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[SOLVED] which is correct capacitor ripple current in simulation

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Zak28

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The 12v power source in the sim outputs ~4.5amps peak without any series current limit resistors.

img.png


But the same circuit with a 1R series resistor to the 12v supply causes the capacitor to have very large ripple.

img2.png

Without the series resistor the capacitor doesn't have such a ripple.

In an actual circuit without the 1R resistor and with a 12v 150w supply will the capacitor have large ripple?
 
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Hi,

How accurate your simulation results get compared to reality depends on your model.

As it is I cannot say whether this ripple will be what it is in this simulation because I can't seem to figure out what exactly you're simulating.

Please explain in great detail what system you're simulating. With that we can say whether your model is correct in the first place or not.

If the model is correct, then we would understand your question and can give directed solution.
 
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    Zak28

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Hi,

The behaviour is quite expectable.

In your first simulation you used
* ideal power supply - with zero impedance
* ideal wiring - with zero impedance
This is not realistic.

Each power supply and each wiring has frequency dependent impedance.
Each equivalent impedance can be modeled as a couple of resistors, inductivities and capacitors.

Klaus
 
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KlausST said:
Hi,

The behaviour is quite expectable.

In your first simulation you used
* ideal power supply - with zero impedance
* ideal wiring - with zero impedance
This is not realistic.

Each power supply and each wiring has frequency dependent impedance.
Each equivalent impedance can be modeled as a couple of resistors, inductivities and capacitors.

Klaus
Click to expand...

A parasitic 3nH inductor with 100mR 200pF series to the supply seems more probable for an actual circuit. This models parasitics found between interconnections from 12v supply into load and the capacitor across it.

img.png

The capacitor is with 25mR 45nH of parasitics. The ripple is likely very much not accurate but much lower even than with a large series resistor between load and supply.

Whats a typical ESL range for 470uf-2200uf elecytrolytic capacitors?
 

You'll realize that the capacitor current with parallel connected ideal voltage source should be zero. In so far, the first setup is showing just simulator artefacts respectively the effect of intentional circuit matrix modifications that the solver is making.

It would be useful to add realistic parasitic parameters (ESR and ESL) to capacitor and voltage source.
 
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    Zak28

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What does your power source represent? A battery?
What does your load represent?
Your load is directly connected to the power source.
You are switching the load in and out.

I seriously think you need to give more detail on what you're doing. Whatever result you got from that simulation is based on what schematic you made on it. SPICE does not know your real system.

- - - Updated - - -

Zak28 said:
...
The ripple is likely very much not accurate but much lower even than with a large series resistor between load and supply.
...
Click to expand...

Higher value resistor would charge the capacitor slower than lower value once. That's just R-C transient response to the step change you create with switching the load.
 

I see about 20 mOhms ESR + 20 nH ESL for a 2200 uF/100V capacitor. Unless you have a low impedance battery, the power supply impedance will considerably higher.

Like Akanimo I wonder what's the real world problem behind the simulation.
 
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To be clear it’s a tricky problem to figure out cap ripple current in a power distribution network with poorly specified or hard to measure impedance.

If you know the ripple current of your load however the safest bet is to specify your local caps to handle all of it.
 

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