AN3106 datasheet(13/34 Pages) STMICROELECTRONICS

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AN3106

Functional check

Doc ID 16775 Rev 2

13/34

4.2

Half-bridge resonant LLC circuit

The following figures show waveforms relevant to the resonant stage during steady-state

operation. The resonant stage switching frequency is about 100 kHz, in order to have

a good trade-off between transformer losses and dimensions.

The LLC converter has been designed to operate at nominal voltage and full load at the

resonance frequency, but due to the PFC output voltage ripple at twice the mains frequency,

it is driven slightly above and below the resonant tank frequency, according to the

instantaneous value of the PFC output voltage.

In Figure 17 some waveforms relevant to the resonant stage ZVS operation are shown. We

note that both MOSFETs are turned on when resonant current is flowing through their body

diodes and drain-source voltage is almost zero, thus achieving good efficiency because the

turn-on losses are negligible. The HB MOSFET voltage de-rating and low operating

temperature allow increasing the board’s MTBF.

The current flowing in the resonant tank is sinusoidal. In Figure 17 we note a slight

asymmetry of operating modes by each half portion of the sine wave. The half cycle is

working at resonant frequency while the other one is working above the resonant frequency.

This is due to a small difference between each half-secondary leakage inductance of the

transformer reflected to the primary side, providing the two slightly different resonant

frequencies. This phenomenon is typically due to a different coupling of the transformer

secondary windings and, in this case, it is not an issue. The slight asymmetry is also visible

in Figure 18 where the small ringing appearing on both secondary rectifiers anode voltage

indicates that for a short time the rectifiers are not conducting. This demonstrates that

during the half cycle the circuit is working below the resonant frequency, while during the

following half cycle it is working at the resonant frequency.

In Figure 18 we also note the rectifier operating voltage and its margin with respect to the

maximum reverse voltage (VRRM). This de-rating with respect to the rectifiers VRRM

guarantees good reliability of the output rectifiers, increasing the board’s total MTBF.

In Figure 19 the high-frequency ripple has been measured. As visible the ripple and noise at

switching frequency is very limited, thanks to the low EMI generated by both stages. In

Figure 17.

EVL130W-STRLIG demonstration

board: primary side LLC waveforms

at 115 V - 60 Hz - full load

Figure 18.

EVL130W-STRLIG demonstration

board: secondary side LLC

waveforms at 230 V- 50 Hz - full load

CH1: HB voltage

CH2: CF pin voltage

CH3: VCC

CH4: res. tank current

CH1: V_D12

CH2: V_D12

CH3: VOUT

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