Can varactor be used as decoupling capacitors?

[rodgerwxh]

I am learning decoupling capacitor (DeCap) strategies. The basic rules are very straightforward to understand. "Making sure that noises carried in high frequencies will always pass through your decoupling capacitor so that the power supply of the IC has no ripples". Another function is to provide IC with fast and sufficient charge rather than ask the IC to pull charges through the long power lines which may introduce inductance and cause delay in phase. Correct me if I am understanding this wrong.

However, my concern is not about the purpose, its about practical strategy in using the decoupling capacitor.

1. DeCap values
Common suggestion online is use "sufficient", which means capacitance needs to be big enough, but how "sufficient", the bigger the better? Some give values like 0.1uF, 0.01uF, but we know Zc=1/(j*omega*C), which is a freq dependent value, a "fixed" or "sufficient" description is definitely not clear. Although manufactures of ICs suggest decoupling capacitor values, but how did they know that what kind of environment that I am going to use their IC? This is quite uncertain and everything has to be based on experience which is hard for newbies.

2. DeCap positioning
From YouTube or Google or some text books, it suggest to put local DeCap close the the power line of the IC. Also put a Bulk Cap close to the source. These all have its purpose. But I am wondering is it really always good to put DeCap as close as possible? Since it generate current loops and might cause resonances as well. So where to put the DeCaps seems to be another undetermined rules. What if I have many DeCaps on the board? Scatter them out or put them together?

3. With all above said, can I use a varactor as a DeCap? Since its capacitance can be controlled by the biased voltage. Is it more flexible? Any one can tell me if it violates some basic design rules or cause some extra troubles or resources? I also want to learn about the specific advantage and disadvantages of using varactor as DeCaps.

Thanks in advance guys.

 

[chuckey]

3, no, its not a very good capacitor and expensive for its performance.
2. If a chip takes pulses of current, then its good that a local cap supplies it rather then the pulses having to run along the board before they get to a cap.
1. For logic circuits, the current transients are very fast and the amplitude of the currents low, so some people say a .01 cap across every chip, other say .1 MF per 10 chips. Its up to you to decide how tolerant your circuit is to noise on the supply lines.
For high frequency amplifiers, its the series inductance of the cap that is more important then its actual value, some designs actually used certain lead lengths to put in a series resonant circuit (very low impedance) over some operating frequency band. For RF amplifiers there is a cap which is called a "feedthrough cap". This is an arrangement whereby you have a tube of metal with a thread on the outside, then an inner tube of dielectric, then a centre conductor that is longer then the outside tube. So in use it is used to feed power supply voltages into a screened compartment, by drilling a hole in the screen and putting nuts either side on the outer tube , hence earthing it . The power line is then fed through the inner conductor which has a capacity of a up 1000pF to the outer tube but has very low inductance.
Frank

 

[HTA]

Hi

A DeCap has the purpose to supply the fast current demand of an integrated circuit to avoid that supply voltage(between VCC and GND) varies too much. Since each connection/line to the two supply pins on a PCB represents an inductance(which resist fast current changes) it should be a small as possible( so near the current demanding IC pins). The DeCap nearest to the IC needs to be a low ESR for high frequencies (good HF capacitor, https://wiki.xtronics.com/index.php/Capacitors_and_ESR ). The capacitance value depend on the current demand. If the DeCap on the ICs provides the fast current changes the central DeCap on the source can be typically a Tantalum cap with higher capacitance The varactor are in relation to the DeCap more expensive and used more for LC-oscillator.

In summary: The design consideration for DeCAP is driven by the current demand(peak value and rise and fall time). This defines how critical the inductance of the connection lines will be (both for ground and supply lines).

Enjoy your design work!

 

[rodgerwxh]

Thank you Frank, for the clear explanation. With that said, it seems to me that the real challenge of using DeCap is at the high frequency devices. The reason I started this post is that I want to find some new use of varactors, I learned that varactors can be used for tuning filters and harmonic multipliers. I want to see if it can be used for any other purposes.

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Hi HTA, thanks for the answer, it is clearer for the design concern of a DeCAP. Per your description, if inductance of the Chip pins is considered, then the DeCAP will resonate with that induced inductance. This will be very bad for the EMC concern right? Especially for the high-frequency devices. Is there a way to balance the conflict? Or the resonate frequency is typically to high to be concerned?

 

[volker_muehlhaus]

Per your description, if inductance of the Chip pins is considered, then the DeCAP will resonate with that induced inductance.

No. Series L and the shunt C will form a low pass filter. This is fine.

 

[crutschow]

To minimize the effects of capacitor inductance on a PCB circuit, use surface-mount capacitors with the ground side connected directly to the ground plane and run the trace being decoupled directly through the capacitor mounting pad. That way the inductance is basically that of the surface-mount cap, which is about as low as you can get.

 

[chuckey]

If you are trying to find a new use for varactors, try searching on " parametric upconverters".
Frank