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Power Electronics Circuits

Which of the following correctly describes continuous conduction mode (CCM) of a DC-DC converter?

  1. AOperation in which a DC current of constant magnitude flows from the input supply at all times, regardless of the state of the switch
  2. BOperation in which the switch stays on all the time and current keeps flowing
  3. COperation in which the inductor current keeps flowing and never falls to zero during a period
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Answer: C. Operation in which the inductor current keeps flowing and never falls to zero during a period

In continuous conduction mode the inductor current ripples but never falls to zero within a period. When the load current decreases, the valleys of the current reach zero and stay there for a while, and the converter moves into discontinuous conduction mode (DCM). Formulas such as Vout = D × Vin for the buck converter and Vout = Vin/(1 − D) for the boost converter hold in continuous conduction mode. In discontinuous conduction mode the output voltage also depends on the load current.

  • A. In a buck converter the input current is pulsating, flowing only while the switch is on. The modes are distinguished by the inductor current.
  • B. The switch turns on and off periodically. A switch that stays on cannot convert the voltage.
  • C. Correct. The condition for continuous conduction mode is that the inductor current never falls to zero.
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More Power Electronics Circuits questions

  1. In an ideal buck converter (freewheeling through a diode) with an input voltage of 12 V, an output voltage of 3 V, an inductance of 15 µH and a switching frequency of 300 kHz, the load current is gradually reduced. Which of the following is the minimum output current that keeps continuous conduction mode (the boundary with discontinuous conduction mode)?
  2. Assume that the entire AC component of the inductor ripple current (0.6 A peak to peak) flows into the output capacitor of a buck converter. With a switching frequency of 300 kHz and an output capacitance of 25 µF (ignore the ESR), which of the following is the peak-to-peak output voltage ripple?
  3. In a DC-DC converter with a low output voltage, which of the following is the main purpose of synchronous rectification, which uses a MOSFET in place of the freewheeling diode?
  4. In a synchronous buck converter, which of the following correctly describes the path of the inductor current during the dead time when both the upper and lower MOSFETs are off?
  5. Assume that the power lost during a transition is, on average, half of voltage × current. For a power device switching 400 V and 10 A with turn-on and turn-off transition times of 50 ns each and a switching frequency of 100 kHz, which of the following is closest to the switching loss?

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