AN3011 datasheet(27/37 Pages) STMICROELECTRONICS

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AN3011

Efficiency

Doc ID 16043 Rev 1

27/37

Figure 28.

Converter power capability

3.4

Secondary winding short-circuit protection

VIPer27 is provided with an adjustable first level of primary overcurrent limitation that

switches off the power MOSFET if this level is exceeded. This limitation acts cycle by cycle

and its main purpose is to limit the maximum deliverable output power. A second level of

primary overcurrent protection is also present but in this case it is not adjustable, it is fixed to

1 A (typical value). If the drain current exceeds this second overcurrent protection threshold,

the device enters a warning state. In the next cycle the MOSFET is switched on, and if the

second level of overcurrent protection is exceeded again, the device assumes that a

secondary winding short-circuit or a hard saturation of the transformer is happening and

stops the operation. In order to re-enable the operation, VDD voltage must be recycled, that

means: VDD must go down, up to VDD_RESTART, then rise up to VDD_ON. When the VIPer27

is switched on again (VDD equals VDD_ON), the MOSFET can start to switch again. If the

cause of the second overcurrent protection activation is not removed, the device goes into

auto-restart mode.

This protection was tested on the demonstration board. The secondary winding of the

transformer was shorted, in different operating conditions. Figure 29 and 30 show the

behavior of the system during these tests.

AN3011 Datasheet(PDF) 27 Page - STMicroelectronics