CODING INTERNATIONAL CO LTD

The current-doubler rectifier is a popular alternative 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 include the push-pull, half bridge and full bridge. Many advantages can be gained by using a current-doubler rectifier, of which the least mentioned is the ability to cancel ripple current seen at the output capacitor.

Output ripple-current cancellation reduces the required output capacitance while 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.

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 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 also provides the ability to better distribute heat dissipation, which is a significant problem with high-current output designs. It’s 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 shouldn’t be a problem, especially with the availability of advanced current-mode PWM controllers, such as the UCC3808 Push-Pull PWM Controller shown in Fig. 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 replaced with a single secondary winding in a current-doubler application.

For high-current, low-voltage applications, the current doubler makes control-driven 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 high-side gate drive, allowing the use of a low-side MOSFET gate driver, such as the UCC37324 shown in Fig. 1. This results in less circuitry and a greater selection of low-side gate driver ICs from which to choose.

Ripple-Current Cancellation

While these benefits are noteworthy, perhaps the greatest motivation for using the current-doubler rectifier is the reduced ripple current seen by the output capacitors. In some way, the current-doubler rectifier can be thought of as a 2-phase, interleaved synchronous buck. As shown in Fig. 2, maximum ripple-current cancellation for a 2-phase synchronous buck occurs when each phase operates at 50% duty cycle. However, because the volt-seconds applied to the transformer primary must be equal for each half of the power-transfer cycle, the current doubler is limited to a maximum duty cycle of 50% per phase. In contrast, the 2-phase synchronous buck can operate at greater than 50% duty cycle per phase.

In terms of ripple-current cancellation, one of the main differences between the 2-phase 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 VOUT to VIN. The current-doubler rectifier-transfer function is given in Equation 1.

Here, D is the duty cycle defined as the total positive and negative power-transfer duty cycle, and N is the primary-to-secondary transformer turns ratio. From Fig. 1, QA and QB are each limited to 50% duty cycle; however, the total power-transfer duty cycle seen at the current doubler can approach 100%.

Using the waveforms of Figs. 3 and 4, taken from the current-doubler rectifier of Fig. 1, an expression for ripple-current cancellation can be derived for the current-doubler 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, IL1 and IL2 can be defined as:

that simplifies to:

Knowing the peak values for the individual inductor currents and the output ripple current, the ripple-cancellation factor, K, is now defined and simplified to give:

And from the current-doubler power-transfer 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 ripple-current cancellation effect for the current-doubler rectifier.

Fig. 5 shows a graphical representation of the ripple-current cancellation effect that would be expected for a current-doubler rectifier. The defined areas for N=3 and N=4 show the effect that transformer turns ratio has on ripple cancellation for a 3.3-V (plus 0.7-V drop) typical telecom converter operating from an input-voltage range of 36 V ≤ VIN ≤ 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 VIN=72 V and 0.88 for VIN=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 ripple-current cancellation that can be achieved.

Benefits of Reduced Ripple

Comparing the waveforms of Fig. 4 to Fig. 3, the frequency of the output-ripple current is twice the frequency of each individual inductor-ripple current. This higher-frequency ac ripple current results in lower output capacitance or lower inductor value for the same output-ripple current obtained using a full-wave rectifier. Having calculated the output-ripple 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 current-doubler 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 push-pull or bridge topology for mid- to high-power, high-current power-supply applications, keep in mind the current-doubler rectifier.

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