Automotive Application of Multi-Phase Coupled-Inductor DC-DC Converter

A set of four coupled inductors is applied to a four-phase interleaved 1 kW bi-directional 14 V to 42 V dc/dc converter for automotive applications. The coupled-inductor structure is optimized, and the performance is examined through simulations and experimental measurements. Although coupled inductors offer bigger advantages in applications that require fast transient response, they also have significant advantages in this type of application. I. I NTRODUCTION Buck converters comprising several parallel sections, operated staggered in phase (interleaved) have become popular, particularly for microprocessor power delivery circuits (often referred to as VRMs—voltage regulator modules). Recent work has shown that introducing coupling between the inductors in the different phases of such a multi-phase converter can offer dramatic advantages for microprocessor power delivery applications, because it can offer very fast transient response without large inductor-current ripple [1], [2]. In this paper, we examine the advantages that coupled inductors can offer for an application in which transient response is not an important consideration: an automotive 14 V to 42 V bi-directional dcto-dc converter. Low-cost, but efficient and reliable 14 V to 42 V bidirectional dc-to-dc converters will soon be in great demand. For the year 2005, reference [3] expects the electrical power needed in a car to be between 3000 W and 7000 W [3]. This electrical power demand can not be handled by the 14 V electrical system. To solve this problem a new 42 V standard has been developed [4]. Before the transition to the 42 V system is completed, many vehicles will require both 42 V and 14 V buses to accommodate the electrical subsystems still working with 14 V [5]. One possible architecture for such a dual voltage electrical system includes A 36 V battery is connected to the 42 V bus and a 12 V battery is connected to the 14 V bus. The generator supplies the 42 V bus, and a bi-directional dc/dc converter is required to charge the 12 V battery or to supply power from the 12 V battery to the 42 V bus [5]. This work was supported in part by the United States Department of Energy under grant DE-FC36-01GO1106 In power converters for microprocessor power delivery, fast response to transients is essential. Small inductor values can help, but that approach leads to large ripple currents and thus high losses. Using coupled inductors can help the designer bypass this tradeoff. In [6], [7] it was shown that coupling the inductors in a two-phase interleaved converter can effect a reduction in ripple. A topology using three gaped legs was introduced. Based on this work, [1], [2] show a different gapping configuration for two-phase converters which leads to higher ripple reduction. Additionally, a topology that allows practical operation with strong coupling and with more than two phases was introduced. The result was significant performance improvements; the ripple current in the inductors and the MOSFETs could be reduced without degrading the transient response time [1], [2]. In our automotive application, however, transient response is not important — batteries are available on both sides of the converter to buffer transients. The most important considerations are simply cost, efficiency and size. In the work presented here, we have found that coupled inductors offer significant advantages even when transient response is not a consideration. II. PRINCIPLE OF OPERATION For the purpose of explaining the basic principle of coupled inductors, we discuss only a two-phase system in this section. In Section III, a model of a multiphase coupled-inductor converter will be presented.