Boost Converter Efficiency Through Accurate Calculations

P ortable battery-operated electronic equipment is becoming packed with more features requiring larger amounts of power, leading to decreased operating times between battery charges. Innovations such as dynamic backlight control, where the liquid crystal display’s (LCD) backlight power is reduced based on screen content, and automatic voltage scaling, where a microprocessor’s frequency and supply voltage are reduced based on computing time requirements, help to gain back those lost milliwatts. This milliwatt level of lost power is important considering the relatively low amounts of power consumed by a typical electronic device, and the small battery capacities that can often be rated at 1000 mAh or less. Essentially, everywhere that power loss can be accounted for helps in optimizing the design and aids in extending battery life. One contributor to power loss in portable electronic devices is the inductive boost-switching regulator. These types of converters are typically used to power white lightemitting diodes (LEDs) in back-lit LCDs, organic LED panels, high-current flash LEDs, or even audio amplifiers. Although inductive boost converters can be highly efficient, improper selection of a converter type, operating frequency and/or external components can lead to an inefficient design. Therefore, making accurate efficiency calculations helps to identify the individual loss contributors and provides the insight necessary in designing an efficient boost converter. To begin with, take a standard asynchronous boost converter. The simple approximation to efficiency can be made using a first-order model where the ideal duty cycle (D) = (VOUT – VIN)/VOUT and the average inductor current, or input current (IIN), IIN = (IOUT/1 – D) is used to estimate the dc losses in the NMOS switch, the Schottky diode and the inductor. Efficiency then becomes: eff P P P P P IN SWITCH INDUCTOR DIODE