Basic concepts of linear regulator and switching mode power supplies - Part Two

September 16, 2013 // By Henry J. Zhang
Basic concepts of linear regulator and switching mode power supplies - Part Two
Henry J. Zhang, Applications Engineering Manager, Linear Technology looked at an intro and the basics of linear regulator and switching power supply operation in part one of this article series. This final part two takes a look at design considerations of the switching power components as well as discuss the importance of the feedback loop, board layout and other key aspects and topologies of switching supply architecture.

Switching Frequency Optimization

In general, higher switching frequency means smaller size output filter components L and CO. As a result, the size and cost of the power supply can be reduced. Higher bandwidth can also improve load transient response. However, higher switching frequency also means higher AC-related power loss, which requires larger board space or a heat sink to limit the thermal stress. Currently, for ≥10A output current applications, most step-down sup-plies operate in the range of 100kHz to 1MHz ~ 2MHz. For < 10A load current, the switching frequency can be up to several MHz. The optimum frequency for each design is a result of careful trade-offs in size, cost, efficiency and other performance parameters.
Output Inductor Selection

In a synchronous buck converter, the inductor peak-to-peak ripple current can be calculated as:

ΔIL(P-P) = ((V IN – V O) • V O/VIN )/ (L • fS)           (14)

With a given switching frequency, a low inductance gives large ripple current and results in large output ripple voltage. Large ripple current also increases MOSFET RMS current and conduction losses. On the other hand, high inductance means large inductor size and possible high inductor DCR and conduction losses. In general, 10% ~ 60% peak-to-peak ripple current is chosen over the maximum DC current ratio when selecting an inductor. The inductor vendors usually specify the DCR, RMS (heating) current and saturation current ratings. It is important to design the maximum DC current and peak current of the inductor within the vendor’s maximum ratings.

Power MOSFET Selection

When selecting a MOSFET for a buck converter, first make sure its maximum VDS rating is higher than the supply V IN(MAX) with sufficient margin. However, do not select a FET with an excessively high voltage rating. For example, for a 16V IN(MAX) supply, a 25V or 30V rated FET is a

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