7.2 SEPIC Buck-Boost Converters

The abbreviation SEPIC comes from the description Single Ended Primary Inductance Converter. A SEPIC is a boost-buck converter, like a Cuk, so its input voltage range can overlap the output voltage. SEPIC circuits can be designed for constant voltage or constant current output.

The SEPIC topology has been known for some time, but only recently has there been a revival in its application because: (a) it needs low ESR capacitors and these are now widely available and (b) it can be used to create AC input power supplies with power-factor correction that are used to meet worldwide EMI standards.

In automotive and portable applications, batteries are used as a power source for DC/DC converters. A 12V supply used in automotive applications can have a wide range of terminal voltage, typically 9V to 16 V during normal operation using a lead-acid battery, but can go as low as 6.5V during cold-crank and as high as 90V during load-dump (when the battery is disconnected). The peak voltage is usually clamped to about 40 V, using a voltage dependent resistor to absorb the energy.

Lithium batteries have been very successful in portable applications, thanks mostly to their impressive energy density. A single lithium cell provides an open voltage of 4.2 V when fully charged, and replaces up to three of the alternative NiCd or NiMH cells. During discharge the cell still retains some energy down to 2.7 V. This input voltage range can be both above and below the...