Impedance in PSU

[Electronman]

Hi,

How a capacitor in PSU circuits affects to reduce the impedance of the power supply and how does this affect the PSU?
I just thought those caps as filter caps that help to reduce the ripples of PSU

 

[MikeMl]

Input caps filter ripple, and sometimes are required for regulator stability. Read specific regulator data sheet.
Output caps are sometimes required for regulator stability, and improve regulation above ~10KHz as the regulator internal opamp circuit usually has a Gain Bandwidth of < 100Khz...

 

[Electronman]

Input caps filter ripple, and sometimes are required for regulator stability. Read specific regulator data sheet.
Output caps are sometimes required for regulator stability, and improve regulation above ~10KHz as the regulator internal opamp circuit usually has a Gain Bandwidth of < 100Khz...

I knew that, But this website gave me another vague view to think about impedance of a cap too when it is used as a ripple reducer too"
**broken link removed**

Any idea if thats true?

 

[crutschow]

As Mike ML noted, the frequency response of the power supply control circuits is limited. Above this response frequency the output impedance of a power supply would rise but for the output capacitors. They provide a high frequency source of current and keep the apparent output impedance low at the high frequencies.

Not quite sure what your question is. The website you referenced talks about needing to reduce the power supply impedance for high frequency circuits, which is certainly true. This requires capacitors that have a low impedance at high frequencies so sometimes a small ceramic or film capacitor is place in parallel with an electrolytic.

 

[Electronman]

As Mike ML noted, the frequency response of the power supply control circuits is limited. Above this response frequency the output impedance of a power supply would rise but for the output capacitors. They provide a high frequency source of current and keep the apparent output impedance low at the high frequencies.

Not quite sure what your question is. The website you referenced talks about needing to reduce the power supply impedance for high frequency circuits, which is certainly true. This requires capacitors that have a low impedance at high frequencies so sometimes a small ceramic or film capacitor is place in parallel with an electrolytic.

Ok thanks,

What I think is this: Why a cap is needed at power supplies? Well because of reducing the ripples.
I Actually do not know why and how a cap (electrolyte or ceramic one) is able to reduce the impedance of a power supply? Or it is better to ask, How a cap lowers the impedance of a power supply and why the lower impedance means the better results at high frequencies?

 

[MikeMl]

You are asking how does a capacitor work.

 

[Sceadwian]

Capacitors store energy in an electric field surrounding the plates. When the voltage of the power supply drops the capacitor energy will flow into the circuit, providing sometimes massive backup power. The power supply still has to recharge the capacitor though and as far as ripple goes all the cap does it smooth out thepeaks a bit based on the RC time constant of the circuit as a whole. Power supplies with a lot of ripple or that need high peak power backup will have massive filter capacitors to provide those high energy low duration pulses.

 

[Electronman]

Capacitors store energy in an electric field surrounding the plates. When the voltage of the power supply drops the capacitor energy will flow into the circuit, providing sometimes massive backup power. The power supply still has to recharge the capacitor though and as far as ripple goes all the cap does it smooth out thepeaks a bit based on the RC time constant of the circuit as a whole. Power supplies with a lot of ripple or that need high peak power backup will have massive filter capacitors to provide those high energy low duration pulses.

Thanks, but I do know those, I myself told and was agreed which a cap in a PSU is used to reduces the ripples of the voltage, But the main question is how a cap reduces the impedance in PSU's if It does so there really, and why that's helpful???

And yes, I do know how a cap does work.

 

[MikeMl]

And yes, I do know how a cap does work.

If you did, you would have accepted what both I and Cruts told you several posts ago. Reread those replies again.

 

[Hero999]

I'll give it once last try.

Read up impedance and hopefully you'll understand that it's not the same as resistance.
Electrical impedance - Wikipedia, the free encyclopedia

The impedance of a supply such as the LM317 is low at DC but high at high frequencies.

In other words the LM317 has a poor transient response, unless a capacitor is connected to the output.

When a large load is connected to the output, the voltage will suddenly fall before returning too the set value over a ms or so.

When a large load is disconnected, the output voltage will suddenly rise before returning to the set value over a ms or so.

The reason for this is because it takes time for the error amplifier in the LM317 to adjust the impedance of the output transistor to compensate for the sudden change in load impedance.

Adding a capacitor solves the problem because it discharges slightly when a large load is connected and absorbs the voltage spike when the load is connected by charging up more. High frequency digital circuits draw a high frequency fluctuating (AC) current so the capacitor stops the supply from falling below a voltage that would make the circuit unstable.