ISL6722A Datasheet, PDF (15/24 Pages) Intersil Corporation – Flexible Single Ended Current Mode PWM

ISL6722A, ISL6723A
For simplicity, only the final design is further described.
An EPCOS EFD 20/10/7 core using N87 material gapped to
an A_L value of 25 nH/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 ΔB is only 0.069T or 690 gauss, a relatively
low value.
L_p
=
μ_----o-----•_----N_----p----²_----•_----A_-----e_----f_--f
lg
μH
(EQ. 25)
Since the number of primary turns, N_p, may be calculated.
The result is N_p = 40 turns. The secondary turns may be
calculated in Equation 26:
N_s
≤
-^I--^g-----^•----^〈---^V-----^o---^u----^t----+----^V-----^d---^〉----^•----^T-----^r
N_p • Ippk • μ_o • Aeff
(EQ. 26)
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 t_sw - t_ON(max) where t_sw = 1/f_sw. 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 N_s
for the 3.3V output using T = t_sw - t_{ON (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 N_s1 = 5 turns and N_s2 = 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 in
Equation 27:
Ip(rms) = Ippk • ^t--^O-----^N----⁽--^m-----^a---^x---⁾
A
3 • t_sw
(EQ. 27)
The peak and RMS current values in the remaining windings
may be calculated in Equation 28:
Ispk
=
2_-----•_-----I_--o_----u_---t_----•_----t_--s---w--
Tr
A
(EQ. 28)
Irms = 2 • Iout • ₃----^t-_•-^s---^w-_T----_r
A
(EQ. 29)
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
1500 VDC 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 is required due to the flyback voltage likely to be seen
on the primary of the isolation transformer.
15
FN9237.1
July 11, 2007