Functional check

AN3407

14/25

Doc ID 018857 Rev 1

5.2

Half-bridge resonant LLC circuit

The waveforms are measured in the resonant stage ZVS operation in Figure 18. 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. The switching frequency

has been chosen around 94 kHz.

Figure 18 shows waveforms during steady-state operation of the circuit at full load. A slight

asymmetry of operating modes by each half portion of the sine wave is visible: one half-

cycle is working at resonant frequency while the other half is working above the resonant

frequency. This is due to a small difference between each half’s secondary leakage

inductance of the transformer reflected to the primary side, providing two slightly different

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

transformer’s secondary windings and in this case it is not an issue.

Figure 19 demonstrates that during one half-cycle the circuit is working below the resonant

frequency, while during the following half-cycle it is working at resonance frequency.

Waveforms relevant to the secondary side are shown: the rectifier’s reverse voltage is

measured by Ch3 and Ch4 on the right of the picture. It is a bit higher than the theoretical

value that would be 2 (VOUT+VF), hence about 100 V.

Figure 18.

Primary side LLC waveforms at

Figure 19.

Secondary side LLC waveforms at

CH1: VCC

CH4: HB

CH3: Res. Tank current

CH1: Vout

CH4: V_D5

CH3: V_D7