AN3339 datasheet(20/44 Pages) STMICROELECTRONICS

20 / 44 page

Functional check

AN3339

20/44

Doc ID 18449 Rev 2

Resonant stage

Figure 31 and 32 show some waveforms relevant to the resonant stage during steady-state

operation. The selected switching frequency is about 110 kHz, in order to have a good

trade-off between transformer losses and size. The converter operates slightly below the

resonance frequency. Figure 31 shows the resonant ZVS operation: both MOSFETs are

turned on when resonant current is flowing through their body diodes and drain-source

voltage is zero. Figure 32 shows the L6599A control signals. The oscillator signal is clean

from noise and symmetrical ensuring proper HB driving.

In Figure 33 and 34 reverse voltages across output rectifiers have been captured. The

maximum voltages applied to the rectifier are well below the components maximum ratings.

Because 12 V and 24 V outputs are obtained by center-tap secondary configuration, reverse

voltages applied to the respective rectifiers are almost twice the output voltage. On the other

hand, 130 V output is obtained by full bridge rectification and voltages across rectifiers

correspond to output voltage.

Figure 31.

Resonant stage waveforms at 115

Vac - 60 Hz, full load

Figure 32.

Resonant stage control signals at

115 Vac - 60 Hz, full load

CH3: LV FET gate

CH1: HB voltage

CH2: HV FET gate

CH4: C39 current

CH3: RFmin (pin #4)

CH1: LV FET gate

CH2: CF (pin #3)

CH4: LINE (pin #7)

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