ISL6744ABZ datasheet(11/18 Pages) INTERSIL

11 / 18 page

FN9147.8

September 22, 2005

MOSFET Selection

The criteria for selection of the primary side half-bridge FETs

and the secondary side synchronous rectifier FETs is largely

based on the current and voltage rating of the device.

However, the FET drain-source capacitance and gate

charge cannot be ignored.

The zero voltage switch (ZVS) transition timing is dependent

on the transformer’s leakage inductance and the

capacitance at the node between the upper FET source and

the lower FET drain. The node capacitance is comprised of

the drain-source capacitance of the FETs and the

transformer parasitic capacitance. The leakage inductance

and capacitance form an LC resonant tank circuit which

determines the duration of the transition. The amount of

energy stored in the LC tank circuit determines the transition

voltage amplitude. If the leakage inductance energy is too

low, ZVS operation is not possible and near or partial ZVS

operation occurs. As the leakage energy increases, the

voltage amplitude increases until it is clamped by the FET

body diode to ground or VIN, depending on which FET

conducts. When the leakage energy exceeds the minimum

required for ZVS operation, the voltage is clamped until the

energy is transferred. This behavior increases the time

window for ZVS operation. This behavior is not without

consequences, however. The transition time and the period

of time during which the voltage is clamped reduces the

effective duty cycle.

The gate charge affects the switching speed of the FETs.

Higher gate charge translates into higher drive requirements

and/or slower switching speeds. The energy required to

drive the gates is dissipated as heat.

The maximum input voltage, VIN, plus transient voltage,

determines the voltage rating required. With a maximum

input voltage of 53V for this application, and if we allow a

10% adder for transients, a voltage rating of 60V or higher

will suffice.

FIGURE 7C. INT. LAYER 2: 2 TURNS PRIMARY WINDING

FIGURE 7D. INT. LAYER 3: 2 TURNS PRIMARY WINDING

FIGURE 7E. INT. LAYER 4: 1 TURN SECONDARY WINDING

FIGURE 7F. BOTTOM LAYER: 1 TURN SECONDARY AND SR

WINDINGS

FIGURE 7G. PWB DIMENSIONS

∅0.689

0.807

0.639

0.403

0.169

0.000

1.054

0.774

0.479

0.184

0.000

∅0.358

ISL6744

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