ripplle factor from fourier analysis

Thread Starter

aragon1971

Joined Apr 7, 2008
140
Hi
i m trying to find the ripple factor in full wave rectifier circuit with a capacitor filter
i find a results can check that ?

thank you
George
 

Attachments

Last edited:

Jerry-Hat-Trick

Joined Aug 31, 2022
544
Hi George,
I struggled to follow the maths, but the way I work out the expected ripple is to consider how fast the capacitor will discharge before the next voltage peak comes along. So long as the capacitor is large enough you can assume that the voltage drop is linear with time and the next peak will come along (for a full rectified AC) after half the period of the full wave. So the ripple voltage is close to Vpeak times the time between peaks divided by the load resistance divided by the capacitor value.

The fact that the voltage drop is not quite linear and the falling voltage is picked up by the next peak before the time between peaks means that the ripple voltage is slightly less than this
 

Papabravo

Joined Feb 24, 2006
21,157
You can also simulate it and get the simulator to measure it for you. If all three methods give approximately the same result, then you can choose the most convenient method.

ETA: Here is a simulation of a DC supply with an undersized capacitor and a high impedance load. The droop is about 41% here. You can not see the full wave rectified waveform without removing the capacitor entirely, but that is why you are seeing a cycle between the positive peaks of the AC waveform.

ETA2: Oh, but thanks to waveform arithmetic there is a way to see it:
1685802339568.png
 
Last edited:

Papabravo

Joined Feb 24, 2006
21,157
hI
I lnow this approximately solution but i m looking if thera an most exact solution
An exact solution may be a difficult hill to climb especially when our modeling of the situation is less than perfect. Is it really your contention that knowing the amount of ripple to 9 or 10 significant figures is better than 3 or 4?
 

Ian0

Joined Aug 7, 2020
9,667
Hi
i m trying to find the ripple factor in full wave rectifier circuit with a capacitor filter
i find a results can check that ?

thank you
George
Your approach would be valid if you had a filter circuit that was been driven by a voltage source V=|Vo.sin(ωt)| with a source impedance of Rs. Unfortunately that is not the case. Current can only flow when Vs>Vo because of the diode. For your maths to be correct current must be able to flow back to the source when Vo>Vs. Fourier analysis works on linear circuits, and this isn't.
In most cases the approximation Vripple_peak-to-peak = It/C will suffice. If Rs is high then it will be less accurate as it assumes that the capacitor charges instantly once per half cycle, and the current is a δ-function.
 

Papabravo

Joined Feb 24, 2006
21,157
Your approach would be valid if you had a filter circuit that was been driven by a voltage source V=|Vo.sin(ωt)| with a source impedance of Rs. Unfortunately that is not the case. Current can only flow when Vs>Vo because of the diode. For your maths to be correct current must be able to flow back to the source when Vo>Vs. Fourier analysis works on linear circuits, and this isn't.
In most cases the approximation Vripple_peak-to-peak = It/C will suffice. If Rs is high then it will be less accurate as it assumes that the capacitor charges instantly once per half cycle, and the current is a δ-function.
In fact, the leading edge of the diode current is sharp and after reaching its peak, the falling "edge" has a distinctly rounded aspect. The width of the current pulse is not approximated very well by a delta function. This can also be seen in the simulation.

ETA: I saw the diode waveform but did not include it in the image I posted. I can do that upon request.
 
Last edited:

Ian0

Joined Aug 7, 2020
9,667
In fact, the leading edge of the diode current is sharp and after reaching its peak, the falling "edge" has a distinctly rounded aspect. The width of the current pulse is not approximated very well by a delta function. This can also be seen in the simulation.
But if the tolerance on your smoothing cap is +80%-20% then it is probably close enough.
 
Top