Forum: Analog Circuits Low-pass-filter after PFC-Stage

Author: kuebli (Guest)
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Hi, Im modelling in LTspice since long time a PFC-stage including a 
Boost-Converter, which works in Boundary Conduction Mode. As control 
scheme I use Peak Current Control mode.

As Im interested on a reaction of the output voltage on load variations, 
I applied a load Jump from 800W to 1000W (at time stamp 33ms). The 
problem is, that the controller is not able to stabilize the output 
voltage after the load Jump. I always get low frequenz oscillations as 
attached. In my model I use ideal switchtes and ideal inductances and 
capacitances. The funtionality does work perfectly, but the settings or 
the controller gains are not made correctly by myself. And I dont know 
where to adjust, to get these ocillations away(or are they in a 
acceptable range.

thx in advance for any hint!

greez kuebli

ps.: red course is the output of the integrator (analog), blue course is 
the output-voltage, and the step represents the time stamp, where the 
load jump occurs

Author: KlaRa (Guest)
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Hallo kuebli,
Gruss Klaus.

Author: Fralla (Guest)
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Do not use ideal components. A capacitor with no ESR will lead to a zero 
(which helps to boost the phase) at a very (or infinit) high frequency. 
Model the Capacitor with a realistic ESR of few 100mOhms.

The low frequenzy oscilation ist the result of to low phase margin of 
the voltage control loop. Which controller do you use? I think a simple 
integrator... The only way to increase phase margin is to decrease to 
integrator gain. But this will lead to larger overshoot of the DC-link 
voltage. The only way to get rid of this increased loop gain with high 
phase boost. A PI-Regulator could do this...

PS:  Need the .asc file for further help

MFG Fralla


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