Re: Cuk converter bizzare control loop

From: Terry Given (my_name_at_ieee.org)
Date: 12/08/04 

Date: Wed, 08 Dec 2004 15:54:00 +1300

Larry Brasfield wrote:
> "ChrisGibboGibson" <chrisgibbogibson@aol.com> wrote in message
> news:20041207193848.08355.00001605@mb-m04.aol.com...
>
>>"robert lafrance" wrote:
>>
>>
>>>Right off the bat I will tell you this is homework - sorta. I got the thing
>>>running pretty much ok. Used the old virtual decade box and it looks about
>>>right. Now I just have to go back and analytically justify the loop that
>>>works. Was supposed to finish this last problem in Mathcad, but Switchercad
>>>works so nice.
>>>
>>>With the thing running I see when I jerk the bus up and down the converter
>>>kinda does opposite what I would expect. When I step it up to 550v from
>>>450v the output actually goes down before catching itself and stabalizing.
>
>
> [to OP]
> That is prima facie evidence of the right half plane zero. So don't let
> anybody tell you it is not there. It is, and this is a well known fact.
>
>
>>>I expect this is probably a characteristic of the species. This version
>>>just uses output inductor in series with 10 ohm load. The problem is to get
>>>it stable at 60Hz out. I'm just running along at DC out to satisfy my own
>>>curiousity.
>>>
>>>Would like to hear a comment on the control loop from someone who has
>>>played with this animal.
>
>
> [to OP]
> I used a convertor with the same response to convey signals at
> frequencies approaching a small submultiple of the switching
> frequency. This used a compensating zero in the right half plane.
>
>
>>Look up "right half plane zero". As you suspect it is a "feature* of the
>>species. It ocurrs in continuous mode mode only.
>
>
> So far, so good.
>
>
>>Run it in discontinuous mode
>>(complete energy transfer) and the problem will go away. Despite what you might
>>read in some texts there's piss all you can do about it other than to swamp it
>>by drastically reducing the bandwidth of the loop. This can be proved
>>mathematically.
>
>
> That, I disagree with, having done much better than that, both
> analytically and in practise. If not for the effects that occur as
> the response approaches the switching frequency, you could
> bring the response poles arbitrarily to the left.
>
>
>>Best analogy I ever heard for a right half plane zero was on this NG. Think
>>about turning a bicycle.
>
>
> Not a bad analogy. Think about balancing an upside down pendulum.
> It is the same problem with respect to balancing the bike.

and a well-tuned inverted penulum contoller will of course move bacwards
initially in response to a "move forward" command. This tips the
pendulum in the direction of desired motion, and the cart then moves to
keep it from falling

>
>
>>Loads of people will argue with this. I don't care, they're wrong. I'm off to
>>the airport to fly to USA.
>
> Good luck.
>
>
>>Gibbo
>
>

Cheers
Terry