AN3286 datasheet(27/42 Pages) STMICROELECTRONICS

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AN3286

Testing the board

Doc ID 17998 Rev 1

27/42

short-circuit or a hard-saturation of the transformer is occurring and stops the PWM activity.

To re-enable the operation, the VDD pin voltage must be recycled, which means that VDD

must go down to VDD(RESTART), then rise up to VDDon. At this point the MOSFET restarts

switching. If the cause of activation of the second overcurrent protection is still present, the

protection is tripped again and the system works in auto-restart mode, resuming normal

operation as soon as the cause of the fault is removed and the VDD recycled.

This protection has been tested in different operating conditions, short-circuiting the output

diode. The following figures show the behavior of the system during these tests.

With faults such as those described, the 2nd OCP protection of the VIPer25 stops the

operation after two switching cycles, therefore avoiding high currents in the primary as in the

secondary windings and through the power section of the VIPer25 itself. Figure 32 shows

the operation when a permanent short-circuit is applied on the secondary winding. Most of

the time the power section of VIPer25 is off, eliminating any risk of overheating.

Figure 31.

2nd OCP protection tripping

Figure 32.

Operating with secondary winding

shorted - restart mode

AN3286 データシート(PDF) 27 Page - STMicroelectronics