The DC/DC Boost Converter

The boost converter, or step-up converter, is switching DC/DC converter that produces an output voltage greater than the source.

Figure 1: The Boost Converter

A basic boost configuration is depicted in Figure 2. Assuming that the switch (the transistor) has been open for a long time, and the components are ideal, the voltage across the capacitor is equal to the input voltage.

Charge

Figure 1 shows the charge phase. When the switch closes, the input voltage is flowing across the inductor. The diode prevents the capacitor from discharging to ground. Because the input voltage is DC, current through the inductor rises linearly with time at a rate that is proportional to the input voltage divided by the inductance. The current through the inductor increases and the energy stored in the inductor builds up.

Figure 2: Boost Converter Charge Phase

Discharge

Figure 2 shows the discharge phase. When the switch opens, the voltage across the inductor changes to whatever is required to maintain current flow. The inductor is discharging its energy and the polarity of inductor voltage is such that its terminal connected to the diode is positive with respect to its other terminal connected to the source. Now the capacitor voltage is higher than the source voltage. The inductor receives energy when the switch is closed and transfers it to the output when the switch is open.

When the capacitor is relatively large, Vout remains relatively constant during the second half of the cycle.

Figure 3: Boost Converter Discharge Phase

If we continue this process over and over, the voltage across the capacitor (VOUT) will rise with every cycle

 Figure 4: Inductor Current

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