Power integrations LinkSwitch-4 LNK4*15D Series Application Note page 10

Application Note
Step 5 – Transformer Core Parameters
The default peak core flux is set to 3900 Gauss. If the users selected
core material requires a different value, enter it into [B86].
The default tolerance on the primary inductance is ±10%. A tighter
tolerance allows better average efficiency across production, an
alternative value may be entered into [B88].
Step 6 – Transformer Primary Winding Design
Parameters
The default number of primary winding layers is 3, however if [H103]
indicates that the current density is low, then 2, or even 1, can be
TRANSFORMER CORE PARAMETERS
BP
LP
LP Tolerance
ALG
BM
BAC
ur
LG
Figure 12. Transformer Core Operational Parameters.
TRANSFORMER PRIMARY DESIGN PARAMETERS
L
OD
INS
DIA
AWG
Primary Current Density (J)
Bias/Feedback Winding
NB
VDB
VB_NOLOAD
PB_NOLOAD
VB_NOLOAD_MEASURED
Figure 13. Transformer Primary Design Parameters.
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Rev. B 10/17
entered into [B98] and the wire diameter will be reduced accordingly.
The lower the number of primary layers, the lower leakage induc-
tance is likely to be. However it may be possible that sandwiching
the secondary between 2 primary layers would result in the lowest
leakage inductance at the cost of a more complicated transformer
construction.
If [H103] indicates that the current density is high, then a higher
value must be entered into [B98]. More than 3 primary layers will
result in high primary to secondary leakage inductance, resulting in
high-voltage stress on the BJT or increased clamp/snubber losses, so
it may be necessary to sandwich the secondary between layers of the
primary.
Peak Flux Density @ max IOCP & max LP. 3900 Gauss. Lower BP may reduce
3900 Gauss
efficiency
1099 uHenries Nominal Primary Inductance
Tolerance of Primary Inductance. Tighter tolerance allows better average
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efficiency across production.
100 nH/T^2 Gapped Core Effective Inductance
2763 Gauss Maximum Flux Density at PO, VMIN, LP (BM<3000)
1381 Gauss AC Flux Density for Core Loss Curves (0.5 X Peak to Peak)
1614 Relative Permeability of Ungapped Core
0.26 mm Gap Length (Lg > 0.1 mm)
3.0 Number of Primary Layers
0.25 mm Maximum Primary Wire Diameter including insulation to fill layers
0.05 mm Estimated Total Insulation Thickness (= 2 * film thickness)
0.20 mm Bare conductor diameter
33 AWG Primary Wire Gauge (Rounded to next smaller standard AWG value)
5.23 Amps/mm^2 Primary Winding Current density (3.8 < J < 10)
9 Turns Suggested number of turns for the bias / feedback winding
0.70 Volts Bias Winding Diode Forward Voltage Drop
7.40 Volts Desired Minimum Bias voltage at no load
4.14 mW Bias winding power consumption estimate at no load
7.40 Volts Measured Bias voltage at no load
AN-69
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