Power Electronics: DC-DC Converters

DC-DC Converters

• A DC-DC converter is an electrical system that converts direct current (DC) sources from one voltage level to another.
• It takes a DC input voltage and outputs a different DC voltage, which can be higher or lower than the input voltage.

Step-down DC-DC Converter (Buck Converter)

• A buck converter converts a DC voltage to a lower voltage.
• It contains two switches (K and D) and a power supply.
• The switch K is periodically controlled with a switching period T, closed for a period Ton=αT, and opened for the rest of the period T (Toff=T-Ton).

Buck Converter Operation

• When K is ON, Vin appears directly at the load terminals vout = Vin.
• When K is OFF, the diode D is ON and takes over the conduction of the load current.
• The output voltage vout is equal to Vin when K is ON and 0 when K is OFF.
• The output current iout is equal to Vin/R when K is ON and 0 when K is OFF.

Resistive Load

• The output voltage vout is equal to Vin when K is ON and 0 when K is OFF.
• The output current iout is equal to Vin/R when K is ON and 0 when K is OFF.
• The mean value of the output current Iout,mean = I max + I min / 2.
• The ripple of the output current ΔIout = I max - I min.

Buck Converter with Resistive Load and Low-Pass LC Filter

• The output voltage vout is a DC voltage with a small ripple.
• The output current iout is a DC current with a small ripple.
• The filter capacitor C is required to reduce the output voltage ripple.
• The minimum value of the inductance L required to operate in Continuous Conduction Mode (CCM) is given by (L f)min = vout / (2L f).

Continuous Conduction Mode (CCM)

• The inductor current is continuous and never reaches zero.
• The output voltage vout is a DC voltage with a small ripple.
• The output current iout is a DC current with a small ripple.
• The mean value of the output current Iout,mean = I max + I min / 2.

Discontinuous Conduction Mode (DCM)

• The inductor current is discontinuous and reaches zero during part of the periods.
• The output voltage vout is a DC voltage with a larger ripple compared to CCM.
• The output current iout is a DC current with a larger ripple compared to CCM.

Boost Converter

• A boost converter converts a DC voltage to a higher voltage.
• It contains two switches (K and D) and a power supply.
• The switch K is periodically controlled with a switching period T, closed for a period Ton=αT, and opened for the rest of the period T (Toff=T-Ton).

Boost Converter Operation

• When K is ON, the input voltage Vin is connected to the inductor L.
• When K is OFF, the diode D is ON and takes over the conduction of the load current.
• The output voltage vout is equal to Vin / (1 - α) when K is ON and 0 when K is OFF.
• The output current iout is equal to Vin / (1 - α)R when K is ON and 0 when K is OFF.

Test your knowledge of DC-DC converters, their function, and applications. Learn how these devices convert direct current sources from one voltage level to another.