What is Continuous conduction mode?

Operation of a DC-DC converter in which the inductor current keeps flowing and never reaches zero during a switching period (CCM). When the load becomes light and the valleys of the current reach zero and stay there for a while, the converter is in discontinuous conduction mode (DCM); operation exactly at the boundary is called boundary (or critical) conduction mode (BCM). Simple conversion ratios such as D × Vin for the buck converter hold in continuous conduction mode, while in discontinuous conduction mode the output voltage also depends on the load. For a buck converter that freewheels through a diode, the boundary output current is half the peak-to-peak inductor ripple current.

Japanese name: 電流連続モード

Continuous conduction mode appears in 5 practice questions across 1 topic, including Power Electronics Circuits.

Field: Power Electronics Circuits

Try it: questions about Continuous conduction mode

For an ideal step-up chopper (boost converter) with an input voltage of 5 V and a switch duty cycle of 0.6, which of the following is the output voltage in continuous conduction mode?

A3 V
B8 V
C12.5 V

Show answer and explanation

Answer: C. 12.5 V

The output voltage of an ideal boost converter is Vout = Vin/(1 − D). 5 V ÷ (1 − 0.6) = 5 ÷ 0.4 = 12.5 V. The formula follows from the balance of volt-seconds across the inductor, with Vin applied while the switch is on and (Vout − Vin) while it is off: Vin × D = (Vout − Vin) × (1 − D). The 3 V of D × Vin is the formula of the buck converter.

For an ideal buck-boost chopper (inverting buck-boost converter) with an input voltage of 12 V and a duty cycle of 0.6, which of the following is the magnitude of the output voltage in continuous conduction mode?

A7.2 V
B18 V
C30 V

Show answer and explanation

Answer: B. 18 V

In an inverting buck-boost converter, energy from the input is stored in the inductor while the switch is on and released to the output through the diode while it is off. From the volt-second balance of the inductor, Vin × D = Vout × (1 − D), the magnitude of the output voltage is Vout = Vin × D/(1 − D) = 12 × 0.6/0.4 = 18 V. The output polarity is opposite to that of the input (negative with respect to ground). It works as a step-down converter for D < 0.5 and a step-up converter for D > 0.5, and at D = 0.5 the output magnitude equals the input.

Continuous conduction mode in the explanations

In continuous conduction mode the inductor current ripples but never falls to zero within a period.
Which of the following correctly describes continuous condu…
Its valley is "output current − ΔI/2", so when the output current falls below ΔI/2 = 0.25 A, the valley reaches zero and the converter enters discontinuous conduction mode.
In an ideal buck converter (freewheeling through a diode) w…