Buck converter question

[allegro]

How come the current is said to be changed linearly, if we learned that in a RL circuit, the current changes exponentially?

 

[georgetwo]

what is the difference between sin wave form and exponential form? If u can draw them, you will see the diffrence

 

[vlad777]

If you have rectangular voltage signal it will produce triangular (linear) current trough the coil.

 

[crutschow]

It is not an RL circuit, it is an LC. But the normal switching frequency of a buck regulator is much higher than the LC resonant frequency (L is the inductor and C is the output filter capacitance) so the LC frequency is not a factor in determining the inductor current.

The current through the inductor di = V*dt/L. When the converter switch is on, the voltage (V) across the inductor is basically constant since the output voltage varies only slightly for each switch cycle (equal to the output ripple voltage). And a constant voltage across an inductor will produce a linear change in inductor current with time.

 

[EngIntoHW]

How come the current is said to be changed linearly, if we learned that in a RL circuit, the current changes exponentially?

As Crutschow saidm there's also a capacitor.

It is not an RL circuit, it is an LC. But the normal switching frequency of a buck regulator is much higher than the LC resonant frequency (L is the inductor and C is the output filter capacitance) so the LC frequency is not a factor in determining the inductor current.

How come it's an LC circuit?
What about the load?

Moreover, the capacitor is charged through the inductor, so how can you disregard that?

 

[crutschow]

How come it's an LC circuit?
What about the load?

Moreover, the capacitor is charged through the inductor, so how can you disregard that?

It's a LC circuit because the dynamics of the inductor current change is dominated by the output inductor and filter capacitor. The load does enter in but only to the extend that it changes the average inductor current. It does not affect the delta change in inductor current caused by the the on and off periods of the switch.

I didn't' disregard the capacitor. As I previously stated the LC resonant frequency of the output inductor and capacitor is well below the normal buck converter switching frequency so that has no significant effect on the inductor current. The capacitor is large enough that there typically is no more than a few tens of millivolts change in the capacitor voltage during one switch cycle which also means it has little effect on the delta inductor current.

If you are still confused, try reading this.