STCH02 Datasheet, PDF (3/24 Pages) STMicroelectronics – Offline PWM controller for ultra-low standby adapters

STCH02
Device description and block diagram
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Device description and block diagram
The STCH02 is a current mode controller designed for offline quasi resonant ZVS (zero
voltage switching at switch turn-on) flyback converters.
It combines a high performance low voltage PWM controller chip and a 650 V HV start-up
cell in the same package.
The device features a unique characteristic: it is capable to provide a constant output
current (CC) regulation using primary-sensing feedback. This eliminates the need for
a dedicated current reference IC, as well as the current sensor, still maintaining a quite
accurate output current regulation.
The quasi resonant operation is achieved by means of a transformer demagnetization
sensing input that triggers MOSFET's turn-on, connected on the ZCD pin. This input serves
also to monitor the output voltage monitor and to achieve the mains independent CC
regulation (line voltage feedforward).
The maximum switching frequency is top-limited below 260 kHz, so that at the medium-
light-load a special function automatically lowers the operating frequency 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 CC regulation, where the flyback voltage generated by the auxiliary winding
drops and may be not enough to supply the internal circuits, the chip is able to power itself
directly from the rectified mains through the high voltage start-up circuit.
During the burst mode operation the self-supply feature is disabled (due to very stringent no
load consumption requirement), and the V_DD supply voltage has to be guaranteed by proper
application design.
In any case, an innovative adaptive UVLO helps minimize the issues related to the
fluctuations of the self-supply voltage with the output load, due to transformer's parasitic and
further reducing the IC's bias consumption.
In addition to the said functions that optimize power handling under different operating
conditions, the device offers also a protection against the transformer saturation and
secondary diode short-circuit and an adjustable output overvoltage protection. All of them
are in the autorestart mode.
An embedded leading edge blanking on the current sense input for greater noise immunity
completes the equipment of this device.
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