AN3329 datasheet(26/47 Pages) STMICROELECTRONICS

26 / 47 page

Functional check

AN3329

26/47

Doc ID 18376 Rev 1

Startup

Figure 50 shows waveforms during startup at 115 Vac and full load of PFC and resonant

stages. It is possible to note the sequence of the two stages; once the on/off signal is

asserted high, voltage on C44 increases up to the Vcc turn-on thresholds of the L6564 and

L6599A. The PFC starts first and its output voltage starts increasing from the mains rectified

voltage to its nominal value. In the meantime, the L6599A is kept inactive by the LINE pin

(#7) until the PFC voltage reaches the nominal output voltage, corresponding to 1.24 V on

the LINE pin, then the resonant converter is enabled and starts switching. Figure 53 reports

details of initial operation by the half bridge. It can be noted that the initial HB pulses are

shorter than the following ones, and the resonant tank current has the correct polarity. This

is achieved by means of the circuit R29, D8, C23, and R23, connected to the L6599A CF pin

(#3). The purpose of this circuit is to provide an asymmetric operation by the half bridge

MOSFETs during the initial pulses to prevent hard switching operation that may occur

because the resonant capacitor C39 is discharged, and could damage the half bridge

MOSFETs.

In Figure 52 the rising of the output voltages at startup has been captured. Note that the

output voltages are reaching the steady-state level after ~90 ms when the on/off signal has

been asserted high. The rising of the output voltages, controlled by C21 (CSS capacitor), is

monotonic, regular, and without overshoots.

Figure 53 details the PFC and resonant converter turn-off. Note that after the on/off signal

has been asserted low, the V_C44 decreases down to the L6564 and L6599A respective

Vcc UVLO voltages and both converters stops switching correctly, without restarting

attempts.

Figure 50.

Startup by on-off signal at full load

and 115 Vac - 60 Hz

Figure 51.

Startup by on-off signal at full load

and 115 Vac - 60 Hz - L6599A

signals

CH3: V_C44

CH1: HB voltage

CH2: PFC drain

CH4: On/off signal

CH3: LVG (pin #11)

CH1: HB voltage

CH2: LINE (pin #7)

CH4: C39 current

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