InnoSwitch3-EP
the increased leakage inductance and the physical fit of the windings
should be considered. A split primary construction may be helpful for
designs where clamp dissipation due to leakage inductance is too
high. In split primary construction, half of the primary winding is
placed on either side of the secondary (and bias) winding in a
sandwich arrangement. This arrangement is often disadvantageous
for low power designs as this typically increases common mode noise
and adds cost to the input filtering.
output voltage is low and little reset of the transformer occurs during
the MOSFET off-time. This allows the transformer flux density to
staircase beyond the normal operating level. A value of 3800 gauss
at the peak current limit of the selected device together with the
built-in protection features of InnoSwitch3-EP IC provide sufficient
margin to prevent core saturation under start-up or output short-
circuit conditions.
Transformer Primary Inductance, (LP)
Maximum Operating Flux Density, BM (Gauss)
Once the lowest operating input voltage, switching frequency at full
load, and required VOR are determined, the transformers primary
inductance can be calculated. The PIXls design spreadsheet can be
used to assist in designing the transformer.
A maximum value of 3800 gauss at the peak device current limit
(at 132 kHz) is recommended to limit the peak flux density at start-up
and under output short-circuit conditions. Under these conditions the
IR
IP
KP ≡ KRP
=
IR
IP
Primary
(a) Continuous, KP < 1
IP
IR
Primary
(b) Borderline Continuous/Discontinuous, KP = 1
PI-2587-103114
Figure 17. Continuous Conduction Mode Current Waveform, KP < 1.
17
Rev. D 08/18
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