TIDUET3 Design guide

TIDUET3 February 2021

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2.4.2 LLC Stage Design

LLC topology has been widely used in telecom and server power supplies. LLC topology can get a wide gain range by changing the frequency. LLC topology usually applies to generation of a constant output voltage for a wide-input DC voltage range or generation of a variable output voltage for a constant input DC voltage. The wider the frequency range is, the more different the LLC design is. The DC/DC stage in this design must support 20-ms holdup time. LLC topology can achieve both ZVS for the primary MOSFET and ZCS for the secondary diode under resonant frequency. Considering efficiency, the converter is designed to operate at a frequency slightly lower than resonance frequency at full load.

Select the Transformer
The transformer can be built or purchased from Wurth. The detail specifications are found on the Wurth website with P/N 750344508.
Select the Resonant Inductor
The resonant inductor can be built or purchased from Wurth. The detail specifications are found on Wurth website with P/N 750344650/
Inductor Terminal AC voltage can be expressed as the following:
Equation 12.
Select the Resonant Capacitor
This capacitor carries the full-primary current at a high frequency. A low dissipation factor part is needed to prevent overheating in the part.
The AC voltage across the resonant capacitor is given by its impedance times the current.
Equation 13.

Equation 14.

Equation 15.

Equation 16.
Select the Primary Side MOSFETs
Each MOSFET detects the input voltage as its maximum applied voltage. Choose the MOSFET voltage rating to be 1.5 times of the maximum bulk voltage.
Equation 17.

Choose the MOSFET current rating to be 1.1 times of the maximum primary side RMS current.
Equation 18.

For the LLC power stage working in ZVS, the turn-on losses can be neglected. The choice of MOSFET must be based on R_DS(on) and Coss. Optimizing the Coss helps in minimizing the dead time required for achieving ZVS, thereby minimizing duty cycle loss. The feature that optimizes the adaptive dead time of the UCC256403 helps in maximizing the duty cycle, thereby improving efficiency.
Select the Secondary Side MOSFETs
The secondary-side rectifier voltage rating is determined by:
Equation 19.

The current rating of the secondary-side MOSFET is determined by:
Equation 20.

This reference design uses 150-V MOS with its low R_DS(on) (7.6 mΩ) and Qg (21 nC). The very low R_DS(on) helps in reducing the overall loss in the synchronous rectifier.