STCH02 Datasheet, PDF (11/24 Pages) STMicroelectronics – Offline PWM controller for ultra-low standby adapters

STCH02
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Application information
Application information
The STCH02 is an offline CC mode primary sensing switching controller, specific for offline
quasi resonant ZVS (zero voltage switching at switch turn-on) flyback converters.
Depending on converter's load condition, the device is able to work in different modes (see
Figure 5):
1. QR mode at the heavy load. Quasi resonant operation lies in synchronizing MOSFET's
turn-on to the transformer's demagnetization by detecting the resulting negative-going
edge of the voltage across any winding of the transformer. Then the system works
close to the boundary between discontinuous (DCM) and continuous conduction
(CCM) of the transformer. As a result, the switching frequency will be different for
different line/load conditions (see the hyperbolic-like portion of the curves in Figure 5).
Minimum turn-on losses, low EMI emission and safe behavior in the short-circuit are
the main benefits of this kind of operation.
2. Valley-skipping mode at the medium/light-load. Depending on voltage on the FB pin,
the device defines the maximum operating frequency of the converter. As the load is
reduced MOSFET's turn-on will not any more occur on the first valley but on the
second one, the third one and so on. In this way the switching frequency will no longer
increase.
3. Burst mode with no or a very light-load. When the load is extremely light or
disconnected, the converter will enter a controlled on/off operation with the constant
peak current. Decreasing the load will then result in frequency reduction, which can go
down even to few hundred hertz, thus minimizing all frequency related losses and
making it easier to comply with energy saving regulations or recommendations. Being
the peak current very low, no issue of audible noise arises.
Figure 5. Multi-mode operation of STCH02
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DocID028766 Rev 1
11/24
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