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AN3180

Basic topologies with zero-ripple current

Doc ID 17273 Rev 1

5/39

All converter topologies are capable of producing zero-ripple current phenomenon, provided

there are two or more inductors which have equal (or, more generally, proportional) voltages

of equal frequency and phase. Some topologies, such as Cuk and SEPIC, have two

inductors, which can be coupled on a common magnetic core: so they immediately lend

themselves to ripple-steering. The other basic topologies - buck, boost, buck-boost, flyback

and single-output forward - have typically a single inductor and then, to reduce its ripple

current to zero, they must be modified with the addition of a second winding wound on the

same inductor core. Moreover, this additional winding must be connected in such a way as

to have the same voltage as the winding where the ripple current is to be cancelled.

Figure 2 shows examples of how to modify some of the basic topologies to achieve zero-

ripple inductor current, to have non-pulsating current at either the input or the output. In all

examples it is possible to recognize the addition of a cell, a two-port circuit commonly

termed a smoothing transformer and shown separately in Figure 3. This cell is able to divert,

or steer, the AC component (the ripple current) from the externally accessible DC winding,

to the AC winding (cancellation winding) whose current conduction path goes directly to the

input port, leaving only DC current flowing through the DC winding and the output port. Note

that the denomination of input and output ports of the smoothing transformer cell is different

from that of input and output of the converter where the cell is applied.

Figure 3.

Smoothing transformer and related currents

This application note, after reviewing the theoretical base, considers the realization of zero-

ripple inductor current to minimize the input ripple in a TM PFC pre-regulator. In these

systems, the large input ripple is one of the major limitations to its use at higher power

levels.

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