Energy-saving features include ‘zero-power’ high-voltage start-up circuitry, as well as burst-mode switching when the load is very low or disconnected (see below), are intended to help chargers meet legislation such as the incoming CoC Tier 2 power supply efficiency spec.

“Low peak current eliminates audible noise at light load or no load, and the STCH03 features an adaptive under-voltage lock-out to ensure consistent operation at low values of transformer auxiliary bias voltage,” said ST.

At heavier loads, the chip moves to a quasi-resonant mode with zero-voltage switching (ZVS) controlled by detecting transformer demagnetization.

Detection circuitry also provides for line-voltage feed-forward control, intended to improve constant-current regulation.

Regulation is described at “primary-side constant-current regulation, although an opto-isolator provides voltage feedback in a typical application (see circuit).

Over-temperature and over-voltage protection is included, as is auto-restart (or optional latching operation in the STCH03L variant).

Soft-start prevents high peak currents at start-up.

It comes in an SO-8 package.

Switching

At heavy loads, operation is quasi-resonant, achieved by synchronising mosfet turn-on to transformer demagnetisation by detecting the resulting negative-going edge of the voltage across any winding of the transformer – causing variable frequency boundary-mode operation.

As load lessens, eventually it enters valley-skipping mode, where mosfet turn-on will no longer occur on the first valley but on the second, the third, and so on – this also prevents switching frequency progressing above the maximum of 167kHz.

Once load is very light or non-existent, it enters controlled on/off operation with constant peak current – resulting in frequency reducing with load, eventually down to a few hundred Hz, cutting switching losses to a minimum. “As the peak current is very low, no issue of audible noise arises,” said ST.

Zero-power start-up

Am internal 650V rated depletion mosfet in the chip, pulling current through the HV pin from the mains rectifier, comes on when HV reaches ~50V, filling the capacitor between Vdd and ground at 7mA ubtil that cacitor reaches ~17V when the mosfet current stops – leaking only “a few hundred nA”, said the firm.