Automatic Tuning of Coupled Inductor Filters

A coupled inductor filter is presented with interesting notch mode filtering characteristics. Although this configuration has been long established, it has received little attention recently. The new contribution of this paper is to provide an automatic tuning process that can make use of the frequency domain characteristics of the coupled inductor building block. In a sense, this becomes an “active” filter technique that uses passive elements for implementation. The combination offers potential performance improvements in comparison with active filters. Introduction Coupled-inductor and other integrated-magnetic techniques have existed for many years [1],[2]. However, the topic is usually incorporated into a discussion of new power converter topologies or approached from a purely magnetic design perspective. In many cases the level of complexity in the treatment of the coupled magnetic device obfuscates the theory. Unlike most common electrical networks, coupled-inductor techniques involve simultaneous energy transfer through electrical and magnetic pathways. Because of this, it is helpful to approach the coupled inductor from a circuit–based filtering perspective. Whereas previously successful implementations of coupled inductors were often illreceived due to the perceived sensitivity of operation resulting in the tuning the magnetic device to tweak the performance, tuning a filter network is more familiar to most engineers than tuning the coupling in a magnetic device. The technique described in this paper replaces a smoothing choke with a “smoothing transformer” (a coupled inductor) and a dc blocking capacitor. Together, this is a basic filter building block shown in Fig. 1. Because these components are a linear two-port filter, the building block can be implemented in any dc circuit to reduce the ripple current wherever a choke is currently used. Thus it may be applied to the dc input of a converter, a dc output, or an internal dc link in applications such as motor drives or HVDC transmission. Dc power distribution systems are one interesting application of coupled inductor filters. It is a well known problem that instabilities can occur on dc power system due to interaction between the dc-dc power converters, passive filter components, and the dc bus itself. These instabilities can occur at high frequencies initiated by the switching action of the converter, or at low frequencies stimulated by "beat frequencies" that occur when two or more converters operate at proximal frequencies. A review of the literature reveals a large body of work establishing stability criterion [3]-[6]. Much of this work has involved design requirements on passive filter networks, or proposals for active filtering schemes. The tunable coupled inductor filter offers the ability to tune the filter network to minimize the propagation of disturbances between the power converter and the distribution bus. This paper introduces a filter block that uses a coupled inductive network and ripple correlation control to automatically tune the frequency response of the filter. In a sense, the approach provides an “active” filter technique that uses passive elements for implementation. The combination offers potential loss and performance improvements in comparison with active filters. Coupled inductor filter building block The technique described in this paper takes advantage of a smoothing choke that is implemented with a coupled inductor. The addition of a dc blocking capacitor, as in Fig. 1, forms a two port linear filter that can be analyzed in the usual way [2]. In a typical application, such as a dc power supply, the port labeled Vn or “noisy port” is connected to the output of a switching circuit and contains a desired dc component with superimposed ac noise. Assuming an infinite and ideal capacitor, the entire ripple component of the input voltage appears across the Lac winding. With perfect coupling and a unity turns ratio, the ripple component will be transferred to the Ldc winding. Kirchhoff’s voltage law tells us that only the dc component of the input voltage will be present at Vq, the output or “quiet port.” It can be misleading to think of the smoothing transformer’s windings in terms of a primary and secondary. Rather, it is convenient to denote the windings as “dc” and “ac” indicating their purpose in the circuit: the dc winding carries the heavy direct current (similar to a smoothing choke), while the ac winding carries only a small ac ripple current. Accordingly, the dc winding size is selected appropriately to carry the large dc current while the ac inductor may be wound using a smaller gauge. C Vn +