Basic concepts of linear regulator and switching mode power supplies, Part two

Henry J. Zhang, Applications Engineering Manager, Linear Technology Corp. -September 10, 2013

Editor’s note: In Part one, we looked at an intro and the basics of linear regulator and switching power supply operation. This final part two will 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) = ((VIN – VO) • VO/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 VIN(MAX) with sufficient margin. However, do not select a FET with an excessively high voltage rating. For example, for a 16VIN(MAX) supply, a 25V or 30V rated FET is a good fit. A 60V rated FET can be excessive, because the FET on-resistance usually increases with rated voltage. Next, the FET’s on-resistance RDS(ON) and gate charge QG (or QGD) are two most critical parameters. There is usually a trade-off between the gate charge QG and on-resistance RDS(ON). In general, a FET with small silicon die size has low QG but high on-resistance RDS(ON), while a FET with a large silicon die has low RDS(ON) but large QG. In a buck converter, the top MOSFET Q1 takes both conduction loss and AC switching loss. A low QG FET is usually needed for Q1, especially in applications with low output voltage and small duty cycle. The lower side synchronous FET Q2 has small AC loss because it is usually turned on or off when its VDS voltage is near zero. In this case, low RDS(ON) is more important than QG for synchronous FET Q2. When a single FET cannot handle the total power, several MOSFETs can be used in parallel.


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