Buck, Boost, and Buck-Boost Converters

Buck Converter

https://en.wikipedia.org/wiki/Buck_converter

https://learnabout-electronics.org/PSU/psu31.php

Type of SMPS (switched-mode power supply)
Converts DC input voltage to a lower DC output voltage

When the switch closes in the on-state, the inductor opposes an increase in current and produces an opposing voltage across its terminals. This steps down the output voltage since Vout = Vin - VL.

When the switch opens in the off-state, the voltage drop across the inductor flips due to decreasing current and the inductor becomes a current source to the load. The diode provides a path for current. As the inductor loses its stored energy, the capacitor helps to maintain current flow to the load.

Power is preserved:

Average output voltage < average input voltage
Average output current > average input current

Continuous Mode

Current through the inductor never falls to zero

Transfer function: Vout = Vin * ton/T = Vin * D
D = duty cycle of switch

Discontinuous Mode

Current through the inductor falls to zero during part of the period
Transfer function is a bit more complicated

Output Voltage Ripple

Due to the charging and discharging of the output capacitor, the output voltage rises during the on-state and falls during the off-state

Inductor Sizing

Lower inductor values = better transient response
Higher inductor value = better efficiency, lower ripple
Inductor size is chosen based on allowed ripple current

The higher the inductance the lower the ripple current
The higher the switching frequency, the smaller the inductance can be for the same allowable ripple current

Boost Converter

https://en.wikipedia.org/wiki/Boost_converter#:~:text=A%20boost%20converter%20is%20a,lower%20than%20the%20source%20current

https://learnabout-electronics.org/PSU/psu32.php

Type of SMPS (switched-mode power supply)
Converts DC input voltage to a higher DC output voltage

When the switch closes in on-state, the inductor stores energy from the increasing current.

When the switch opens in off-state, the inductor produces a back EMF of opposite polarity to when the switch was on, due to the decrease in current. This produces a higher output voltage: Vout = Vin + VL. This also charges the capacitor up to Vout.

When the switch closes again in on-state, the diode is reverse biased and the capacitor continues to supply the load while the inductor continues to store energy from the input source.

Continuous Mode