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Design >> Analog Design >> LC pulse response
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Message started by Mikay on Dec 13^th, 2013, 6:35am

Title: LC pulse response
Post by Mikay on Dec 13^th, 2013, 6:35am

I have a question. It may be stupid, but I'm stuck for several days. can any help me and give it a good explaination? Thanks.

Here is the question, it will be the scenario of ideal loseless LC(as attached). L =1uF C=1uF. vin=1V. duty cycle=50%. The frequency of the pulse is 10Mhz which is much morn than LC natural frequency 160Khz.The LC will output a oscillation since it's loseless.

My question is: is this model the ideal case of a bulk dc dc converter? If it is, the output will be a DC value equal to (Duty cycle)*vin=0.5V as every book about DCDC converter will tell you. However, the output is not a DC but a oscillation wave.

If the attached is not the ideal model for the bulk dc cd converter. What is wrong with it? I know complete dc dc converter will introduce compensation for loop stability but I still can't understand why the output canl be a DC value for a pulse input.

Thanks,
-Mikay

Title: Re: LC pulse response
Post by Mikay on Dec 13^th, 2013, 6:36am

The output wave is as following:

Title: Re: LC pulse response
Post by raja.cedt on Dec 13^th, 2013, 11:20am

How does it models buck converters without switch, it's a nothing but Low pass filter!!!

Title: Re: LC pulse response
Post by Mikay on Dec 13^th, 2013, 3:21pm

Thanks Raja. The purpose of the switches are to feed periodic voltage into the inductance. Now since the pulse signal is generated using Vpulse component. So is it ok to get off the switches if they are loesless? IMHO, this is also a resonable model for bulk dc dc.
raja.cedt wrote on Dec 13^th, 2013, 11:20am:

How does it models buck converters without switch, it's a nothing but Low pass filter!!!

Title: Re: LC pulse response
Post by AnilReddy on Dec 25^th, 2013, 11:05pm

Hi Mikay,

I have a few thoughts which might answer your question.

First of all, i don't think it is the model of ideal loss less buck converter. Because, without modelling the load this circuit is just a low-pass filter as raja pointed out. It is ideal from the view point of not having any resistances (DCR or ESR) associated with inductance and capacitance. But, not having load component does not mean an ideal buck converter and it should be there.

Without the load resistance what happens is the entire inductor current will flow through the capacitor (DC+ ripple). But, in normal buck converter only the ripple component will flow through the capacitor which will integrate it and give us the voltage ripple. But, here what is happening is there is dc current flowing through the cap and it will keep on integrating it and it will manifest itself as huge oscillations at the output. I think that is why you are observing those oscillations at the output.

Connect some load res at o/p and see the magnitude of oscillations.

Thanks
Anil Reddy