as input
voltage and transformer turns ratio need careful attention.
1 Introduction
As illustrated in Figure 1, the current doubler uses two output inductors, each carrying half the
total load current and operating at half the switching frequency. Initially, this may not seem like
an advantage, especially considering that the full-wave approach uses only one output inductor.
However, from an energy storage point of view, the total area product required for each case is
the same, so the total magnetic core volume is the same. Using two inductors in the output
stage additionally provides the ability to better distribute heat dissipation, which is especially a
problem with high current output designs. It is also critical that the currents in each output
inductor remain equal under all operating conditions. For this reason, current mode control is a
requirement with the current doubler rectifier. To the designer, this should not be a problem,
especially with the availability of advanced current mode PWM controllers such as the UCC3895
Phase Shift PWM Controller or the UCC3808 Push Pull PWM Controller shown in Figure 1.
Another benefit of the current doubler is the simplification of the transformer design. Without the
center-tapped secondary, characteristic of all full-wave rectifiers, the transformer output is more
easily terminated to the rest of the power stage. Also, a finer resolution in the transformer turns
ratio is possible since the two secondaries of the full-wave rectifier are now replaced with a
single secondary winding in a current doubler application.
For higher current, low voltage applications, the current doubler also makes control driven
synchronous rectification much simpler because each output rectifier is referenced directly to
secondary ground. Having the output rectifiers both referenced to ground eliminates the need to
develop a high-side gate drive, allowing the use of a low-side MOSFET gate driver such as the
UCC37324 shown in Figure 1. This results in less circuitry, saving precious board space, and a
greater selection of low-side gate driver devices to choose from.
