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?

<p>To reduce the ripple of the output voltage (B)</p> Signup and view all the answers

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

<p>During the time period 0 &lt; t &lt; αT (A)</p> Signup and view all the answers

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

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

What is the output voltage when switch K is OFF?

<p>0 (C)</p> Signup and view all the answers

What is the relationship between ton and α?

<p>α = ton / T (A)</p> Signup and view all the answers

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

<p>Vin / R (D)</p> Signup and view all the answers

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

<p>-Vin (A)</p> Signup and view all the answers

What is the period of the switching signal?

<p>1 / f (A)</p> Signup and view all the answers

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

<p>Forward biased (A)</p> Signup and view all the answers

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

<p>Imin = 0 (B)</p> Signup and view all the answers

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

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

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

<p>IL,mean = ΔiL / 2 (C)</p> Signup and view all the answers

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

<p>ΔiL &lt; 2IL,mean (C)</p> Signup and view all the answers

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

<p>To convert DC to another DC voltage level (B)</p> Signup and view all the answers

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

<p>Step-down DC-DC converter (B)</p> Signup and view all the answers

What is the power equivalence in an ideal Buck converter?

<p>Pin = Pout (D)</p> Signup and view all the answers

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

<p>C = (1 - α) / (8L(Δvout / vout)f^2) (A)</p> 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?

<p>Step-up DC-DC converter (C)</p> Signup and view all the answers

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

<p>To control the switching period (B)</p> Signup and view all the answers

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

<p>Vin appears directly at the load terminals (A)</p> 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?

<p>Buck converter (C)</p> Signup and view all the answers

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

<p>I min = 0 (B)</p> Signup and view all the answers

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

<p>Lf = 2L/(1 - α)2R (A)</p> 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?

<p>Pin = Pout (B)</p> 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?

<p>vout = Vin/(1 - α) (B)</p> Signup and view all the answers

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

<p>To filter the output voltage (B)</p> Signup and view all the answers

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

<p>The current is continuous in CCM and discontinuous in DCM (B)</p> Signup and view all the answers

Flashcards

DC-DC Converter

An electrical system that converts direct current (DC) voltage from one level to another.

Step-Down Converter (Buck)

A DC-DC converter that outputs a lower voltage than its input.

Buck Converter: K is ON

When the switch 'K' is ON, the input voltage (Vin) is directly applied to the output.

Buck Converter: K is OFF

With Switch K off, Diode D conducts, load current continues, output voltage is zero.

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Filter Capacitor (C)

Reduces output voltage ripple in a buck converter.

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Continuous Conduction Mode (CCM)

Inductor current is always flowing, never dropping to zero.

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Discontinuous Conduction Mode (DCM)

Inductor current drops to zero during part of each switching cycle.

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Boost Converter

A DC-DC converter that outputs a higher voltage than its input.

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Boost Converter: K is ON

When K is ON, the input voltage is connected to the inductor L.

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Boost Converter: K is OFF

Output voltage is equal to Vin / (1 - α) when K is OFF

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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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