Cs Give Sizeable Boost To DC/DC Converter Efficiency

By using low on-resistance MOSFETs as secondary synchronous rectifiers, the Si9122, Si9123 and Si9124 500-kHz PWM controller ICs are said to provide a significant increase in dc/dc converter efficiency—as high as 92%, even for low output voltages—compared to using Schottky diodes. The devices meld on-chip a secondary-side FET drive signal for synchronous rectification and a direct primary MOS drive (1A max.) on high and low sides, as well as a high-voltage start-up circuit. Targeted for use in fixed telecomm applications requiring a 50W to 150W supply, the half-bridge Si9122 and Si9123 and push/pull Si9124 devices operate over a 12V to 72V input voltage range, drive both primary side FETs, and provide timing for the secondary synchronous rectifiers. The ICs can withstand input voltages of up to 100V for 100 ms and offer voltage-mode control with voltage feed-forward compensation to reportedly provide excellent line transient response. Other features include programmable input voltage detector and an oscillator spanning a 200 to 600 kHz frequency range. The Si9122 comes in TSSOP-20 and SO-20 packages and the Si9123 and Si9124 in TSSOP-16s, with all three devices priced at $1.95 each/10,000. VISHAY SILICONIX, Santa Clara, CA. (619) 336-0860.