NCV898031

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11

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:

From this the ideal minimum and maximum duty cycles can

be calculated as follows:

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.

NCV898031, the conversion will not be possible. It is

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

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:

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

After choosing a peak current ripple value, calculate the

inductor value as follows:

L +

The maximum average inductor current can be calculated as

follows:

The Peak Inductor current can be calculated as follows: