Power Electronics

SMPS Primary Regulators Use Soft Switching

Allegro’s line of low-power off-line SMPS primary regulators, the STR-W67xx series, includes quasi-resonant topology ICs designed for SMPS power supplies. These units show lower EMI noise characteristics, especially at more than 2 MHz, higher than conventional PWM solutions. They utilize a soft switching mechanism to turn on the internal MOSFET at a close point to ground level (Vds bottom point) through the resonant phenomenon of a primary inductor and resonant capacitor.



The bottom-skip function skips the first bottom of Vds and turns on the MOSFET at the second bottom point to minimize the increase of operational frequency at light output load, improving system-level efficiency over the entire load condition. Two standby functions reduce the input power at very light output load conditions; one is an auto-burst mode operation internally triggered by sensing on-time, and the other is a manual standby mode that is executed by clamping the secondary output.



The package is a fully molded TO-220, which contains the controller chip and MOSFET, and enables output powers up to 160 W at 120 Vac input, or 240 W at 230 Vac input. Pricing in 1000-unit quantities is as follows: STR-W6735, $1.56; STR-W6753, $1.33; STR-W6754, $1.67; STR-W6756, $1.85; STR-W6765, $1.72.

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