Dynamic Models for Passive Components

A year ago, my Quiet Power column described the possible large loss of capacitance in multilayer ceramic capacitors (MLCC) when DC bias voltage is applied. However, DC bias effect is not the only way we can lose capacitance. Temperature, aging, and the magnitude of the AC voltage across the ceramic capacitor also can change its capacitance.

Finally, the initial tolerance needs to be considered as well. In the worst case, we may lose up to 90% of the capacitance for an X5R capacitor, and even for an X7R capacitor. This column will show you the details and also how the most advanced manufacturers are helping the users with new simulation models to take these effects into account.

As an actual example, let us look at one of the capacitors that was extensively tested, where 1uF 0603-size 16V capacitors were tested from various vendors. We further assume that we want to use the part on a 12V supply rail, where the AC noise is low (this will be important later when we take the AC bias dependence into account). Some of the samples were chosen with X5R, some with X7R temperature characteristics. As showed with actual test data , X7R capacitors are sometimes worse for DC bias sensitivity than X5R parts.

If we take the part from Vendor B (labeled B7) in Figure 1, we see that at 12V DC bias we can lose 60% or 70% of the capacitance, dependent on which way the DC bias changes. But when we need to consider the worst-case capacitance loss, we have to consider the cumulative effect of all of the following factors:

  • Initial tolerance
  • Temperature effect
  • DC bias effect
  • AC bias effect
  • Aging

The sample had +-10% initial tolerance. The X7R temperature characteristics comes with an additional +-15% tolerance window for the temperature variation.

To read this entire article, which appeared in the March issue of The PCB Design Magazine, click here.

Back

2016

Dynamic Models for Passive Components

05-11-2016

A year ago, my Quiet Power column described the possible large loss of capacitance in multilayer ceramic capacitors (MLCC) when DC bias voltage is applied. However, DC bias effect is not the only way we can lose capacitance. Temperature, aging, and the magnitude of the AC voltage across the ceramic capacitor also can change its capacitance.

View Story
Back

2015

Avoid Overload in Gain-Phase Measurements

07-01-2015

There is a well-established theory to design stable control loops, but in the case of power converters, we face a significant challenge: each application may require a different set of output capacitors coming with our loads. Since the regulation feedback loop goes through our bypass capacitors, our application-dependent set of capacitors now become part of the control feedback loop. Unfortunately, certain combination of output capacitors may cause the converter to become unstable, something we want to avoid. This raises the need to test, measure, or simulate the control-loop stability. Istvan Novak has more.

View Story

Effects of DC Bias on Ceramic Capacitors

04-01-2015

The density of multilayer ceramic capacitors has increased tremendously over the years. While 15 years ago a state-of-the-art X5R 10V 0402 (EIA) size capacitor might have had a maximum capacitance of 0.1 uF, today the same size capacitor may be available with 10 uF capacitance. This huge increase in density unfortunately comes with a very ugly downside. Istvan Novak has more.

View Story
Back

2014

Vertical Resonances in Ceramic Capacitors

12-03-2014

Because of their small size, we might think that structural resonances inside the ceramic capacitors do not exist in the frequency range where we usually care for the PDN. The unexpected fact is that the better PDN we try to make, the higher the chances that structural resonances inside ceramic capacitors do show up. This column tells you why and how.

View Story

Quiet Power: Vertical Resonances in Ceramic Capacitors

12-03-2014

Because of their small size, we might think that structural resonances inside the ceramic capacitors do not exist in the frequency range where we usually care for the PDN. The unexpected fact is that the better PDN we try to make, the higher the chances that structural resonances inside ceramic capacitors do show up. This column tells you why and how.

View Story

Checking Cable Performance with VNA

04-02-2014

In a previous column, Columnist Istvan Novak showed that poor cable shields can result in significant noise pickup from the air, which can easily mask a few mV of noise voltage needed to measure on a good power distribution rail. In this column, he looks at the same cables in the frequency domain, using a pocket-size vector network analyzer (VNA).

View Story

Quiet Power: Checking Cable Performance with VNA

04-02-2014

In a previous column, Columnist Istvan Novak showed that poor cable shields can result in significant noise pickup from the air, which can easily mask a few mV of noise voltage needed to measure on a good power distribution rail. In this column, he looks at the same cables in the frequency domain, using a pocket-size vector network analyzer (VNA).

View Story

Comparing Cable Shields

01-08-2014

In his last column, Istvan Novak looked at the importance of properly terminating cables even at low frequencies and also showed how much detail can be lost in PDN measurements when bad-quality cables are used. This month, he analyzes a step further the shield in cables.

View Story
Back

2013

Quiet Power: Cable Quality Matters

11-20-2013

In his August column Istvan Novak looked at the importance of properly terminating the cables that connect a measuring instrument to a device under test. He writes that we may be surprised to learn that even if the correct termination is used at the end of the cable, the measured waveform may depend on the quality of the cable used.

View Story

Quiet Power: Don't Forget to Terminate Cables

10-23-2013

In high-speed signal integrity measurements, the first rule is to properly terminate traces and cables. However, many PDN measurements may be limited to lower frequencies, such as measuring the switching ripple of a DC-DC converter. Do you really need to terminate measurement cables if the signal you want to measure is the switching ripple of a converter running at 1 MHz?

