NCV898031_16 Datasheet, PDF (9/19 Pages) ON Semiconductor – 2 MHz Non-Synchronous SEPIC/Boost Controller

NCV898031
SEPIC TOPOLOGY APPLICATION INFORMATION
Oscillator
PWM Comparator ^{S Q}
R
+
CSA
Gate
Drive
GDRV
I_SNS
Voltage Error
V_FB
NCV898031
VEA
L1
V_IN
C_CPL
L2
C_o
R_L
Compensation
Figure 10. SEPIC Current Mode Schematic
SEPIC Design Methodology
This section details an overview of the component
selection process for the NCV898031 in continuous
conduction mode SEPIC. It is intended to assist with the
design process but does not remove all engineering design
work. Many of the equations make heavy use of the small
ripple approximation. This process entails the following
steps:
1. Define Operational Parameters
2. Select Current Sense Resistor
3. Select SEPIC Inductors
4. Select Coupling Capacitor
5. Select Output Capacitors
6. Select Input Capacitors
7. Select Feedback Resistors
8. Select Compensator Components
9. Select MOSFET(s)
10. Select Diode
1. Define Operational Parameters
Before beginning the design, define the operating
parameters of the application. These include:
V_IN(min): minimum input voltage [V]
V_IN(max): maximum input voltage [V]
V_OUT: output voltage [V]
I_OUT(max): maximum output current [A]
I_CL: desired typical cycle−by−cycle current limit [A]
From this the ideal minimum and maximum duty cycles
can be calculated as follows:
D_min
+
V_OUT
V_IN(max) )
V_OUT
D_max
+
V_OUT
V_IN(min) ) V_OUT
Both duty cycles will actually be higher due to power loss
in the conversion. The exact duty cycles will depend on
conduction and switching losses.
If the calculated D_WC (worst case) is higher than the D_max
limit of the NCV898031, the conversion will not be
possible. It is important for a SEPIC converter to have a
restricted D_max, because while the ideal conversion ratio of
a SEPIC converter goes up to infinity as D approaches 1, a
real converter’s conversion ratio starts to decrease as losses
overtake the increased power transfer. If the converter is in
this range it will not be able to regulate properly.
If the following equation is not satisfied, the device will
skip pulses at high V_IN:
D_min
f_s
w
t_on(min)
Where: f_s: switching frequency [Hz]
t_on(min): minimum on time [s]
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