AN2950 datasheet(24/37 Pages) STMICROELECTRONICS

24 / 37 page

Testing the board

AN2950

24/37

Doc ID 15494 Rev 1

Since the value of R3 has already been selected as a consequence of the maximum output

power that the converter has to deliver, R14 can be chosen according to the following

formula to reach the desired output overvoltage threshold, VOUT_OVP:

Equation 1

where VγD2 and VγD4 are the forward drop of the diodes D2 and D4 respectively, NAUX and

NSEC are the number of turns of the auxiliary and secondary winding respectively. If the

overvoltage protection is not required, D2 and R14 can be removed.

The above formula, solved for VOUT_OVP, gives about 7.2 V for the setting of the presented

board.

This value has been verified experimentally, short-circuiting the lower resistor (R9) of the

output voltage divider and thus producing an output overvoltage, as shown in the figures

below. In Figure 37 one can see that as VOUT reaches the value of 7.2 V, the converter

stops switching. In the same figure, the CONT pin voltage and the FB pin voltage are

reported. The crest value of the CONT pin voltage during the MOSFET’s off time tracks the

output voltage. The converter is shut down when the CONT pin voltage reaches the 3 V

threshold, as shown in Figure 38.

The OVP protection is in auto-restart mode: after the shutdown for protection tripping, the

VDD recycles and, if the overvoltage is still present, the protection is tripped again,

indefinitely, until the cause of the fault is removed, at which point the converter resumes

normal operation, as shown in Figure 39.

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

⋅

OVP

OUT

SEC

AUX

OVP

)

LIM

)

OVP

)

(

Figure 37.

Output overvoltage protection at

115 VIN_AC 0.2 A

Figure 38.

Output voltage ripple at 230 VIN_AC

no load

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