ISL6744_14 Datasheet, PDF (10/18 Pages) Intersil Corporation – Intermediate Bus PWM Controller

ISL6744
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
I_RMS = I_OUT • D = 10 • 0.5 = 7.07
A
(EQ. 10)
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.
R = -------²----^π----^ρ--------
Ω
t
•
ln
⎛
⎜
⎝
r^-r--21-_{⎠⎟⎞}
(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 2.21mΩ. The winding that shares the layer
with the SR winding has a DC resistance of 2.65mΩ. 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 (I_OUT x N_S/N_P 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
4.25mΩ, and the outer trace has a resistance of 5.52mΩ.
The resistance of the primary then is 19.5mΩ 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.
FIGURE 7A. TOP LAYER: 1 TURN SECONDARY AND SR
WINDINGS
FIGURE 7B. INT. LAYER 1: 1 TURN SECONDARY WINDING
10
FN9147.8
September 22, 2005