ISL6722A_15 Datasheet, PDF (15/26 Pages) Intersil Corporation – Flexible Single Ended Current Mode PWM Controllers

ISL6722A, ISL6723A
Design Criteria
The following design requirements were selected:
Switching Frequency, f_sw: 200kHz
V_IN: 36V to 75V
V_OUT(1): 3.3V @ 2.5A
V_OUT(2): 1.8V @ 1.0A
V_OUT(Bias): 12V @ 50mA
P_OUT: 10W
Efficiency: 70%
Maximum Duty Cycle, Dmax: 0.45
Transformer Design
The design of a flyback transformer is a non-trivial affair. It is
an iterative process which requires a great deal of
experience to achieve the desired result. It is a process of
many compromises, and even experienced designers will
produce different designs when presented with identical
requirements. The iterative design process is not presented
here for clarity.
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.
• 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. For flyback transformers, the ability to store
energy is the critical factor in determining the core size.
The cross sectional area of the core and the length of the
air gap in the magnetic path determine the energy storage
capability.
• 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 required based on flux density and energy storage
calculations.
• Determine the number of primary turns.
• Determine the turns ratio.
• Select the wire gauge for each winding.
• Determine winding order and insulation requirements.
• Verify the design.
Input Power:
P_OUT/Efficiency = 14.3W (use 15W)
Max On Time: t_ON(max) = Dmax/fsw = 2.25µs
Average Input Current: Iavg(in) = Pin/Vin(min) = 0.42A
Peak Primary Current:
Ippk = -_f--_s---^2w-----^-_---^I-_t-^a-_O---^v--_N--^g--_-^-_m--ⁱ--ⁿ--_a-^--_x----_ = 1.87
A
(EQ. 21)
Maximum Primary Inductance:
Lpmax = -^V----ⁱ--ⁿ----^--^m-----ⁱ--ⁿ----_I^--_p-^---_p--^t-_k-^O----^N----^---^m-----^a----^x----^ = 43.3
H
(EQ. 22)
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 = Pout  -^-_F--^V--_s---^ow---^u---_-^t---+_V-----_oV----_u-^d--_t-^-
joules
(EQ. 23)
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 = Aeff  lg = ²-----^-----^---^o-_B----^₂----^----^w---
m³
(EQ. 24)
where Aeff is the effective cross sectional area of the core in
m², lg is the
permeability
length
of free
osfptahceea(i4rg_a_p_1i0n-m_7)e, taenrds,_µB_o
is
is
the
the
change
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).
15
FN9237.2
September 29, 2015