Primary-side control boosts efficiency of ac/dc switching ICs
By Margery Conner, Technical Editor -- EDN, June 6, 2008
Most consumer-targeted power adapters for devices such as cell phones and MP3-player chargers use optocoupler circuits to feed back voltage information from the secondary side to the regulator circuitry. Designers use these optocouplers because most ac/dc-power-supply transformers have a tolerance range of 10% or more, making it difficult to use a transformer winding to sense and regulate input and output power levels. The devices in Power Integrations’ LinkSwitch-II ac/dc-power-conversion family of ICs address this problem by measuring the inductance of the transformer using a known current. In this way, the chip can rely on a primary-side transformer winding to sense voltage and current variations, eliminating the need for optocoupler-feedback circuitry and providing constant-voltage and constant-current regulation. This regulation is important in applications such as ac/dc adapters and chargers for cell phones and MP3 players, which rely on constant-voltage supplies, and high-brightness LEDs, which require constant-current supplies.
The elimination of secondary feedback-sensing components increases LinkSwitch-II-powered chargers to efficiency levels as high as 80%. No-load power consumption is less than 30 mW. For example, in a 5V/1A, 5W cell-phone-charger reference design, LinkSwitch-II achieves Energy Star EPS (external-power-supply) 2.0 compliance with average efficiency levels as high as to 75% at 25, 50, 75, and 100% loads across a 115 to 230V-ac input range. The chip integrates all control and protection circuitry plus a 700V MOSFET in an eight-pin package and sells for 60 cents (10,000).
