Introduction <br> Single-phase step-down controllers are very effective for low-voltage converter applications with currents around 25 A. If the current is large, power consumption and efficiency begin to have problems. A better approach is to use a multiphase buck controller. This article will briefly compare the benefits of using multiphase buck converters and single-phase converters, and explain what values ​​a multiphase buck converter can provide when implemented.
Figure 1 shows a two-phase circuit. The waveforms of the circuit (shown in Figure 2) clearly show that they are interlaced. This interleaving reduces input and output ripple currents. In addition, it reduces hot spots on printed circuit boards or on specific components. In fact, the two-phase buck converter reduces the RMS-current power dissipation of FETs and inductors by half. Interleaving also reduces conduction losses.
Figure 1 two-phase buck converter
Figure 2 Phase 1 and 2 node waveforms
Output Filter Considerations <br> Since the power stage current is lower for each phase, the output filter requirements for multiphase implementation are also reduced. For a 40-A two-phase solution, the average current supplied to each inductor is only 20A. Compared to the 40-A single-phase method, both inductor and inductor volumes are greatly reduced due to lower average current and saturation current.
Output Ripple Voltage <br> Ripple current cancellation in the output filter stage provides a lower output capacitor ripple voltage than a single-phase converter. This is why multiphase converters are the preferred choice. Equations 1 and 2 calculate the percentage of ripple current that is cancelled in each inductor.
m = D x Phases (1)
with
Where D is the duty cycle, IRip_norm is the normalized ripple current, which is a function of D, and mp is an integer of m. Figure 3 is a graph of these equations. For example, using 2 phases with a 20% duty cycle (D) reduces the ripple current by 25%. The magnitude of the ripple voltage that the capacitor must withstand can be calculated by multiplying the ripple current by the equivalent series resistance of the capacitor. Clearly, the maximum current and voltage requirements are reduced.
Figure 3 normalized capacitor ripple current as a function of duty cycle
Figure 4 shows the simulation results for a two-phase buck converter at 25% duty cycle. The inductor ripple current is 2.2A, but the output capacitor current is only 1.5A due to ripple current cancellation. When two phases are used at 50% duty cycle, the capacitor has no ripple current at all.
Figure 4 : Inductive ripple current cancellation at D = 25%
Figure 1 shows a two-phase circuit. The waveforms of the circuit (shown in Figure 2) clearly show that they are interlaced. This interleaving reduces input and output ripple currents. In addition, it reduces hot spots on printed circuit boards or on specific components. In fact, the two-phase buck converter reduces the RMS-current power dissipation of FETs and inductors by half. Interleaving also reduces conduction losses.
Figure 1 two-phase buck converter
Figure 2 Phase 1 and 2 node waveforms
Output Filter Considerations <br> Since the power stage current is lower for each phase, the output filter requirements for multiphase implementation are also reduced. For a 40-A two-phase solution, the average current supplied to each inductor is only 20A. Compared to the 40-A single-phase method, both inductor and inductor volumes are greatly reduced due to lower average current and saturation current.
Output Ripple Voltage <br> Ripple current cancellation in the output filter stage provides a lower output capacitor ripple voltage than a single-phase converter. This is why multiphase converters are the preferred choice. Equations 1 and 2 calculate the percentage of ripple current that is cancelled in each inductor.
m = D x Phases (1)
with
Where D is the duty cycle, IRip_norm is the normalized ripple current, which is a function of D, and mp is an integer of m. Figure 3 is a graph of these equations. For example, using 2 phases with a 20% duty cycle (D) reduces the ripple current by 25%. The magnitude of the ripple voltage that the capacitor must withstand can be calculated by multiplying the ripple current by the equivalent series resistance of the capacitor. Clearly, the maximum current and voltage requirements are reduced.
Figure 3 normalized capacitor ripple current as a function of duty cycle
Figure 4 shows the simulation results for a two-phase buck converter at 25% duty cycle. The inductor ripple current is 2.2A, but the output capacitor current is only 1.5A due to ripple current cancellation. When two phases are used at 50% duty cycle, the capacitor has no ripple current at all.
Figure 4 : Inductive ripple current cancellation at D = 25%
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