AN3011 datasheet(29/37 Pages) STMICROELECTRONICS

29 / 37 page

AN3011

Efficiency

Doc ID 16043 Rev 1

29/37

3.5

Output overvoltage protection

Output overvoltage protection is implemented, monitoring the voltage across the auxiliary

winding during the MOSFET off time, through the D2 diode and the R3 and R14 resistor

dividers (see schematic of Figure 31) connected to the CONT pin of the VIPer27. If the

voltage on the CONT pin exceeds the VOVP thresholds (3 V typ.), VIPer assumes an

overvoltage event and the power section is no longer allowed to be switched-on. To re-

enable operation, the VDD voltage has to be recycled. In order to provide high noise

immunity and avoid spikes erroneously tripping the protection, a digital filter was

implemented so the CONT pin must sense a voltage higher than VOVP for four consecutive

cycles before it stops operation (see device datasheet for details).

Figure 31.

OVP circuit

The value of the output voltage when the protection must be tripped can be properly fixed by

selecting the R3 and R14 resistor dividers. When R3 is selected, considering the maximum

power that the converter must manage, R14 output must be selected, according to the

formula:

Equation 1

The protection was tested by shorting the voltage reference pin of the TS431 device (pin 3

of VR1 in the schematic) with pin 2 of the same device during converter operation. In this

way the converter operates in open loop and delivers the maximum power possible to

output. The excess of power, in respect to the maximum load, charges the output

capacitance increasing the output voltage, as the OVP is tripped and the converter stops

working.

In Figure 32 it is possible to see that the output voltage increases, and as it reaches the

value of 5.58 V the converter stops switching. In the same figure the CONT pin voltage

(Ch3, fuchsia waveform) and the drain current (Ch4 green waveform) are shown (see

Figure 32

and 33). The crest value of the CONT pin voltage tracks the output voltage.

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

⋅

)

ovp

drop

OVP

OUT

AUX

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LIM

)

OVP

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