DocID028766 Rev 1

11/24

STCH02

Application information

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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