Boost converter is third switching mode power supply topology without transformer, with one inductor and one capacitor (apart from buck and buck-boost converter). Boost or step-up converter topology is given in figure bellow.
In boost topology, one terminal of the inductor is fixed to power supply, and another is swinging from power ground to the output. During “ON” time, the inductor is disconnected from the load and shortly connected to the ground, like in figure bellow.
Inductor current is increasing according to formula:
In here, Î”t1 is duration of “ON” time and is equal to DÂ·T, where D is duty cycle and T is switching period. When main switch goes to “OFF” state, the inductor terminal that was on the power ground, according to Lenz’s law induces voltage high enough to forward bias clamping diode. Induced voltage is higher then input voltage and higher then the output voltage.
Now, inductor current flows from the place of the lower electrical potential (input voltage, VCC) to the place with higher electrical potential, VOUT. Only inductor has this unique feature that for short term, current can flow from lower electrical potential to the node of higher electrical potential. During this short time, current is decreasing and decrement is:
In here, Î”t2 is duration of “OFF” time and is equal to (1-D)Â·T. If main switch is turned “ON” before the inductor current drops to zero, boost dc-dc converter is operating in continuous conduction mode, and the output voltage is related with the input voltage with:
Because 1>D>0, the output voltage of boost dc-dc converter is always higher then the input, and that’s the reason why boost dc-dc converter is also called step-up converter.
This feature that the output voltage is higher then the input voltage is useful for battery power systems and particularly for power factor corrector (PFC) circuits. PFC circuits suppose to deliver power to the load from rectified sine voltage, even when rectified sine voltage is few volts only (after passing zero), to the load that is usually on 400V potential. It is possible only with switching power supply converter that have such topology that during “ON” time, the inductor is disconnected from the load and is connected between the input voltage and power ground. Together with boost converter topology, this feature exists in buck-boost and flyback topology. However, buck-boost converter produces negative output voltage, and flyback has limited power due to possible transformer saturation, so boost or step-up convertor is most popular choice for PFC converters.