AN3339 datasheet(14/44 Pages) STMICROELECTRONICS

14 / 44 page

Functional check

AN3339

14/44

Doc ID 18449 Rev 2

In Figure 13 the 5 V 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 14 shows the waveforms during the startup of only the standby converter at full load.

At power-on, the VIPER27LN internal startup current source charges the Vcc capacitor until

voltage increases up to the turn-on threshold. At this point the VIPER27LN starts operating

and the output voltage reaches the nominal value. During the converter startup the

VIPER27LN internal digital fixed time-based (8.5 ms) soft-start limits the drain current which

gradually increases to the maximum value. In this way the stress on the secondary diode is

considerably reduced and transformer saturation is prevented. The brownout circuits

prevent the VIPER27LN from operating if the input voltage is too low.

Figure 11.

Standby supply waveforms at 400

Vdc, full load

Figure 12.

Standby supply o/p rectifiers PIV at

400 Vdc, full load

CH2: VDD (pin #2)

CH1: DRAIN (pin #8)

CH3: CONT (pin #3)

CH4: FB (pin #4)

CH3: PIV D22

CH4: PIV D18

Figure 13.

Standby supply 5 V ripple at 115

Vac - 60 Hz, full load

Figure 14.

Standby supply startup at 115 Vac -

60 Hz, full load

CH3: 5 V ripple voltage

CH2: 5 V stby

CH1: DRAIN (pin #8)

CH3: BO (pin #5)

CH4: VDD (pin #2)

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