AN3329 datasheet(14/47 Pages) STMICROELECTRONICS

14 / 47 page

Functional check

AN3329

14/47

Doc ID 18376 Rev 1

In Figure 17 the 5 V standby output voltage ripple has been captured during operation at

115 Vac of the standby converter only. Ripple and noise measured at full load are very

limited.

Figure 18 shows the waveforms during the startup of the standby converter only, at 115 Vac

and full load. At Viper27LN power-on the Vcc capacitor is charged by the internal startup

current source until its voltage reaches the turn-on threshold. At this point the Viper27LN

starts operating and the output voltage rises to reach the nominal value. During the

converter startup, the Viper27LN internal digital fixed time-based (8.5 ms) soft-start

gradually increases the drain current to its maximum value. In this way the stress on the

secondary diode is considerably reduced and transformer saturation is prevented. The

brownout circuit prevents the Viper27LN from starting up during abnormal mains conditions.

Figure 15.

Standby supply waveforms at

400 Vdc, full load

Figure 16.

Standby supply output rectifiers

PIV at 400 Vdc, full load

CH1: DRAIN (pin #8)

CH3: CONT (pin #3)

CH2: VDD (pin #2)

CH4: FB (pin #4)

CH3: PIV D22

CH4: PIV D18

Figure 17.

Standby supply 5 V ripple at

115 Vac - 60 Hz, full load

Figure 18.

Standby supply startup at 115 Vac -

60 Hz, full load

CH3: + 5 V standby ripple voltage

CH1: DRAIN (pin #8)

CH2: +5 V standby

CH3: BO (pin #5)

CH4: VDD (pin #2)

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