AN3014

Functional check

Doc ID 16064 Rev 2

13/29

4

Functional check

Figure 6 and Figure 7 are waveforms relevant to the resonant stage during steady state

operation. The selected switching frequency is approximately 100 kHz in order to achieve a

good trade-off between transformer losses and dimensions. The converter operates above

the resonance frequency. Figure 7 shows the resonant ZVS operation. Both MOSFETs are

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

voltage is zero.

In Figure 8, typical waveforms relevant to the secondary side are shown. It can be noted that

the rectifier MOSFETs are switched on and off according to the current flowing through

them. Figure 9 shows the waveforms during startup at 90 Vac and full load. Here, the

voltages increase up to the turn-on thresholds of the two ICs. The PFC starts and its output

voltage increases from the mains rectified voltage to its nominal value. In the meantime, the

L6599A is kept inactive by the LINE pin (pin 7) until the PFC voltage reaches the set

threshold. Then the resonant starts operating and the output voltage reaches the nominal

level.

Figure 6.

Resonant stage oscillator at

230 V - 50 Hz - full load

Figure 7.

Resonant stage waveforms at

230 V - 50 Hz - full load