Magnetics Design for Switching Power Supplies

Figure 1-1 Transformer Equivalent Circuit Introduction Experienced SwitchMode Power Supply designers know that SMPS success or failure depends heavily on the proper design and implementation of the magnetic components. Parasitic elements inherent in high frequency transformers or inductors cause a variety of circuit problems including: high losses, high voltage spikes necessitating snubbers or clamps, poor cross regulation between multiple outputs, noise coupling to input or output, restricted duty cycle range, etc. Figure I represents a simplified equivalent circuit of a two-output forward converter power transformer, showing leakage inductances, core characteristics including mutual inductance, dc hysteresis and saturation, core eddy current loss resistance, and winding distributed capacitance, all of which affect SMPS performance. With rare exception, schools of engineering provide very little instruction in practical magnetics relevant to switching power supply applications. As a result, magnetic component design is usually delegated to a self-taught expert in this "black art". There are many aspects in the design of practical, manufacturable, low cost magnetic devices that unquestionably benefit from years of experience in this field. However, the magnetics expert is unlikely to be sufficiently aware of the SMPS circuit problems caused by the various parasitic elements and the impact of the specific circuit locations of these elements. This often results in poor decisions in the magnetic component design. This collection of topics on magnetics is intended to give the SMPS designer the confidence and the ability to: (I) Develop a reasonably accurate electrical circuit model of any magnetic device, to enable prediction of circuit performance, (2) Relate the electrical circuit model to the magnetic device structure, thus providing the insight needed to achieve an optimized design, (3) Collaborate effectively with experts in magnetics design, and possibly (4) Become a "magnetics expert" in his own right.