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The STCH03 current-mode controller is designed for an offline quasi-resonant ZVS (zero-voltage-switching at switch turn-on) flyback converter.
It combines a high-performance low-voltage PWM controller chip and a 650V HV start-up cell in the same package.
Constant output voltage (CV) regulation loop is achieved by secondary side feedback, the output voltage information is transfered via an optocoupler on the FB pin of the STCH03 to get the selected output voltage.
The device features a unique characteristic: it is capable of providing constant output current regulation (CC) using primary-sensing feedback. This eliminates the need for dedicated current reference IC, as well as the current sensor, still maintaining quite accurate output current regulation.
The quasi-resonant operation is achieved by means of a transformer demagnetization sensing input that triggers the turn-on of the MOSFET, connected on the ZCD pin. This input serves also as both output voltage monitor, to achieve mains-independent CC regulation (line voltage feedforward), and to implement OVP and UVP.
The maximum switching frequency is top-limited below 154 kHz, so that at the medium-light load a special function automatically lowers the operating frequency whilst still maintaining the operation as close to ZVS as possible. At the very light load, the device enters a controlled burst-mode operation that, along with the zero-power high-voltage start-up circuit, the extremely low quiescent current of the device, helps minimize the residual input consumption, thus meeting the requirements of the most stringent standards.
During the burst-mode operation the VDD supply voltage has to be guaranteed by optimum application design. In any case, an innovative adaptive UVLO helps to minimize the issues related to the fluctuations of the auxiliary biasing voltage with the output load, due to parasitic capacitance of the transformer and further reducing the IC's bias consumption.
In addition to these functions that optimize power handling under different operating conditions, the device also offers the output overvoltage protection (OVP), overtemperature protection (OTP), hiccup-mode protection that is invoked when the transformer saturates or the secondary diode fails short and the output undervoltage protection (UVP) that limits the average output current in case of the output short-circuit. All the protections are auto-restart mode, except the OVP protection, that can be internally selected to be auto-restart or latched mode.
The embedded leading-edge blanking on the current sense input for greater noise immunity completes the equipment of this device.