# Super Lift Luo Converter

Discussion in 'Homework Help' started by Janna, Sep 1, 2010.

1. ### Janna Thread Starter New Member

Sep 1, 2010
2
0
Hi

I need to analyze the circuit that attached in the file. It is called "Super Lift Luo Converter"
Because it has switch it has 2 topologies (also in the file). I can't understand how the voltage on capacitor should behave.

It seems to me that their are on opposite direction in these 2 topologies and when I plot it with Spice, I expect to see it +Vin,-Vin,+Vin and so on, depends on a switch.

BUT for some reason on SPICE it is always +Vin.

Does anyone have any idea why it is?

Thanks
Janna

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2. ### t_n_k AAC Fanatic!

Mar 6, 2009
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If you look at the capacitor plates (one curved and one straight - suggesting a polarized type) in either topology you will see that the polarity remains the same with respect to the plate shape. What you are 'seeing' as reversal in the two schematics is an artifact of the simplified circuit visualization according to the assumed switch state - the polarity [probably] does not reverse.

I say a qualified "probably" because the current flow through the capacitor in the switch off state will tend to decrease the capacitor voltage drop. I assume the switching duty is arranged to ensure the polarity does not reverse in practice - if indeed that were possible. I think the diode D1 would tend to prevent that likelihood.

I haven't fully analyzed the circuit operation to test the matter. I guess your SPICE simulation can do that work.

Last edited: Sep 1, 2010
3. ### t_n_k AAC Fanatic!

Mar 6, 2009
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I have subsequently run a simulation on the circuit.

It is possible to get the capacitor voltage to change polarity under dynamic conditions - for example at start-up. It depends on the switch duty cycle and other factors. The negative excursions seem to vanish under steady-state conditions.

Edit: Actually, depending on the choice of capacitor value the capacitor voltage can be made to have negative transitions even under steady state operation .... an interesting circuit.

Last edited: Sep 2, 2010
4. ### Janna Thread Starter New Member

Sep 1, 2010
2
0
Hi

Thanks a lot for your help. Indeed
" I assume the switching duty is arranged to ensure the polarity does not reverse in practice - if indeed that were possible. I think the diode D1 would tend to prevent that likelihood."

This converter should work while there is no voltage drop on C1, (and no change in polarity) because otherwise the output will not be stable.

Sorry for very dumb question.....
What is in SPICE "dynamic conditions " ? transient?

Thanks a lot for your help. Now I need to show the same graphsbut calculating them by myself and ploting it on Matlab :-(

Janna

5. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
Yes dynamic and transient are synonymous terms for me. I would qualify this by suggesting that the circuit behavior is always dynamic but that after a suitable time duration the circuit parameters take on "steady-state" average values. So what one observes at the start of a simulation usually isn't the same as what one observes after a simulation has been in progress for some time.

Interesting that you say the capacitor C1 sees no voltage drop for stable operation. I would rather state that (once the circuit settles down) C1 will have an average steady-state voltage value with some ripple superimposed thereon. It wont be a 'perfect' DC value.