LT3959
15
3959fa
For more information www.linear.com/LT3959
SEPIC CONVERTER APPLICATIONS
The LT3959 can be configured as a SEPIC (single-ended
primary inductance converter), as shown in Figure 1. This
topology allows for the input to be higher, equal, or lower
than the desired output voltage. The conversion ratio as
a function of duty cycle is:
OUT
D
V
=
1−D
In continuous conduction mode (CCM).
In
a SEPIC converter, no DC path exists between the input
and output. This is an advantage over the boost converter
for applications requiring the output to be disconnected
from the input source when the circuit is in shutdown.
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 diode forward
voltage (V
D
).
The maximum duty cycle (D
MAX
) occurs when the converter
has the minimum input voltage:
D
MAX
=
OUT
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 voltages
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)
•
MAX
1– D
MAX
I
L2(MAX)
= I
O(MAX)
Due to the current limit of it’s internal power switch,
the LT3959 should be used in a SEPIC converter whose
maximum output current (IO(MAX)) is less than the output
current capability by a sufficient margin (10% or higher
is recommended):
I
O(MAX)
< (1–D
MAX
) • (6A – 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 and greater core
losses and reduces output current capability.
Given an operating input voltage range, and having chosen
the operating frequency and ripple current in the inductor,
the inductor value (L1 and L2 are independent) of the SEPIC
converter can be determined using the following equation:
L1 = L2 =
IN(MIN)
0.5 • ∆I
SW
• ƒ
OSC
•D
MAX
For most SEPIC applications, the equal inductor values
will fall in the range of 1µH to 100µH.
By making L1 = L2, and winding them on the same core, the
value of inductance in the preceding equation is replaced
by 2L, due to mutual inductance:
L =
IN(MIN)
∆I
SW
• ƒ
OSC
•D
MAX
This maintains
the same ripple current and energy storage
in the inductors. The peak inductor currents are:
I
L1(PEAK)
= I
L1(MAX)
+ 0.5 • ∆I
L1
I
L2(PEAK)
= I
L2(MAX)
+ 0.5 • ∆I
L2
The maximum RMS inductor currents are approximately
equal to the maximum average inductor currents.
applicaTions inForMaTion
