ISL6744ABZ datasheet(10/18 Pages) INTERSIL

10 / 18 page

FN9147.8

September 22, 2005

windings would also be acceptable, but the gate drive losses

would increase.

The next step is to determine the equivalent wire gauge for

the planar structure. Since each secondary winding

conducts for only 50% of the period, the RMS current is

where D is the duty cycle. Since an FR-4 PWB planar

winding structure was selected, the width of the copper

traces is limited by the window area width, and the number

of layers is limited by the window area height. The PQ core

selected has a usable window area width of 0.165 inches.

Allowing one turn per layer and 0.020 inches clearance at

the edges allows a maximum trace width of 0.125 inches.

Using 100 circular mils(c.m.)/A as a guideline for current

density, and from EQ. 10, 707c.m. are required for each of

the secondary windings (a circular mil is the area of a circle

0.001 inches in diameter). Converting c.m. to square mils

yields 555mils2 (0.785 sq. mils/c.m.). Dividing by the trace

width results in a copper thickness of 4.44mils (0.112mm).

Using 1.3mils/oz. of copper requires a copper weight of

3.4oz. For reasons of cost, 3oz. copper was selected.

One layer of each secondary winding also contains the

synchronous rectifier winding. For this layer the secondary

trace width is reduced by 0.025 inches to 0.100 inches(0.015

inches for the SR winding trace width and 0.010 inches

spacing between the SR winding and the secondary

winding).

The choice of copper weight may be validated by calculating

the DC copper losses of the secondary winding. Ignoring the

terminal and lead-in resistance, the resistance of each layer

of the secondary may be approximated using EQ. 11.

where

R = Winding resistance

ρ = Resistivity of copper = 669e-9Ω-inches at 20°C

t = Thickness of the copper (3 oz.) = 3.9e-3 inches

r2 = Outside radius of the copper trace = 0.324 or 0.299

inches

r1 = Inside radius of the copper trace = 0.199 inches

The winding without the SR winding on the same layer has a

DC resistance of 2.21m

Ω. The winding that shares the layer

with the SR winding has a DC resistance of 2.65m

Ω. With

the secondary configured as a 4 turn center tapped winding

(2 turns each side of the tap), the total DC power loss for the

secondary at 20°C is 486mW.

The primary windings have an RMS current of approximately

5 A (IOUT x NS/NP at ~ 100% duty cycle). The primary is

configured as 2 layers, 2 turns per layer to minimize the

winding stack height. Allowing 0.020 inches edge clearance

and 0.010 inches between turns yields a trace width of

0.0575 inches. Ignoring the terminal and lead-in resistance,

and using EQ. 11, the inner trace has a resistance of

4.25m

Ω, and the outer trace has a resistance of 5.52mΩ.

The resistance of the primary then is 19.5m

Ω at 20°C. The

total DC power loss for the primary at 20°C is 489mW.

Improved efficiency and thermal performance could be

achieved by selecting heavier copper weight for the

windings. Evaluation in the application will determine its

need.

The order and geometry of the windings affects the AC

resistance, winding capacitance, and leakage inductance of

the finished transformer. To mitigate these effects,

interleaving the windings is necessary. The primary winding

is sandwiched between the two secondary windings. The

winding layout appears below.

IRMS

IOUT

•

0.5

•

7.07

(EQ. 10)

πρ

-----







•

------------------------

Ω

(EQ. 11)

FIGURE 7A. TOP LAYER: 1 TURN SECONDARY AND SR

WINDINGS

FIGURE 7B. INT. LAYER 1: 1 TURN SECONDARY WINDING

ISL6744

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