SEPIC Switching States

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SEPIC Switching States
SEPIC Capacitor



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1. The initial state of a SEPIC before the
switch closes is shown above. The SEPIC
capacitor has charged to VIN. The output is 0V,
and no current is flowing in any of the
components. 2. When the switch closes, VIN is
applied across the inductor L1. Current ramps up
through L1, which stores energy (just like a
boost). VIN is also applied across L2! This
voltage comes from the SEPIC capacitor. The
SEPIC capacitor starts dumping its energy into
L2 by ramping current through L2. The diode is
reverse biased during this time. Now current is
flowing in both inductors which can not change
instantaneously when the switch opens again. 3.
The switch opens. Current through L1 has no
place to go but through the SEPIC capacitor to
the output capacitor and the output. The current
through L2 must also go to the output. In order
for the current to continue to flow through L1,
the voltage on the switch boosts up to
VINVOUTVDIODE. The current flowing through the
SEPIC capacitor charges it up again to enable it
to transfer this energy to L2 when the switch
closes. There is an energy balance between the
SEPIC capacitor and L2, which helps determine the
value of the SEPIC capacitor. Keeping the value
of the SEPIC capacitor low helps ensure stable
operation.