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Basic topologies with zero-ripple current
AN3180
4/39
Doc ID 17273 Rev 1
1
Basic topologies with zero-ripple current
Coupled magnetic devices have been around since the early days of electronics, and their
application to power switching circuits dates back to the late 70's with the experiments on
the Cuk converter, from which “magnetic integration” originated. With this technique,
inductors and transformers are combined into a single physical structure to reduce the
component count, usually with little or no penalty at all on the converter's characteristics,
sometimes even enhancing its operation. During initial experiments on the Cuk converter
the zero-ripple current phenomenon was first observed. The technique derived by the use of
this phenomenon is known as ripple-steering or ripple cancellation. Besides providing an
excellent discussion, also gives an interesting historical outline of the subject (see
application to power switching circuits dates back to the late 70's with the experiments on
the Cuk converter, from which “magnetic integration” originated. With this technique,
inductors and transformers are combined into a single physical structure to reduce the
component count, usually with little or no penalty at all on the converter's characteristics,
sometimes even enhancing its operation. During initial experiments on the Cuk converter
the zero-ripple current phenomenon was first observed. The technique derived by the use of
this phenomenon is known as ripple-steering or ripple cancellation. Besides providing an
excellent discussion, also gives an interesting historical outline of the subject (see
). The application of the zero-ripple current phenomenon is of considerable
interest in switching converters, where there are at least two reasons why it is desirable to
minimize inductor ripple currents. Firstly, lowering ripple current in inductors reduces the
stress on converter capacitors, resulting in either lower associated power loss or more
relaxed filtering requirements. Secondly, and often more importantly, most converter
topologies have pulsating current at either input or output or both, and most applications
require low conducted noise at both ports, because of EMC requirements or load
requirements.
minimize inductor ripple currents. Firstly, lowering ripple current in inductors reduces the
stress on converter capacitors, resulting in either lower associated power loss or more
relaxed filtering requirements. Secondly, and often more importantly, most converter
topologies have pulsating current at either input or output or both, and most applications
require low conducted noise at both ports, because of EMC requirements or load
requirements.
Figure 2.
Some basic topologies with zero-ripple current characteristics
This issue is commonly addressed with the use of additional LC filters, whose impact on
both the overall converter size and cost is not at all negligible, not to mention their interaction
with the small-signal dynamics which sometimes cause poor dynamic response issues or
even stability issues. In particular, in offline converters, where EMC regulations specify limits
to the amount of conducted and radiated emissions, a technique like ripple-steering which
makes the input current non-pulsating or nearly so, therefore eliminating most of the
differential mode conducted noise, is advantageous as it enables the reduction in EMI filter
size and complexity, especially in its differential filtering section (Cx capacitors and
differential mode inductors).
both the overall converter size and cost is not at all negligible, not to mention their interaction
with the small-signal dynamics which sometimes cause poor dynamic response issues or
even stability issues. In particular, in offline converters, where EMC regulations specify limits
to the amount of conducted and radiated emissions, a technique like ripple-steering which
makes the input current non-pulsating or nearly so, therefore eliminating most of the
differential mode conducted noise, is advantageous as it enables the reduction in EMI filter
size and complexity, especially in its differential filtering section (Cx capacitors and
differential mode inductors).
Reducing Cx capacitors to a minimum brings an additional benefit to applications with tight
specifications on standby consumption: Cx capacitors cause a considerable reactive current
to flow through the filter, which is a source of additional and unwanted loss (even 0.1 W or
more at high line); furthermore, the discharge resistor which, for safety, must be placed in
parallel to Cx can be higher. As a result, both losses are minimized.
specifications on standby consumption: Cx capacitors cause a considerable reactive current
to flow through the filter, which is a source of additional and unwanted loss (even 0.1 W or
more at high line); furthermore, the discharge resistor which, for safety, must be placed in
parallel to Cx can be higher. As a result, both losses are minimized.
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