# Buck Converter Inductor Current in Discontinuous Mode

#### hoyyoth

Joined Mar 21, 2020
224
Dear Team,

I am going through the DCM operation of Buck Converter.In DCM it says that inductor current is zero for certain time.

https://en.wikipedia.org/wiki/Buck_converter

In the figure you can see that inductor current waveform is continuous. I checked in some other materials also,I can see that inductor current waveform is continuous.
May I know why inductor current waveform is continuous when current is zero for a certain time.

Regards
Hari

#### DickCappels

Joined Aug 21, 2008
9,509
With Discontinuous Mode part of the current waveform is flat at zero amps.

The waveform is only a graph of current vs time so you should see a continuous waveform but it will be zero for part of the time, which means the current stopped for that time and is therefore discontinuous.

#### Papabravo

Joined Feb 24, 2006
19,570
In CCM, the word continuous has a slightly different meaning than it does in mathematics. The current is both continuous in the mathematical sense and NON-ZERO.
In DCM, the current is both continuous in the mathematical sense, and is ZERO some of the time. The inductor is idle and all of the current is supplied to the load from the capacitor.

• hoyyoth

#### Ian0

Joined Aug 7, 2020
6,680
In a buck converter, when the switch transistor is ON the current through the inductor increases at dI/dt=(Vsupply-Vload)/L and when the switch transistor is OFF the current decreases at dI/dt=-Vload/L
If the converter is made with a transistor and a diode, then, if the output current is low, the inductor current will reach zero before the end of the OFF period. This is discontinuous current mode.

In the case of a synchronous buck regulator (with two transistors, not a transistor and a diode) there is nothing to stop the current continuing to decrease so that it goes negative, where current is flowing back from the output capacitor into the power supply.

• hoyyoth

#### hoyyoth

Joined Mar 21, 2020
224
Dear Team,

when the inductor current is zero at that time the load current is completely given by the capacitor.
Correct me if I am wrong.

Regards
AHri

• DickCappels

#### Ian0

Joined Aug 7, 2020
6,680
Dear Team,

when the inductor current is zero at that time the load current is completely given by the capacitor.
Correct me if I am wrong.

Regards
AHri
Correct.

• hoyyoth

#### Delta Prime

Joined Nov 15, 2019
1,311
Hello there! This link below will reinforce what others have already stated. Seems you're off to a good start, but there is a state in continuous mode called Dead time. you're more than capable of digesting this information without confusion.control circuits are responsible to not to turn on both transistors at the same time, creating a short-circuit to the input voltage.
In practice, a dead time is added between switching off one transistor and turning on the other switching transistor.