ISL6721 datasheet(14/23 Pages) RENESAS

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ISL6721

FN9110 Rev 9.00

Page 14 of 23

May 20, 2016

gap occurs. This has the effect of increasing the airgap volume.

Some fringing is usually acceptable, but excessive fringing can

cause increased losses in the windings around the gap resulting

in excessive heating. Once a suitable core and gap combination

are found, the iterative design cycle begins. A design is

developed and checked for ease of assembly and thermal

performance. If the core does not allow adequate space for the

windings, then a core with a larger window area is required. If

the transformer runs hot, it may be necessary to lower the flux

density (more primary turns, lower operating frequency), select

a less lossy core material, change the geometry of the windings

(winding order), use heavier gauge wire or multi-filar windings,

and/or change the type of wire used (Litz wire, for example).

For simplicity, only the final design is further described.

An EPCOS EFD 20/10/7 core using N87 material gapped to an

AL value of 25nH/N2 was chosen. It has more than the

required air gap volume to store the energy required, but was

needed for the window area it provides.

Aeff = 31

 10-6 m2

lg = 1.56

10-3 m

The flux density

B is only 0.069T or 690 gauss, a relatively

low value.

Since:

The number of primary turns, Np, may be calculated. The result

is Np = 40 turns. The secondary turns may be calculated as

follows:

Where tr is the time required to reset the core. Since

discontinuous MMF mode operation is desired, the core must

completely reset during the off time. To maintain

discontinuous mode operation, the maximum time allowed to

reset the core is tsw - tON(MAX) where tsw = 1/fsw. The

minimum time is application dependent and at the designers

discretion knowing that the secondary winding RMS current

and ripple current stress in the output capacitors increases

with decreasing reset time. The calculation for maximum Ns

for the 3.3 V output using t = tsw - tON (MAX) = 2.75µs is 5.52

turns.

The determination of the number of secondary turns is also

dependent on the number of outputs and the required turns

ratios required to generate them. If Schottky output rectifiers

are used and we assume a forward voltage drop of 0.45V, the

required turns ratio for the two output voltages, 3.3V and 1.8V,

is 5:3.

With a turns ratio of 5:3 for the secondary windings, we will

use Ns1 = 5 turns and Ns2 = 3 turns. Checking the reset time

using these values for the number of secondary turns yields a

duration of Tr = 2.33µs or about 47% of the switching period,

an acceptable result.

The bias winding turns may be calculated similarly, only a

diode forward drop of 0.7V is used. The rounded off result is 17

turns for a 12V bias.

The next step is to determine the wire gauge. The RMS current

in the primary winding may be calculated using Equation 15:

The peak and RMS current values in the remaining windings

may be calculated using Equations 16 and 17:

The RMS current for the primary winding is 0.72A, for the 3.3V

output, 4.23A, for the 1.8V output, 1.69A, and for the bias

winding, 85mA.

To minimize the transformer leakage inductance, the primary

was split into two sections connected in parallel and

positioned such that the other windings were sandwiched

between them. The output windings were configured so that

the 1.8V winding is a tap off of the 3.3V winding. Tapping the

1.8V output requires that the shared portion of the secondary

conduct the combined current of both outputs. The secondary

wire gauge must be selected accordingly.

The determination of current carrying capacity of wire is a

compromise between performance, size, and cost. It is

affected by many design constraints such as operating

frequency (harmonic content of the waveform) and the winding

proximity/geometry. It generally ranges between 250 and

1000 circular mils per ampere. A circular mil is defined as the

area of a circle 0.001” (1 mil) in diameter. As the frequency of

operation increases, the AC resistance of the wire increases

due to skin and proximity effects. Using heavier gauge wire

may not alleviate the problem. Instead multiple strands of wire

in parallel must be used. In some cases, Litz wire is required.

The winding configuration selected is:

Primary #1: 40T, 2 #30 bifilar

Secondary: 5T, 0.003” (3 mil) copper foil tapped at 3T

Bias: 17T #32

Primary #2: 40T, 2 #30 bifilar

The internal spacing and insulation system was designed for

1500VDC dielectric withstand rating between the primary and

secondary windings.

Power MOSFET Selection

Selection of the main switching MOSFET requires

consideration of the voltage and current stresses that will be

encountered in the application, the power dissipated by the

device, its size, and its cost.

The input voltage range of the converter is 36VDC to

75VDC. This suggests a MOSFET with a voltage rating of 150V



o Np

2 Aeff



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

H

(EQ. 13)

Vout Vd

 tr



Np Ippk o Aeff



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



(EQ. 14)

IPRMS



IPPK

tON MAX



3tsw



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



(EQ. 15)

ISPK

2IOUT tsw



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

(EQ. 16)

IRMS

2IOUT



tsw

3Tr



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



(EQ. 17)

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