InnoSwitch3-CP
Maximum Operating Flux Density, BM (Gauss)
As a minimum, the following tests are strongly recommended:
1. Maximum Drain Voltage – Verify that VDS of InnoSwitch3-CP and
SR FET do not exceed 90% of breakdown voltages at the highest
input voltage and peak (overload) output power in normal
operation and during start-up.
2. Maximum Drain Current – At maximum ambient temperature,
maximum input voltage and peak output (overload) power.
Review drain current waveforms for any signs of transformer
saturation or excessive leading-edge current spikes at start-up.
Repeat tests under steady-state conditions and verify that the
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
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-CP IC provide sufficient
margin to prevent core saturation under start-up or output short-
circuit conditions.
leading edge current spike is below ILIMIT(MIN) at the end of tLEB(MIN)
.
Under all conditions, the maximum d rain current for the
primary MOSFET should be below the specified absolute maximum
ratings.
Transformer Primary Inductance, (LP)
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.
3. Thermal Check – At specified maximum output power, minimum
input voltage and maximum ambient temperature, verify that
temperature specification limits for InnoSwitch3-CP IC,
transformer, output SR FET, and output capacitors are not
exceeded. Enough thermal margin should be allowed for
part-to-part variation of the RDS(ON) of the InnoSwitch3-CP IC.
Under low-line, maximum power, a maximum InnoSwitch3-CP
SOURCE pin temperature of 110 °C is recommended to allow for
these variations.
Quick Design Checklist
As with any power supply, the operation of all InnoSwitch3-CP
designs should be verified on the bench to make sure that component
limits are not exceeded under worst-case conditions.
18
Rev. D 08/18
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