Cuk converter bizzare control loop

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

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

Ascii art:

L1 * C1 * L2 -----)))))---------+-----!!----+----))))))------- ! ! !!- V C1 ---!!- Q1 --- !!- ! ! ! GND GND

I assume that the circuit is this one. Notice how C1 provides a route from the input side to the output side. A positive step on the input tends to cause a positive step on the output. This should be no real surprise.

At low frequencies of feedback, the duty cycle of Q1 determines the Vin to Vout ratio. If you walk around the servo loop and add up all the gains and phases, you can work out a Vode plot for the system. Depending on how you PWM Q1, you may need to add a "transport delay" (exp(-ST)) to the assumed transfer function to get it to match real life. The transport delay usually has to be about 0.5 to 1 cycle of the PWM.

You can then prove this by inserting the disturbance on the spice model and seeing what comes around the loop.

Look up "right half plane zero". As you suspect it is a "feature* of the species. It ocurrs in continuous mode mode only. 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.

Best analogy I ever heard for a right half plane zero was on this NG. Think about turning a bicycle.

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

Gibbo

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

Cheers Terry