Buck-Converter Design Demystified

Jun 1, 2006 12:00 PM
By Donald Schelle and Jorge Castorena, Technical Staff, Maxim Integrated Products, Sunnyvale, Calif.

Stepdown-Converter Efficiency

Minimizing power loss throughout the converter will extend battery life and reduce heat dissipation. The following equations calculate power loss in each section of the converter.

Input capacitor ESR loss: P_CIRMS =I_CIRMS² x ESR_CI.

Refer to Eqs. 6, 9 and 12 for losses due to the diode, the MOSFET on-resistance and the MOSFET switching loss.

Inductor DCR loss:

Pc-board copper Loss: Pc-board copper loss is difficult to calculate accurately, but Fig. 5 provides a rough estimate of the amount of resistance per square area of pc-board copper. With Fig. 5, you can use a simple I²R power dissipation equation to calculate the power loss.

The following equation sums all of the power losses throughout the converter, and accounts for those losses in the expression for converter efficiency:

Assuming a reasonable net copper loss of approximately 0.75 W, the efficiency for this converter is 69.5%. Replacing the silicon diode with a Schottky diode increases the efficiency to 79.6%, and replacing the diode with a MOSFET synchronous rectifier further increases the efficiency to 85% at full load.

Fig. 6 depicts a breakdown of power losses in the converter. Doubling the copper weight to 2 oz or tripling it to 3 oz minimizes the copper loss and thereby increases the efficiency to approximately 86% to 87%.

Careful pc-board layout is critical in achieving low switching losses and stable operation for a stepdown converter. Use the following guidelines as a starting point:

• Keep the high-current paths short, especially at the ground terminals.

• Minimize connection lengths to the inductor, MOSFET and diode/synchronous MOSFET.

• Keep power traces and load connections short and wide. This practice is essential for high efficiency.

• Keep voltage- and current-sensing nodes and traces away from switching nodes.

Verifying Performance

When designing or modifying a stepdown switching-regulator circuit (one that operates in CCM, using PWM), you can use the equations in this article to calculate values for the critical components and characteristics required. You should always lab-test the circuit to verify final electrical and thermal specifications. For acceptable circuit operation, a proper pc-board layout and judicious component placements are as critical as choosing the right components.

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More on Buck Converters

• Buck-Converter Design Demystified
• Optimizing Voltage Selection in Buck Converters
• Power Conversion Synthesis Part 1: Buck Converter Design
• Improving Efficiency in Synchronous Buck Converters

> Buck Converter Archive 

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