Quiet Power: Dynamic Models for Passive Components


Reading time ( words)

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.

Share


Suggested Items

Streamlining Thermal Design of PCBs

01/10/2018 | Dr. John Parry, CEng, Mentor
When designing a PCB, thermal issues are often locked in at the point of selecting and laying out the chip package for the board. After that, only remedial actions are possible if the components are running too hot. Assumptions made about the uniformity of the airflow in these early design stages can mean a disaster for the commercial viability of a PCB if those assumptions are incorrect. A different approach is needed to improve reliability and to optimize board performance. Dr. John Parry of Mentor explains.

Resins: Cutting Through the Technical Jargon

08/21/2017 | Alistair Little, Electrolube
This month, I’m going to cut through some of the more heavy-going tech-speak, taking a few of my customers’ more frequently asked questions about resins to try to help you refine your selection process. There’s a lot of ground to cover, but for the purposes of this column, let’s concentrate on the PCB’s operating environment, caring for the components that are to be encapsulated, and the special needs of applications like LED lighting and RF systems.

Lightning Speed Laminates: Smaller Circuits--Material Properties and Thermal Issues

08/29/2016 | John Coonrod, Rogers Corporation
Coefficient of thermal expansion (CTE) is typically considered for PCB reliability, but it can also have an impact on circuit performance for applications exposed to varying temperatures. Due to CTE, a circuit will change physical dimensions when the temperature changes. If the circuit has small features or tightly coupled features, the physical change of the circuit dimensions can cause a shift in electrical performance.



Copyright © 2018 I-Connect007. All rights reserved.