AN2950 datasheet(23/37 Pages) STMICROELECTRONICS

23 / 37 page

AN2950

Testing the board

Doc ID 15494 Rev 1

23/37

With faults like the ones described, the second OCP protection of the VIPer28 stops the

operation after two switching cycles, thus avoiding high currents in both the primary and

secondary windings and through the power section of the VIPer28 itself. Figure 33 shows

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

the time the power section of the VIPer28 is off, eliminating any risk of overheating.

2.8

Output overvoltage protection

During the power MOSFET’s off time, the voltage generated by the auxiliary winding tracks

the converter's output voltage through the transformer’s auxiliary-to-secondary turn ratio.

The diode D2 is forward biased, and the voltage divider made up by R14 and R3 (see

Figure 2 and Figure 36) between the auxiliary winding and the CONT pin performs an

output voltage monitor function: if the voltage applied to the CONT pin exceeds the internal

VOVP threshold (3 V typical) for four consecutive switching cycles, the controller recognizes

an overvoltage condition and shuts down the converter. This is done to provide high noise

immunity and avoid that spikes erroneously trip the protection. To re-enable operation the

VDD voltage has to be recycled.

Figure 36.

OVP circuit

Figure 34.

Operating with secondary winding

shorted. Steady state

Figure 35.

Operating with secondary winding

shorted. Steady state (zoom)

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