Power Electronics: DC-DC Converters

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Questions and Answers

What is the condition for the buck converter to operate in Continuous Conduction Mode (CCM)?

The output voltage is purely DC (correct)

The output voltage is not regulated

The output voltage is purely AC

The output voltage is a mixture of AC and DC

What happens to the diode D during the time period 0 < t < αT?

The diode D is short-circuited

The diode D is in a neutral state

The diode D is ON and conducts the load current

The diode D is blocked and does not conduct (correct)

What is the effect of increasing the inductance L on the current ripple?

The current ripple increases

The current ripple becomes zero

The current ripple decreases (correct)

The current ripple remains unchanged

What is the purpose of the LC filter in the buck converter circuit?

To reduce the ripple of the output voltage Signup and view all the answers

What is the condition for the switch K to be ON?

During the time period 0 < t < αT Signup and view all the answers

What is the formula for the mean value of the output current in a buck converter?

Iout,mean = (Imax + Imin) / 2 Signup and view all the answers

What is the output voltage when switch K is OFF?

0 Signup and view all the answers

What is the relationship between ton and α?

α = ton / T Signup and view all the answers

What is the current through the load when switch K is ON?

Vin / R Signup and view all the answers

What is the voltage across the diode D when switch K is ON?

-Vin Signup and view all the answers

What is the period of the switching signal?

1 / f Signup and view all the answers

What is the state of the diode D when switch K is OFF?

Forward biased Signup and view all the answers

What is the condition for the boundary between CCM and DCM modes?

Imin = 0 Signup and view all the answers

What is the expression for the minimum inductance Lf in CCM mode?

Lf = (1 - α) / (2R) Signup and view all the answers

What is the expression for the mean current IL,mean in CCM mode?

IL,mean = ΔiL / 2 Signup and view all the answers

What is the condition for a Buck converter to operate in DCM mode?

ΔiL < 2IL,mean Signup and view all the answers

What is the main function of a DC-DC converter?

To convert DC to another DC voltage level Signup and view all the answers

What type of DC-DC converter supplies an output DC voltage lower than the original input voltage?

Step-down DC-DC converter Signup and view all the answers

What is the power equivalence in an ideal Buck converter?

Pin = Pout Signup and view all the answers

What is the expression for the capacitance C required in terms of the specified voltage ripple Δvout?

C = (1 - α) / (8L(Δvout / vout)f^2) Signup and view all the answers

What is the name of the DC-DC converter that outputs a DC voltage higher than the original input voltage?

Step-up DC-DC converter Signup and view all the answers

What is the purpose of the control circuit in a DC-DC converter?

To control the switching period Signup and view all the answers

What happens when switch K is ON in a buck DC-DC converter?

Vin appears directly at the load terminals Signup and view all the answers

What is the name of the DC-DC converter that converts a DC voltage to another voltage of lower value?

Buck converter Signup and view all the answers

What is the condition for the limit between the CCM and DCM modes?

I min = 0 Signup and view all the answers

What is the expression for the minimum inductance value Lf in CCM mode?

Lf = 2L/(1 - α)2R Signup and view all the answers

What is the relationship between the input power Pin and the output power Pout in an ideal Boost converter?

Pin = Pout Signup and view all the answers

What is the expression for the output voltage vout in terms of the input voltage Vin and the duty cycle α?

vout = Vin/(1 - α) Signup and view all the answers

What is the purpose of the capacitance C in the Boost converter?

To filter the output voltage Signup and view all the answers

What is the difference between the CCM and DCM modes in the Boost converter?

The current is continuous in CCM and discontinuous in DCM Signup and view all the answers

Study Notes

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.

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Description

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.