PWM controller boosts stability of ultra-low standby power supplies

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By Nick Flaherty

At heavier loads, the STCH03 maximises efficiency by operating in quasi-resonant mode with zero-voltage switching (ZVS) controlled by detecting transformer demagnetisation. The detection circuitry also provides for line-voltage feed-forward control, which ensures accurate constant-current regulation. Maintaining operation at the boundary of continuous (CCM) and discontinuous (DCM) transformer current means the controller limits the converter’s maximum operating frequency to 167kHz depending on line/load conditions and implements valley-skipping for best efficiency as the load reduces.

Energy-saving features including zero-power high-voltage startup circuitry, as well as low quiescent current and burst-mode switching when the load is very low or disconnected, keep standby power below 10mW to help chargers and adapters meet tough legislation such as the incoming CoC Tier 2 power supply efficiency specifications. Low peak current eliminates audible noise at light load or no load, and the device uses an adaptive under-voltage lockout (UVLO) to ensure consistent operation at low values of transformer auxiliary bias voltage.

With accurate primary-side constant-current regulation, the STCH03 eliminates output-current sensing components, thereby reducing the charger bill of materials (BOM). The integrated startup circuitry, which consumes zero power when inactive, also saves external components.

Over-temperature and over-voltage protection (OTP, OVP) reduce the bill of materials there is an option of latching operation in the STCH03L variant. The soft-start prevents high peak currents at startup, and output under-voltage protection lets the device operate safely in case of an output short-circuit or low output voltage under constant-current control.

The STCH03 is in production now, in an SO-8 package, priced from $0.54 for orders of 1000 pieces.


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