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Ground ring setup when simulating inductors in HFSS

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leander

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Hi, every one,

When simulating inductors in HFSS, there is a ground ring surrounding the inductor, providing a return loop for the ports. This ground ring is required especially when the two ports are placed in different side.

I found that the result is very sensitive to the size of the ground ring. The inductance decreases when shrinking the ground ring. I guess that maybe it is because there is current on the ground ring induced by the central inductor, and this current degrades the inductance. When the size of ground ring increase, the effect will be diminished. So that I set the ground ring to be very large, but the result is much larger than reasonable (I have computed the same structure by analytical formula and other solvers).

How can I get a reliable result in HFSS?

In my HFSS simulation, I followed a design kit developed by Ansoft and UMC:
https://www.ansoft.com/registration/index.cfm?campaign=72
Here is a brief introduction:
https://www.onchippassivedesign.com/
 

GoaGosha

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Hi,

This is one of the problems with HFSS you cannot do stand alone inductors without
parasitic effects.You can minize them but you never get rid of them.

On the otherhand when you have testchip (you always measure with GND ring
otherwise you will have nonesense results) if you put the problem as is
you can attain good results.

In your case when ports are on different sides current loop closed thru these rings.
When you shrink the ring the self inductance of ring got bigger.
This mean that also mutual inductance between ring and inductor will be more
significant.Now,mutual inductance can sum up as positive or negative depending
on current direction when you excite.(opposite direction = negative mutual ind)

Note :

1) In High Freq current tends to close the loop with minimal path
therefore in HF current will choose mainly one side of the ring.

2) I advise you for planar inductor use planar EM simulators.

Regards
 

Ruritania

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Hi, Leander and GoaGosha,

I'm so glad that someone put forward this subject, coz I'm having the same issue. I found that using HFSS to simulate the inductor with ground ring is really troublesome.

Leander, did you notice that the UMC spiral have cut in the ground rings? The weird thing is that putting the cut in different places results in different inductance and quality factor!! REALLY confusing...

GoaGosha, what did you mean by putting:"On the otherhand when you have testchip (you always measure with GND ring otherwise you will have nonesense results) if you put the problem as is you can attain good results. "?

And if I do not have test chip, and my circuit do need inductors with ground rings for noise isolation, how to deal with this situation while using HFSS??

You mentioned that planar structure should use planar EM solver for simulation, but for those 3D inductor (e.g. Solenoid inductor), is HFSS reliable?

Thanks,

Ruri
 

leander

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Hi, GoaGosha and Ruri,

Thanks very much for your quick reply.

GoaGosha, I agree with you with you that in HFSS we cannot do a stand alone simulation without parasitics. When I set the ground ring small, the simulated inductance is about 10% smaller than expected. And after I enlarged the ground ring, I thought the result will converge to the expected one, but it became 20% larger when the ground ring was set to be quite large. This is what confuses me.

Actually I just want to do a comparison between HFSS and other simulators to find out the best one. I do this for quite a long time, and they never get the same result. I don't know which is reliable.

Among plannar simulators, I have try Momemtum. To my experience, it works very well only when the inductor is squared. For circular or octagonal one, the result is much different from other simulators and measure, at the same time, the simulating time becomes much longer.


Ruri,
I am glad that someone have the same issue with me. I didnot found the cut in the UMC design kit before you mentioned because I only refered to Ansoft spirals. But I have try adding a cut in my simulation, and I still cannot get a satisfactory result.

I am wondering if someone can give a simple testbench for different simulators.

Leander
 

Ruritania

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leander said:
Hi, GoaGosha and Ruri,

Thanks very much for your quick reply.

GoaGosha, I agree with you with you that in HFSS we cannot do a stand alone simulation without parasitics. When I set the ground ring small, the simulated inductance is about 10% smaller than expected. And after I enlarged the ground ring, I thought the result will converge to the expected one, but it became 20% larger when the ground ring was set to be quite large. This is what confuses me.

Actually I just want to do a comparison between HFSS and other simulators to find out the best one. I do this for quite a long time, and they never get the same result. I don't know which is reliable.

Among plannar simulators, I have try Momemtum. To my experience, it works very well only when the inductor is squared. For circular or octagonal one, the result is much different from other simulators and measure, at the same time, the simulating time becomes much longer.


