NCV898031(2012) Просмотр технического описания (PDF) - ON Semiconductor
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NCV898031 Datasheet PDF : 13 Pages
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NCV898031
BOOST TOPOLOGY APPLICATION INFORMATION
Design Methodology
This section details an overview of the component
selection process for the NCV898031 in continuous
conduction mode boost. 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 Output Inductor
4. Select Output Capacitors
5. Select Input Capacitors
6. Select Feedback Resistors
7. Select Compensator Components
8. Select MOSFET(s)
9. Select Diode
Define Operational Parameters
Before beginning the design, define the operating
parameters of the application. These include:
VIN(min): minimum input voltage [V]
VIN(max): maximum input voltage [V]
VOUT: output voltage [V]
IOUT(max): maximum output current [A]
ICL: desired typical cycle−by−cycle current limit [A]
From this the ideal minimum and maximum duty cycles can
be calculated as follows:
Dmin
+
1
*
VIN(max)
VOUT
DWC
+
1
*
VIN(WC)
VOUT
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 maximum input
voltage is higher than the output voltage, the minimum duty
cycle will be negative. This is because a boost converter
cannot have an output lower than the input. In situations
where the input is higher than the output, the output will
follow the input, minus the diode drop of the output diode
and the converter will not attempt to switch.
If the calculated DWC is higher than the Dmax limit of the
NCV898031, the conversion will not be possible. It is
important for a boost converter to have a restricted Dmax,
because while the ideal conversion ratio of a boost 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 VIN:
Dmin
fs
w
ton(min)
Where: fs: switching frequency [Hz]
ton(min): minimum on time [s]
Select Current Sense Resistor
Current sensing for peak current mode control and current
limit relies on the MOSFET current signal, which is
measured with a ground referenced amplifier. The easiest
method of generating this signal is to use a current sense
resistor from the source of the MOSFET to device ground.
The sense resistor should be selected as follows:
RS
+
VCL
ICL
Where: RS: sense resistor [W]
VCL: current limit threshold voltage [V]
ICL: desire current limit [A]
Select Output Inductor
The output inductor controls the current ripple that occurs
over a switching period. A high current ripple will result in
excessive power loss and ripple current requirements. A low
current ripple will result in a poor control signal and a slow
current slew rate in case of load steps. A good starting point
for peak to peak ripple is around 20−40% of the inductor
current at the maximum load at the worst case VIN, but
operation should be verified empirically. The worst case VIN
is half of VOUT, or whatever VIN is closest to half of VIN.
After choosing a peak current ripple value, calculate the
inductor value as follows:
L
+
VIN(WC) 2 DWC
DIL,max fsVOUT
Where: VIN(WC): VIN value as close as possible to half of
VOUT [V]
DWC: duty cycle at VIN(WC)
DIL,max: maximum peak to peak ripple [A]
The maximum average inductor current can be calculated as
follows:
IL,avg
+
VOUTIOUT(max)
VIN(min)
The Peak Inductor current can be calculated as follows:
IL,peak
+
IL,avg
)
VIN(min) 2 DWC
LfsVOUT
Where: IL,peak: Peak inductor current value [A]
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