Re: Cuk converter bizzare control loop

From: Genome (ilike_spam_at_yahoo.co.uk)
Date: 12/08/04 

Date: Wed, 08 Dec 2004 01:50:31 GMT

"robert lafrance" <yzordderrex@verizon.net> wrote in message
news:jNntd.2501$x26.113@trndny03...
> 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.
> 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.
>
> Sorry I haven't been paying much attention to the group here. Started
MSEE
> program in September at UW Madison over the web. Last HW assignment in
> progress. I would imagine lots of OT conversation (battle mode) weeks
> before the election.
>
> regards,
> Bob
>
>

Hmmmm... I'd guess you are doing some form of PFC and controlling switch
current to avoid the LC resonance.

Your current loop will be quite fast but your voltage loop, if that's what's
dealing with the output will be quite slow.

Switch current is the sum of input and output current multiplied by the
operating duty cycle which averages to input current.

That's nice for PFC because you want to control input current.

However.

If you wack the input voltage up with your slow voltage loop demanding what
is effectively a fixed switch current.....

The input current will stomp up and the current loop, being fast, will spot
it and adjust things so the switch current, sum of input and output current,
remains the same and so the output current will drop.

So the output voltage will drop and, sometime later, the voltage loop will
realise this and bring things back into regulation.

If you've used a proper PFC IC then the VFF multiplier should take care of
it.... but it's filtered to buggery to avoid line current distortion so it
doesn't.

IN THE MEANTIME

I'd just like to point out that a CUK converter doesn't suffer a transport
delay and it doesn't suffer from a right half plane zero.

Ken Smith and ChrisGibboGibson are blowing it out of their collective
Assholes.

DNA

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