ISL6721 Intersil Corporation, ISL6721 Datasheet - Page 12

• Manufacturer Part Number: ISL6721
• Description: Flexible Single Ended Current Mode PWM Controller
• Manufacturer: Intersil Corporation

Peak Primary Current:
Ippk
Maximum Primary Inductance:
Lp max
Choose desired primary inductance to be 40µH.
The core structure must be able to deliver a certain amount
of energy to the secondary on each switching cycle in order
to maintain the specified output power.
∆w
where ∆w is the amount of energy required to be transferred
each cycle and Vd is the drop across the output rectifier.
The capacity of a gapped ferrite core structure to store
energy is dependent on the volume of the airgap and can be
expressed as:
Vg
where Aeff is the effective cross sectional area of the core in
m
permeability of free space (4π • 10
in flux density in Tesla.
A core structure having less airgap volume than calculated
will be incapable of providing the full output power over
some portion of its operating range. On the other hand, if the
length of the airgap becomes large, magnetic field fringing
around the 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 A
required air gap volume to store the energy required, but
was needed for the window area it provides.
2
Aeff = 31 • 10
(
, lg is the length of the airgap in meters, µ
=
=
L
=
Aeff lg
Pout
value of 25 nH/N
--------------------------------------------- -
Fsw Ton max
)
=
2 Iavg in
•
•
•
Vin min
----------------------------------------------------------- -
•
---------------------------------
〈
Fsw Vout
Vout
=
(
-6
2 µ
-----------------------------
( )
(
•
•
+
) Ton max
Ippk
∆B
m
•
Vd
o
2
)
•
2
〉
2
∆w
=
was chosen. It has more than the
(
1.87
12
joules
)
m
=
3
-7
43.3
A
), and ∆B is the change
µH
o
is the
(EQ. 10)
(EQ. 11)
(EQ. 12)
(EQ. 9)
ISL6721
The flux density ∆B is only 0.069T or 690 gauss, a relatively
low value.
Since
L
the number of primary turns, N
result is N
calculated as follows:
N
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 N
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 N
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 from:
Ip rms
The peak and RMS current values in the remaining windings
may be calculated from:
Ispk
Irms
p
s
lg = 1.56 • 10
(
≤
=
Ig
-------------------------------------------------------- -
N
=
µ
----------------------------------------
=
s1
p
o
)
•
2 Iout Tsw
------------------------------------- -
2 Iout
•
•
=
〈
= 5 turns and N
•
Ippk µ
•
N
Vout
p
Ippk
p
lg
= 40 turns. The secondary turns may be
2
Tr
•
•
+
•
•
Aeff
•
-3
Vd
o
-------------- -
3 Tr
•
Ton max
----------------------------
Tsw
〉 Tr
3 Tsw
•
Aeff
m
•
•
(
s2
µH
A
= 3 turns. Checking the reset time
)
A
p
, may be calculated. The
A
(EQ. 13)
(EQ. 14)
(EQ. 15)
(EQ. 16)
(EQ. 17)
s