Ruri,
I am glad that someone have the same issue with me. I didnot found the cut in the UMC design kit before you mentioned because I only refered to Ansoft spirals. But I have try adding a cut in my simulation, and I still cannot get a satisfactory result.

I am wondering if someone can give a simple testbench for different simulators.

Leander

Hi, Leander,

It seems that we're doing pretty much the same thing, glad to hear that...

I am using momentum too, but I think the Mom results sometimes is far too optimistic. Since UMC has released its design kit in HFSS, I think HFSS is still reliable, what's your opinion? We may try more on HFSS ... and tell all of the guys here what is found out.

Rgds,

Ruri
 

pengboy

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Hi,

did you try to increase the simulation accuracy? this might help..

also when you increas the groudring which is in series with your inductor, apparently it will increase the inductance.. try to use large metal not just a thin trace as the ground ring.
 

GoaGosha

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Hi,

Ruri :

If you have actual ground ring in design, it's perfect for HFSS. In this
case ground ring is not parasitic due to simulation or test chip
but real part of wich will be included in your design.


I don't know if HFSS is reliable for solenoid (i don't have expirience )
but it surely better to use HFSS in this case than any other planar tool.

Leander :

For planar inductors HFSS will never be better nor in accuracy neither in
the time solution - if you not achieved accurate results in Mom I advise you to contact their support.


Regards
 

smithchart

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Dear all:

Out of curiosity, I registered and downloaded the design kit. However, I found that a password is needed to access the designer project. Anyone knows what is that? If I do not use the foundary of UMC but have the commercial license of both HFSS and Designer, who should I approach for the password if it is not freely distributed?

By the way, what is the project variable "Gap" stands for in the project when you customize the spirals? I chaned its value from the default 0.1 to as large as 10, seems nothing happens to the geometry.

Thanks for your reply in advance.

Best regards,
 

itaifrenkel

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Leander shalom,

My name is Itai and I am work for Ansoft's rep in Israel, and fammiliar with Gosha's work on this subject.

Instead of a "one-size-fits-all" answer, please consider the following 6.5 points:

1. What percent of your custom inductor designs will use symmetric/differential spirals (that do not require a ring retrun path at all) ?

2. Assuming you do not have the layout yet and you're using a non-symmetric spiral, you'll need the ring back.
Start the smallest possible ring (like in the UMC example), and take a typical non-symmetric inductor that you would be using (the same number of turns/typical size). Next, replace your inductor with just one straight line. Compare the results with the inductor and without the inductor.

How much L (in percent) has the ring added to the original model ?

3. Look at the layout of the specific design that you intend to use the non-symmetric spiral. Can you identify the "real" retrun path ?

For example, let's assume that you have GND on one side of the inductor, VCC on the other side, and a bypass capacitor between them. Import the inductor layout with the GND,VCC nets and capacitor geometry. If you do not want to simulate the capacitor, just fill its airgap with metal. First, you'll be using the "real" return path in the simulation. Second, the non-ideal EM field will take the capacitor plate into account. The inductor results will include this return path effect.

In a benchmark, here in Israel, a customer has achieved a 2% accuracy of the VCO frequency shift, after extracting the VCO layout in HFSS. The circuit simulation and lab measurements were done by the customer ! With HFSS what you put in is what you get out, no need to "tweak" it.

4. Had you chosen another simulator over HFSS,
Would it be able to correctly simulate the return path through the silicon. In high frequencies some of the return current flows in the silicon, since it's the shortest way to go.
Would it be able to correctly calculate the losses inside the metal?
Unless it is a quasi-static extractor it has a return path (there has to be one, even if it's implicit). Are the simulator assumptions correct (or does it assume that the return path is 300um under the silicon were no current has ever gone before).
Does it simulate correctly above the inducor resonance frequency (3rd harmonic) ?

5. Have you consulted your local Ansoft AE? I can tell you that Ansoft is very proud of is their worldwide technical support engineers. When you buy Ansoft, you also buy their technical support. "Benchmark" them too. If your local AE does not have enough expirience with exporting layout from your favorite layout tool into HFSS, ask him to drop me an email to itaifrenkel@yahoo.com

6.5. Have a great day.

Itai
 

smithchart

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Hi,

Anyone here can help to answer my questions regarding "password" and "gap" I raised in my last post? (just one-level above this post).

Thank you in advance.

Best regards,
 

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