How an Inductor Steps Up DC Voltage in a Boost Converter

This article provides a comprehensive overview of how a boost converter increases direct current (DC) voltage above the level of the input power supply. By examining the physics of an inductor and its magnetic field, we will explore how energy is temporarily stored and then released to create a higher voltage. You will learn about the two main operational phases of the circuit—the charging phase and the discharging phase—and how the rapid collapse of a magnetic field forces voltage to stack on top of the original supply rail.

The Core Component: The Inductor

To understand how voltage is stepped up, you must first understand the behavior of an inductor. An inductor is essentially a coil of wire around a magnetic core. In electronics, inductors resist sudden changes in electrical current.

When current flows through the coil, it generates a magnetic field around it, storing energy in the process. If the current is suddenly interrupted, the inductor tries to keep the current flowing at the exact same rate. It does this by converting its stored magnetic energy back into electrical energy.

Phase 1: The Charging State (Switch Closed)

A basic boost converter consists of a DC power source, an inductor, an electronic switch (usually a MOSFET transistor), a diode, and a capacitor.

In the first phase of operation, the electronic switch closes. This creates a direct path from the positive terminal of the DC power source, through the inductor, and straight back to the ground, bypassing the rest of the circuit.

As current flows through the inductor, it ramps up over time. The inductor absorbs this electrical energy and uses it to build a strong magnetic field. During this time, the polarity of the inductor is positive on the side connected to the power source and negative on the side connected to the switch.

Phase 2: The Collapsing Field (Switch Opened)

Once the magnetic field is sufficiently charged, the control circuit rapidly opens the switch. This instantly breaks the path to the ground.

Because an inductor physically cannot allow current to drop to zero instantly, it fights to keep the current moving in the same direction. To maintain this current flow against the newly created resistance of the open switch, the magnetic field begins to collapse rapidly.

Reversing Polarity and Stacking Voltage

According to Faraday’s Law of Induction, a rapidly changing magnetic field induces a voltage. The faster the magnetic field collapses, the higher the voltage the inductor generates to force the current through the remaining path—which is through the diode and into the output capacitor and load.

Crucially, to keep the current flowing in the same direction as before, the inductor reverses its electrical polarity. The side connected to the power source becomes negative, and the side connected to the diode becomes positive.

Because the inductor's polarity has flipped, it now acts like a second battery wired in series with the original DC power source. The voltage generated by the collapsing magnetic field adds directly to the voltage of the supply rail. If the supply rail is 5 volts, and the collapsing field generates 7 volts, the total output pushed through the diode is 12 volts.

Smoothing the Output

This process of opening and closing the switch happens tens of thousands, or even millions, of times per second.

Every time the switch opens, a high-voltage pulse from the supply rail and the collapsing magnetic field is pushed through the diode. The diode prevents this higher voltage from flowing backward into the input circuit. Finally, an output capacitor catches these high-voltage pulses and smooths them out into a continuous, steady DC voltage that remains strictly higher than the original input supply.