AN2950 datasheet(22/37 Pages) STMICROELECTRONICS

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Testing the board

AN2950

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Doc ID 15494 Rev 1

2.7

Second OCP protection

The VIPer28 is provided with a first adjustable 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, which is not adjustable but fixed to 1.2 A

(typical value). If the drain peak current exceeds this second overcurrent protection

threshold, the device enters a warning state. If, during the next on time of the power

MOSFET, 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 occurring

and stops the PWM activity. To re-enable the operation, the VDD pin voltage has to be

recycled, which means that VDD has to 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.

Figure 30.

OLP: steady-state, short removal

and restart

Figure 31.

OLP: restart after short removal

(zoom)

Figure 32.

Second OCP protection tripping

Figure 33.

Operating with secondary winding

shorted. Restart mode

AN2950 データシート(PDF) 22 Page - STMicroelectronics