LM3478MA_15 Datasheet, PDF (21/26 Pages) Texas Instruments – High-Efficiency Low-Side N-Channel Controller for Switching Regulator

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LM3478MA
SNVS705B – FEBRUARY 2011 – REVISED FEBRUARY 2013
Figure 35. Typical SEPIC Converter
POWER MOSFET SELECTION
As in a boost converter, parameters governing the selection of the MOSFET are the minimum threshold voltage,
V_TH(MIN), the on-resistance, R_DS(ON), the total gate charge, Q_g, the reverse transfer capacitance, C_RSS, and the
maximum drain to source voltage, V_DS(MAX). The peak switch voltage in a SEPIC is given by:
V_SW(PEAK) = V_IN + V_OUT + V_DIODE
(35)
The selected MOSFET should satisfy the condition:
V_DS(MAX) > V_SW(PEAK)
(36)
The peak switch current is given by:
(37)
The RMS current through the switch is given by:
(38)
POWER DIODE SELECTION
The Power diode must be selected to handle the peak current and the peak reverse voltage. In a SEPIC, the
diode peak current is the same as the switch peak current. The off-state voltage or peak reverse voltage of the
diode is V_IN + V_OUT. Similar to the boost converter, the average diode current is equal to the output current.
Schottky diodes are recommended.
SELECTION OF INDUCTORS L1 AND L2
Proper selection of inductors L1 and L2 to maintain continuous current conduction mode requires calculations of
the following parameters.
Average current in the inductors:
(39)
I_L2AVE = I_OUT
(40)
Peak to peak ripple current, to calculate core loss if necessary:
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