Buck, Boost, and Buck-Boost Converters

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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

  • Transfer function: Vout = Vin / (1 - D)




Buck-Boost Converter

https://en.wikipedia.org/wiki/Buck%E2%80%93boost_converter 

https://learnabout-electronics.org/PSU/psu33.php 


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

Inverting Topology:

  • The output voltage is of the opposite polarity than the input

When the switch closes in the on-state, the inductor stores up energy due to increasing current.

When the switch opens in the off-state, the inductor becomes a current source for the load and capacitor.

Continuous Mode

  • Transfer function: Vout = - D * Vin / (1 - D)

Buck Converter Combined with Boost Converter:

  • The output voltage is typically the same polarity as the input

A controller controls which MOSFET is active, putting the circuit in either buck mode (Tr1 on, Tr2 off) or boost mode (Tr1 off, Tr2 on).


Buck Converter Youtube Video

Boost Converter Youtube Video

Buck-Boost Converter Youtube Video

DIY Buck-Boost Converter Youtube Video

Buck Converter vs Linear Voltage Regulator Youtube Video