Circuit description and test results

AN2331

10/24

The resonant stage is also equipped against over voltage or loop disconnections. In fact, the

zener diode D8 detects the auxiliary voltage and in case of over voltage the latched enable

pin of the L6598 will be activated and it will stop the operations. In this case the L6563 will

not be latched but will remain active.

2.5

Start-up sequence

Figure 4.

Start-up @90Vac - full load

Figure 4 shows the waveforms during the start at 90Vac and full load. It is possible to note

the sequence of the two stages: at power-on the L6563 Vcc increase up to the turn-on

threshold and the PFC output voltage increases from the mains rectified voltage to its

nominal value, with a small overshoot. In the meantime, the L6598 Vs capacitor (C26) is

charged by the L6563 Vcc and, as soon as it reaches the L6598 turn-on threshold, the

resonant starts to operate. Hence the output voltage rises according to the soft-start and

reaches the nominal level.

This sequence provides for the advantages of a perfect sequencing of the circuit at start-up

with the PFC acting as master and avoids complex additional circuitry for the correct start-

up of the circuit in all conditions. The circuit has been tested in all line and load conditions

showing a correct start-up sequence.

CH1: +400V PFC Output voltage

CH4: +24V Output voltage