Hi!
I don't know if this is the right place to post or ask but I am stuck in a university course problem that seems really simple but I can't get my head around it. Can anyone give me tips on how to start and go forwards with the problem and possible answer for it? I want to learn it, not one fast answer. I have drawn the circuit and read through material about CCM/DCM and it seems easy but I don't get how this is so hard. Thank you in advance.

 

Do you know when and why the inductor "enters" DCM ? At "light" load or at "heavy" load?

 

Do you know when and why the inductor "enters" DCM ? At "light" load or at "heavy" load?

The DC-component current I depend on load? But then the inductor current does not. I am kind of confused about it

 

So how you define the DCM and CCM ?

 

So how you define the DCM and CCM ?

DCM: "Occurs because switching ripple in inductor current or capacitor voltage causes polarity of applied switch current or voltage to reverse, such that the current- or voltage-unidirectional assumptions made in realizing the switch are violated." This sentence starts to become a little language barrier for me. I'm Finnish.

"The CCM mode is such that the inductor current has to different derivatives or slopes within the switching cycle." This also but not so much.
I try to understand these sentences better from graphic presentation but it is hard

 

To be honest I totally don't understand your definition.

Continuous Current Mode (CCM) means that current through inductor flows continuously throughout the switching period (always >0).
ΔIon = ΔIoff

Discontinuous Current Mode (DCM) means that current through inductor reaches zero before the end of the switching period (when switch is OFF)

https://forum.allaboutcircuits.com/threads/smps-experiment-problem.73116/#post-512842

https://forum.allaboutcircuits.com/threads/buck-converter-problem.145072/#post-1231833

 

To be honest I totally don't understand your definition.

Continuous Current Mode (CCM) means that current through inductor flows continuously throughout the switching period (always >0).
ΔIon = ΔIoff

Discontinuous Current Mode (DCM) means that current through inductor reaches zero before the end of the switching period (when switch is OFF)

https://forum.allaboutcircuits.com/threads/smps-experiment-problem.73116/#post-512842

https://forum.allaboutcircuits.com/threads/buck-converter-problem.145072/#post-1231833

That sounds really simple, I understood that from graphs where it showed the voltage on-time and off-time slopes + the time when inductor current is zero in DCM. From the problems starting values I should compute the resistance first? Don't know how and then the K = 2L/RTs to get the voltage going in?

 

I should compute the resistance first? Don't know how

But you know the output power and output voltage. So you can find the load resistance from there

 

But you know the output power and output voltage. So you can find the load resistance from there

If the output power varies, how do I select the correct value? Or do I have to just guess and start computing from there?

 

As I said early if you really understand why DCM occurs in the buck converter you will already know where to begin.
http://images.elektroda.net/9_1344534972.png
Also notice that the output current Ilout is equal to the average value of the inductor current IL.

 

Here you have the tip

The situation at the boundary between the CCM and DCM will look like this:



Where :
VL - voltage across the incutor.
IL - inductor current
I_lout - output (load) current equal to the average value of the inductor current IL.

And this
https://forum.allaboutcircuits.com/...lator-theory-of-operation.113526/#post-883379

And this is all you need to find the solution You do not have to even bother to use your K factor equation.
Simply understand the basics they are really simple.

 

Thank you for all the help so quickly, still trying to work this.
Edit: I can't solve this...

 

Show us what you have done so far?

 

Show us what you have done so far?

Ignore he DCM graph just tried to understand it. I just don't know how to begin with this

 

OK, Can you find the minimum load current for Vin = 35V so that the converter stays in CCM?

 

OK, Can you find the minimum load current for Vin = 35V so that the converter stays in CCM?

I got the answer already from the course leader but I still don't get it completely. It's the 2a exercise

 

If I understood it correctly, the circuit operates always in CCM with the given Vin values and only in DCM when Pout=10 W and Vin=<29,23 V? Why is it outside the given Vin values. And why isn't there anything mentioned about Iout??

 

If I understood it correctly, the circuit operates always in CCM with the given Vin values and only in DCM when Pout=10 W

I disagree with this conclusion.

For Vin = 35V and Vo = 28V in CCM mode the duty cycle is equal to:
D = Vo/Vin = 28V/35V = 0.8,

Hence at the boundary (see post #11) the inductor current will rise to:

ΔIL = VL/L *Δt = (35V - 28V)/22μH * 0.8*1/75kHz = 3.394A

And because of a fact that at the boundary between CCM and DCM the inductor current is rising from 0A and ends at 3.394A.
So the load current is equal to 3.394A/2 = 1.697A (load current is equal to the average value of the inductor current)

RL = 28V/1.697A = 16.5Ω and the power is 28V*1.697A = 47.5W

As you can see any load current smaller than 1.7A will bring the converter into DCM mode (any RL > 16.5Ω).

On the other hand for Vin = 70V the duty cycle is 28V/70V = 0.4 (in CCM)

Therefore the minimum load current needed if we want to stay in CCM mode is equal to:

ΔIL = VL/L *Δt = (70V - 28V)/22μH * 0.4*1/75kHz = 10.18A --->IL = 10.18A/2 = 5.09A

Which correspond to the output power equal to 28V*5.09A = 142.52W

And this means that conwerter will work in CCM mode only if Po ≥ 142.5W