Design and realization of a DC/DC converter with a partially saturated inductor

DC/DC converters, in some types of applications such as portable equipments, can require more space than it is actually available. The inductor is typically the most bulky element and the possibility to reduce its size can save up to 50% of the converter volume and area [1][2], thus increasing the power density. This reduction, however, comes with nonlinear effects caused by the saturation of the ferromagnetic core. An appropriate modelling of the inductor and of the converter circuit is needed for guaranteeing a good output power quality (Fig. 1). A Boost converter with an inductor in the partially saturated roll-off operating zone was designed and realized to study the behaviour of DC/DC converters with nonlinear inductors (Fig. 2). The main components of the circuit are listed in Table I. An innovative polynomial model of the inductor [3] has been used for the circuit simulations. The Boost circuit model is derived from [4]. The converter control system has been implemented on a Nucleo64- STM32F401RE microcontroller; this system manages the startup of the converter, regulates the output voltage and implements a control of the inductor current via a current loop (Fig. 3). An example of the system stability under a 10% step variation of the load is shown in Fig. 4. For the experimental measurement a test rig has been set up, the elements of which are listed in Table II. Initially, the converter is operated at high frequency (f=15 kHz) with low current ripple. By changing the average current, the inductor is characterized in its linear and partial saturation zones. Then, the current ripple is increased to 2A by decreasing the switching frequency to 2 kHz, in order to simulate an equivalent reduction of the inductor value (volume reduction); in this case, the cusp-shaped current waveform, typical of saturated inductors, can be noted (Fig. 5). Finally the converter has been forced to work in discontinuous conduction mode (DCM) by increasing the load resistance; even in this case the current control system guarantees the output voltage regulation and the system stability (Fig.6). In conclusion, in this work the reduction of the inductor volume has been considered together with its two main consequences: the non-linear behaviour for heavy loads and the discontinuous conduction mode (DCM) for light loads. In both cases, it has been shown that the converter is able to handle partial saturation and to operate correctly in DCM thanks to the current control with consequent beneficial effects on power density.