Boost Converter Efficiency Through Accurate Calculations

Sep 1, 2008 12:00 PM
By Travis Eichhorn, Senior Applications Engineer, National Semiconductor, Grass Valley, Calif.

Switching Losses

Where does the problem lie? For boost converters such as white LED drivers that operate with such low output currents and high duty cycles, the switching losses are no longer trivial compared to the circuit's conduction losses. The switching-loss components must be included in the second-order power-balance equation to give a more accurate estimate of the duty cycle and a closer calculation of circuit efficiency.

To start with, the primary switching-power loss components in an asynchronous boost converter include the power due to the MOSFET cross conduction (P_CC), which is the overlap between the current and voltage during the switch turn-on and turn-off, the charging of the Schottky diode's capacitance with each switching cycle (P_CD), and the charging of the NFET switch's drain-to-source capacitance each switching cycle (P_CDS).

Adding these to the power-balance equation gives a more complicated (but accurate) equation of the duty cycle. The new second-order power-balance equation becomes:

This results in a modified second-order equation for a duty cycle of:

Using C_DS equals 40 pF, C_D equals 20 pF and t_RISE equals t_FALL equals 8 ns results in an estimated duty cycle of 83.8% for the LM3528. This results in a calculated efficiency of 85.5%, which is much closer to the measured value.

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