Datasheet

LT3958
20
3958fa
For more information www.linear.com/LT3958
applicaTions inForMaTion
Compared to the flyback converter, the SEPIC converter
has the advantage that both the power MOSFET and the
output diode voltages are clamped by the capacitors (C
IN
,
C
DC
and C
OUT
), therefore, there is less voltage ringing
across the power MOSFET and the output diodes. The
SEPIC converter requires much smaller input capacitors
than those of the flyback converter. This is due to the fact
that, in the SEPIC converter, the current through inductor
L1 (which is series with the input) is continuous.
SEPIC Converter: Switch Duty Cycle and Frequency
For a SEPIC converter operating in CCM, the duty cycle
of the main switch can be calculated based on the output
voltage (V
OUT
), the input voltage (V
IN
) and the diode
forward voltage (V
D
).
The maximum duty cycle (D
MAX
) occurs when the converter
has the minimum input voltage:
D
MAX
=
V
OUT
+ V
D
V
IN(MIN)
+ V
OUT
+ V
D
SEPIC Converter: The Maximum Output Current
Capability and Inductor Selection
As shown in Figure 1, the SEPIC converter contains two
inductors: L1 and L2. L1 and L2 can be independent, but
can also be wound on the same core, since identical volt
-
ages are applied to L1 and L2 throughout the switching
cycle.
For the SEPIC topology
, the current through L1 is the
converter input current. Based on the fact that, ideally
, the
output power is equal to the input power, the maximum
average inductor currents of L1 and L2 are:
I
L1(MAX)
=I
IN(MAX)
=I
O(MAX)
•
D
MAX
1− D
MAX
I
L2(MAX)
=I
O(MAX)
In a SEPIC converter, the switch current is equal to I
L1
+
I
L2
when the power switch is on, therefore, the maximum
average switch current is defined as:
I
SW(MAX)
=I
L1(MAX)
+I
L2(MAX)
=I
O(MAX)
•
1
1− D
MAX
Due to the current limit of its internal power switch,
the LT3958 should be used in a SEPIC converter whose
maximum output current (I
O(MAX)
) is less than the output
current capability by a sufficient margin (10% or higher
is recommended):
I
O(MAX)
< (1 – D
MAX
) • (3.3A – 0.5 • ∆I
SW
)
The inductor ripple currents ∆I
L1
and ∆I
L2
are identical:
∆I
L1
= ∆I
L2
= 0.5 • ∆I
SW
The inductor ripple current ∆I
SW
has a direct effect on the
choice of the inductor value and the converter’s maximum
output current capability. Choosing smaller values of ∆I
SW
requires large inductances and reduces the current loop
gain (the converter will approach voltage mode). Accepting
larger values of ∆I
SW
allows the use of low inductances,
but results in higher input current ripple, greater core
losses, lower output current capability and in some cases,
subharmonic oscillation. A good start point for ∆I
SW
is
0.6A though careful evaluation of system stabililty should
be made to ensure adequate design margin.
Given an operating input voltage range, and having chosen
the operating frequency and ripple current in the induc
-
tor, the inductor value (L1 and L2 are independent) of the
SEPIC converter can be determined using the following
equation:
L1= L2 =
V
IN(MIN)
1.5A • ∆I
SW
• ƒ
• D
MAX
For most SEPIC applications, the equal inductor values
will fall in the range of 1µH to 100µH.
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