View Story

Quiet Power: Do Not Measure PDN Noise Across Capacitors!

08-07-2013

PDN noise can be measured in a variety of ways, but measuring across a capacitor will attenuate the high-frequency burst noise. Keep in mind that by measuring across a capacitor, the converter output ripple reading could be several times higher--or many times smaller--than the actual ripple across our loads.

View Story

Quiet Power: How to Read the ESR Curve

01-15-2013

To use bypass capacitors properly, any designer must understand ESR (effective series resistance). A designer must understand what it means and how to read the ESR curve in measured or simulated plots.

View Story
Back

2012

Quiet Power: What's the Best Method for Probing a PDN?

08-15-2012

Recently, one of Istvan Novak's friends asked him about the preferred method of probing a power distribution network: "Which probe should I use to measure power plane noise?" Although, as usual, the correct answer begins with "It depends," in this case the generic answer is more clear-cut: For many PDN measurements, a simple passive coaxial cable is better.

View Story

Quiet Power: Will Power Planes Disappear?

04-04-2012

Istvan Novak takes a look at an award-winning paper presented at DesignCon 2012, and he discusses the apparent disappearance of power planes from PCBs. In the future, the need for power planes may diminish or go away altogether. The change is already under way, and power planes, full-layer planes in particular, are disappearing fast from printed circuit boards.

View Story

Do Bypass Capacitors Change Plane Resonances?

02-01-2012

My friend Greg recently asked me, "If I add surface-mount capacitors to a bare pair of planes, I am told that the resonant frequency will drop. On the other hand, someone with expertise is telling me that this is not the case. What would you expect to see?" As happens many times, both observations have elements of the truth in them, and a third scenario is not out of the question.

View Story
Back

2011

Be Careful with Transmission Lines in Plane Models

11-16-2011

Last month, we learned how we can determine the grid equivalent circuit parameters for a plane pair. You may wonder: Is it better to use LC lumped components in the SPICE netlist or to make use of SPICE's built-in transmission line models? In short, we can use either of them, but we need to set up our models and expectations correctly.

View Story

Quiet Power: Simulating Planes with SPICE

10-12-2011

There are several excellent commercial tools available for simulating power distribution planes. However, you don't need a commercial tool to do simple plane analysis. You can, for instance, write your SPICE input file and use the free Berkeley SPICE engine to get result. If you want to do your own plane simulations, there are a couple of simple choice.

View Story

Quiet Power: Does Dk Matter for Power Distribution?

08-16-2011

We know that in signal integrity, the relative dielectric constant (Dk) of the laminate is important. Dk sets the delay of traces, the characteristic impedance of interconnects and also scales the static capacitance of structures. Is the same true for power distribution? The answer is yes, but for power distribution all this matters much less.

View Story
Back

2010

Do Not Perforate Planes Unnecessarily

11-03-2010

For this column, I will take a quick detour from the series on the inductance of bypass capacitors. I will devote this column to a few comments about via placement and its potentially detrimental impact on signal and power integrity when antipads heavily perforate planes.

View Story

Inductance of Bypass Capacitors, Part III

08-18-2010

In Part III of a series, we'll take a look at loop or mounted inductance. Loop inductance is important, for instance, when we need a reasonably accurate estimate for the Series Resonance Frequency (SRF), or for the anti-resonance peaking between two different-valued capacitors or between the capacitor's inductance and the static capacitance of the power/ground planes it connects to.

View Story

Quiet Power: Inductance of Bypass Capacitors, Part II

07-21-2010

We finished the last Quiet Power column with a few questions about the inductance of bypass capacitors: Why do different vendors sometimes report different inductance values for nominally the same capacitor? Start by asking the vendors how they obtained these inductance values.

View Story

Why PI Design is More Difficult Than SI

05-19-2010

Why is power integrity design more difficult than signal integrity design? Reasons abound, and unlike SI, we've only begun to study PI. Collective wisdom and experience gained over the coming years will help to alleviate the pain somewhat, but we should expect the challenge to stay with us for some time.

View Story

Why S11 VNA Measurements Don't Work for PDN Measurements

04-14-2010

In this edition of Quiet Power, Istvan Novak continues to examine one-port and two-port vector network analyzer set-ups for PDN measurements, and other tricks and techniques for measuring impedance values below 5 milliohms.

View Story

PDN Measurements: Reducing Cable-Braid Loop Error

02-24-2010

At low and mid frequencies, where the self-impedance of a DUT may reach milliohm values, a fundamental challenge in measurement is the connection to the DUT. Unless we measure a single component in a well-constructed fixture, the homemade connections from the instrument to the DUT will introduce too much error. What's the solution? By Istvan Novak.

View Story

Quiet Power: Calculating Basic Resonances in the PDN

01-27-2010

In my last column, I showed that the piecewise linear Bode plots of various PDN components can create peaking at some interim frequencies. Today, I must cover peaking in more detail, because, even today, certain articles, books and CAD tools provide the wrong answers to this problem.

View Story
Copyright © 2017 I-Connect007. All rights reserved.