User`s guide

Hardware Design

FIGURE 2-8: ZVT FULL-BRIDGE TRANSFORMER WINDING

CONSTRUCTION

The last check is to assess whether an additional inductor is needed for ZVT operation.

The leakage inductance (see Reference 8 in Appendix C. “References”) for a

standard construction transformer is given by Equation 2-22.

EQUATION 2-22:

The ideal computed leakage inductance is 32 µH, which meets the criteria of

Equation 2-9 with the MOSFET output capacitance without requiring an additional

resonant inductor. Provision has been made on the reference design PCB for an

external inductor and parallel capacitors across the MOSFETs if the ZVT operation

needs tuning.

2.2.2 Synchronous Rectifier Design

Synchronous rectification is the technique used for reducing the power loss in the

output stage of switched mode power supplies (see Reference 9 in Appendix

C. “References”). The conventional diode is replaced by a MOSFET and is controlled

so that current will flow into the third quadrant in the source-to-drain direction when the

equivalent R

DS(ON) voltage drop is lower than the intrinsic diode voltage drop. This

means that with very low R

DS(ON) MOSFETs, the power supply efficiency can be

significantly increased. This is especially true in low output voltage power supplies.

3 mm

margin

3 mm

margin

3 mm

margin

3 mm

margin

64 turns of 8 x 0.2 mm Litz wire

3 layers 0.05 mm Melinex

5 turns of 0.1 mm x 13 mm foil

3 layers 0.05 mm Melinex

410

mp p s

lN h h

−

×+

⎛⎞

⎜⎟

⎝⎠

µH

where c is the space between the primary and secondary.

All dimensions are in mm.