ISL6740IV-T Datasheet, PDF (15/28 Pages) Intersil Corporation – Flexible Double Ended Voltage and Current Mode PWM Controllers

ISL6740, ISL6741
The abbreviated design process follows:
• Select a core geometry suitable for the application.
Constraints of height, footprint, mounting preference, and
operating environment will affect the choice.
• Determine the turns ratio.
• Select suitable core material(s).
• Select maximum flux density desired for operation.
• Select core size. Core size will be dictated by the capability of
the core structure to store the required energy, the number of
turns that have to be wound, and the wire gauge needed. Often
the window area (the space used for the windings) and power
loss determine the final core size.
• Determine maximum desired flux density. Depending on the
frequency of operation, the core material selected, and the
operating environment, the allowed flux density must be
determined. The decision of what flux density to allow is often
difficult to determine initially. Usually the highest flux density
that produces an acceptable design is used, but often the
winding geometry dictates a larger core than is indicated
based on flux density alone.
• Determine the number of primary turns.
• Select the wire gauge for each winding.
• Determine winding order and insulation requirements.
• Verify the design.
n_SR
n_S
n_P
n_S
n_SR
FIGURE 7. TRANSFORMER SCHEMATIC
For this application we have selected a planar structure to
achieve a low profile design. A PQ style core was selected
because of its round center leg cross section, but there are many
suitable core styles available.
Since the converter is operating open loop at nearly 100% duty
cycle, the turns ratio, N, is simply the ratio of the input voltage to
the output voltage divided by 2.
N
=
-------^V----^I--^N--------
V_OUT • 2
=
-----⁴----⁸------
12 • 2
=
2
(EQ. 14)
The factor of 2 divisor is due to the half-bridge topology. Only half
of the input voltage is applied to the primary of the transformer.
A PC44HPQ20/6 “E-Core” plus a PC44PQ20/3 “I-Core” from TDK
were selected for the transformer core. The ferrite material is
PC44.
The core parameter of concern for flux density is the effective
core cross sectional area, Ae. For the PQ core pieces selected:
Ae = 0.62cm² or 6.2e -5m²
Using Faraday’s Law, V = N dΦ/dt, the number of primary turns
can be determined once the maximum flux density is set. An
acceptable Bmax is ultimately determined by the allowable
power dissipation in the ferrite material and is influenced by the
lossiness of the core, core geometry, operating ambient
temperature, and air flow. The TDK datasheet for PC44 material
indicates a core loss factor of ~400mW/cm³ with a ±2000
gauss 100kHz sinusoidal excitation. The application uses a
235kHz square wave excitation, so no direct comparison
between the application and the data can be made. Interpolation
of the data is required. The core volume is approximately
1.6cm³, so the estimated core loss is
P_loss
≈
-^m------^W----
cm³
•
cm³
•
----^f--^a---^c---^t---
f_meas
=
0.4
•
1.6
•
2_----0_----0_---k_----H_----z_-
100 k H z
=
1.28
W
(EQ. 15)
1.28W of dissipation is significant for a core of this size.
Reducing the flux density to 1200 gauss will reduce the
dissipation by about the same percentage, or 40%. Ultimately,
evaluation of the transformer’s performance in the application
will determine what is acceptable.
From Faraday’s Law and using 1200 gauss peak flux density (ΔB
= 2400 gauss or 0.24 tesla)
N
=
---^V---^I--^N-----^•----^T---^O----^N----
2 • A_e • ΔB
=
----------⁵----³-----^•----²-----^•----¹----⁰----^–--⁶-----------
2 • 6.2 • 10^–5 • 0.24
=
3.56
turns
(EQ. 16)
Rounding up yields 4 turns for the primary winding. The peak flux
density using 4 turns is ~1100 gauss. From Equation 1, the
number of secondary turns is 2.
The volts/turn for this design ranges from 5.4V at V_IN = 43V to
6.6V at V_IN = 53V. Therefore, the synchronous rectifier (SR)
windings may be set at 1 turn each with proper FET selection.
Selecting 2 turns for the synchronous rectifier 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. 17)
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 Equation 17, 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
15
FN9111.6
December 2, 2011