Use coupled inductors in multiphase buck converters to improve efficiency
captures the extent of ripple cancellation as a function of
various parameters. Specifically, the FOM depends on ρ, N ph , and D. Equation 3:
Where: j = floor (D × N ph )
While the FOM depends on many factors, the coupling coefficient ρ plays a significant role. To illustrate this point, it is helpful to consider a practical example. Evaluating ripple current for coupled inductors Figure 1 illustrates FOM values for an application with a V IN of 12 V and a V O of 1 V, with a D of ~0.083 and conventional DL values of 100 nanohenries (nH). To upgrade this design to a CL while maintaining the transient performance with the same C O tank, the L k for the CL must be 100 nH. This leaves L m as the design variable. Higher values of L m lead to lower ripple, but a conservative L m of 260 nH is sufficient to achieve most of the desired benefits.
Figure 1: Shown are the FOM values for a 4-phase CL for various L m /L k values as a function of D; the region of interest is highlighted. (Image source: Analog Devices, Inc.)
Figure 2: Current ripple for DL = 100 nH (800 kHz) and CL = 4 × 100 nH (800 kHz, 400 kHz) for V IN = 12 V as a function of V O . (Image source: Analog Devices, Inc.)
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