AN3329 datasheet(21/47 Pages) STMICROELECTRONICS

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AN3329

Functional check

Doc ID 18376 Rev 1

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Resonant stage

Some waveforms relevant to the resonant stage during steady-state operation are reported

in Figure 37, 38, 39, 40, and 41. The switching frequency at full load and nominal input

voltage is around 75 kHz, in order to achieve a good trade-off between transformer losses

and size. Figure 37 shows the resonant ZVS operation. The converter operates slightly

below resonance, therefore, the resonant current lags the voltage applied, as input

impedance of the resonant network is inductive. The current is negative during the rising

edge of half bridge voltage and positive during the falling-edge, providing, in both cases, the

energy to allow the node HB swinging. Because of the dead time, the MOSFETs are turned

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

almost zero. In Figure 38 voltages on the L6599A LINE (#7), RFmin (#4), CF (#3), and LVG

(#11) pins are observed. The switching frequency is programmed by the resistors connected

to the RFmin (#4) pin, which provides an accurate 2 V reference. The current flowing out

from the RFmin (#4) pin is internally mirrored and alternately charges and discharges the

capacitor connected to CF (#3). LINE (#7) pin voltage enables and disables the resonant

stage, preventing operation in capacitive mode.

In Figure 39 the reverse voltages across secondary rectifiers are represented. The system

operates slightly below resonance and therefore the output rectifiers are working in

discontinuous conduction mode, as energy transfer ends before each node HB voltage

swinging. Therefore, at primary side there are some short intervals where both diodes are

reverse-biased and the magnetizing inductance Lp, being no longer shunted by the output

capacitance reflected to the primary, becomes part of resonance. This corresponds to the

primary side current waveform to the portion circled in blue.

In the same image, it can be noted that the rectifiers PIV are operating within their voltage

rating and well below the BV, so ensuring long term reliability.

Figure 37.

Resonant stage waveforms at 115 V

- 60 Hz, full load

Figure 38.

Resonant stage waveforms at 115 V

- 60 Hz, full load

CH2: LVG

CH3: HVG

CH4: C39 current

CH1: HB voltage

CH3: RFmin (pin #4)

CH1: LVG (pin #11)

CH2: CF (pin #3)

CH4: LINE (pin #7)

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