Switching converters operating in CCM and DCM

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analog_match

Joined: 04 Nov 2008
Posts: 41
Location: USA

30 Oct 2009 17:01 Switching converters operating in CCM and DCM

Friends,

When a converter is operated such that it operates in CCM and DCM depending on load, my understanding is that duty cycle will have to change when the converter is in DCM as compared to when it is in CCM. Furthermore, since M(=Vout/Vin) is a function of the load in DCM, the control will have to be be different in order to regulate the output to the same voltage. But how is this achieved when the same converter is operating in CCM and DCM? In other words we obviously cannot change the control loop on the fly? What other cicuit parameters can be changed to able to work in DCM?

Also it will be very helpful if someone can recommend same paper/books/appnotes that talk about designs to operate in CCM and DCM.

Thanks

rdrdtheta

Joined: 30 Oct 2009
Posts: 1
Location: Si valley

30 Oct 2009 22:58 Re: Switching converters operating in CCM and DCM

Hello,

This topic is discussed in Pressman's fine book "Switching Power Supply Design".

A converter can run in DCM or CCM with no change in feedback control. But as you point out, the two modes are different and the transisition from one to the other can cause stability problems.

Lets look at the simplest example; a buck converter

Typically, a converter with limited load and input voltage range can be designed to always run in CCM gaining the advantage of lower inductor ripple current. Current feedback is easy to implement here because the inductor current range is well defined.

For the widest possible operating range, the DCM operating mode is inevatible. Making current feedback work with the CCM/DCM mode transition is quite a problem and plain voltage feedback might give the best results.

Now to make this more complicated, think about an energy storage topology like the boost converter.

Here the inductor transfers power to the load when the input current is turned off. With CCM you still get lower ripple, but now you have to deal with the possibility of a RHP zero. If you run DCM, no RHP zero, but then inductor will ring down when the input current is switched off which will increase EMI.

So depending on what you are doing CCM or DCM are possible and even the transition is manageable.

If you can provide more details I can try to be more specific.

rdrdtheta

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analog_match

Joined: 04 Nov 2008
Posts: 41
Location: USA

01 Nov 2009 16:49 Re: Switching converters operating in CCM and DCM

Thanks for your reply rdrdtheta. I am trying to understand DCM-CCM transition and DCM steady state operation for a syc buck converter using simple voltage-mode feedback. In CCM my duty cycle would be about 10% but I think it will have to be a bit higher in DCM. Also I want to understand how transitions between modes can be detected and implemented.

marthinus

Joined: 04 Oct 2009
Posts: 5

02 Nov 2009 6:49 Re: Switching converters operating in CCM and DCM

The loop gain of the converter changes between DCM and CCM.
When you monitor the feedback amp output on a scope,you can see the dip when
the converter changes from CCM to DCM as you lower the load.