CurrentDoubler Rectifier Reduces Ripple Current
The currentdoubler rectifier is a popular alternative for the output stage of a buckderived power converter, which would otherwise use a conventional centertapped transformer with secondaryside, fullwave rectification. Power topologies within this class include the pushpull, half bridge and full bridge. Many advantages can be gained by using a currentdoubler rectifier, of which the least mentioned is the ability to cancel ripple current seen at the output capacitor.
Output ripplecurrent cancellation reduces the required output capacitance while producing less noise at the powersupply output. The degree of ripplecurrent cancellation is dutycycle dependent, so design specifications, such as input voltage and transformer turns ratio, need careful attention.
As illustrated in Fig. 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 advantageous, especially considering that the fullwave approach uses only one output inductor. However, from an energystorage 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 also provides the ability to better distribute heat dissipation, which is a significant problem with highcurrent output designs. It's also critical that the currents in each output inductor remain equal under all operating conditions. For this reason, currentmode control is a requirement with the currentdoubler rectifier. To the designer, this shouldn't be a problem, especially with the availability of advanced currentmode PWM controllers, such as the UCC3808 PushPull PWM Controller shown in Fig. 1.
Another benefit of the current doubler is the simplification of the transformer design. Without the centertapped secondary, characteristic of all fullwave 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 fullwave rectifier are replaced with a single secondary winding in a currentdoubler application.
For highcurrent, lowvoltage applications, the current doubler makes controldriven synchronous rectification 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 highside gate drive, allowing the use of a lowside MOSFET gate driver, such as the UCC37324 shown in Fig. 1. This results in less circuitry and a greater selection of lowside gate driver ICs from which to choose.
RippleCurrent Cancellation
While these benefits are noteworthy, perhaps the greatest motivation for using the currentdoubler rectifier is the reduced ripple current seen by the output capacitors. In some way, the currentdoubler rectifier can be thought of as a 2phase, interleaved synchronous buck. As shown in Fig. 2, maximum ripplecurrent cancellation for a 2phase synchronous buck occurs when each phase operates at 50% duty cycle. However, because the voltseconds applied to the transformer primary must be equal for each half of the powertransfer cycle, the current doubler is limited to a maximum duty cycle of 50% per phase. In contrast, the 2phase synchronous buck can operate at greater than 50% duty cycle per phase.
In terms of ripplecurrent cancellation, one of the main differences between the 2phase buck and the current doubler is how the duty cycle is defined by the power transfer function. For the buck regulator, the transfer function is simply the ratio of V_{OUT} to V_{IN}. The currentdoubler rectifiertransfer function is given in Equation 1.
Here, D is the duty cycle defined as the total positive and negative powertransfer duty cycle, and N is the primarytosecondary transformer turns ratio. From Fig. 1, QA and QB are each limited to 50% duty cycle; however, the total powertransfer duty cycle seen at the current doubler can approach 100%.
Using the waveforms of Figs. 3 and 4, taken from the currentdoubler rectifier of Fig. 1, an expression for ripplecurrent cancellation can be derived for the currentdoubler rectifier.
The peak current for each individual inductor can be defined as:
where F is the PWM switching frequency and each inductor operates at F/2. Equation 2 simplifies to:
The output peak ripple current, which is really the sum of the two inductor currents, I_{L1} and I_{L2} can be defined as:
that simplifies to:
Knowing the peak values for the individual inductor currents and the output ripple current, the ripplecancellation factor, K, is now defined and simplified to give:
And from the currentdoubler powertransfer function defined by Equation 1, the duty cycle is given as:
Substituting the value of D from (7) into the expression for K from (6) and simplifying gives:
Equation 8 can now be plotted against D for 0 ≤ D ≤ 1 to graphically show the ripplecurrent cancellation effect for the currentdoubler rectifier.
Fig. 5 shows a graphical representation of the ripplecurrent cancellation effect that would be expected for a currentdoubler rectifier. The defined areas for N=3 and N=4 show the effect that transformer turns ratio has on ripple cancellation for a 3.3V (plus 0.7V drop) typical telecom converter operating from an inputvoltage range of 36 V ≤ V_{IN} ≤ 72 V.
Notice that for larger values of N, the ripple cancellation ratio moves closer to zero. For example, when N is equal to four, the duty cycle varies between 0.44 for V_{IN}=72 V and 0.88 for V_{IN}=36 V, corresponding to a minimum cancellation ratio of 0.7 where the ripple current is reduced by 30%, to a maximum cancellation ratio of 0.25 equaling a reduction of 75%. The transformer turns ratio deserves attention as it has direct effect upon the amount of output ripplecurrent cancellation that can be achieved.
Benefits of Reduced Ripple
Comparing the waveforms of Fig. 4 to Fig. 3, the frequency of the outputripple current is twice the frequency of each individual inductorripple current. This higherfrequency ac ripple current results in lower output capacitance or lower inductor value for the same outputripple current obtained using a fullwave rectifier. Having calculated the outputripple current in (5), we can derive an expression for the required output capacitance.
Equating (9) to (5) gives:
And solving for C gives:
Factors such as transient response and ESR come into play when determining the minimum required output capacitance. However, from (11) note that when the frequency is doubled the required capacitance is reduced by a factor of four. Also, from (9) we can show capacitance is directly proportional to ripple current.
Using a currentdoubler rectifier minimizes the required output capacitance by reducing the amount of output ripple current seen by the output capacitor. The degree to which this is achievable increases with duty cycle. While many factors must be considered when designing the power transformer, a higher turns ratio produces a greater degree of ripple cancellation.
When considering a pushpull or bridge topology for mid to highpower, highcurrent powersupply applications, keep in mind the currentdoubler rectifier.
References

Laszlo Balogh, “The Current Doubler Rectifier: An Alternative Rectification Technique For PushPull and Bridge Converters,” TI Application Note SLUA121.

Laszlo Balogh, “Design Review: 100W, 400KHz, DCDC Converter with Current Doubler Synchronous Rectification Achieves 92% Efficiency,” TI Literature Number SLUP111.

Steve Mappus, “Configuring the UCC3895 for Direct Control Driven Synchronous Rectifier Applications,” TI Literature Number SLUU109A.

UCC3808 Current Mode PushPull PWM Datasheet, TI Literature Number SLUS488B.

UCC37324 Dual 4A Low Side Driver Datasheet, TI Literature Number SLUS492B.