Knowledge Explanation of Inductive Boost DC/DC Converter

What is an Inductive Boost DC/DC Converter?

As shown in Figure 1, a simplified inductor-type DC-DC converter circuit, closing the switch causes the current through the inductor to increase. Opening the switch causes the current to flow through the diode to the output capacitor. Due to the storage of current from the inductor, the voltage of the output capacitor increases after multiple switching cycles, resulting in an output voltage higher than the input voltage.

What factors determine the output voltage of an inductive step-up DC-DC converter?

In the actual circuit shown in Figure 2, an IC with an integrated power MOSFET replaces the mechanical switch, and the MOSFET is turned on and off by a pulse-width modulation (PWM) circuit. The output voltage is always determined by the PWM duty cycle, and at a 50% duty cycle, the output voltage is twice the input voltage. Doubling the voltage will cause the input current to be twice the output current, and for a practical lossy circuit, the input current will be slightly higher.

How does the inductor value affect the performance of an inductive boost converter?

Because the inductor value affects the input and output ripple voltage and current, the choice of inductor is critical to the design of an inductive voltage converter. Inductors with low equivalent series resistance values ​​have the best power conversion efficiency. The inductor saturation current rating should be selected to be greater than the circuit's steady-state inductor current peak.

What are the principles for selecting the output diode of an inductive boost converter IC circuit?

For boost converters, choose fast Schottky rectifier diodes. Compared with ordinary diodes, Schottky diodes have a small forward voltage drop, which makes them low power consumption and high efficiency. The average current rating of Schottky diodes should be greater than the maximum output voltage of the circuit.

How to choose the input capacitor of the inductive boost converter IC circuit?

The input of the boost regulator is a triangular voltage waveform, so the input capacitor must reduce the input ripple and noise. The amplitude of the ripple is inversely proportional to the value of the input capacitor, that is, the larger the capacitor value, the smaller the ripple. If the converter load changes little and the output current is small, it is safe to use a small input capacitor. If the difference between the converter input and the source output is small, small capacitors can also be selected. If the circuit is required to have little interference with the input voltage source ripple, large capacitors may be required and/or the equivalent series resistance (ESR) may be reduced.

What factors should be considered when selecting the output capacitor in an inductive boost converter IC circuit?

The choice of output capacitor is determined by the output voltage ripple. In most cases, low ESR capacitors such as ceramic and polymer electrolytic capacitors should be used. If high ESR capacitors are used, the converter frequency compensation needs to be carefully reviewed and additional capacitance may be required at the output circuit.

What factors should be considered when laying out an inductive boost converter IC circuit?

First, the input capacitor should be as close to the IC as possible to reduce the copper trace resistance that affects the IC input voltage ripple. Second, place the output capacitor close to the IC. The length of the copper trace connecting the output capacitor affects the output voltage ripple. Third, minimize the length of the trace connecting the inductor and output diode to reduce power dissipation and improve efficiency. Finally, keep the output feedback resistor away from the inductor to minimize the impact of noise.

Where are inductive boost converters used?

One major application area for inductive boost converters is in powering white LEDs that provide backlighting for liquid crystal display (LCD) panels in battery-powered systems. They can also be used in general-purpose DC-DC voltage regulators where a voltage boost is required.