Power-integrity simulation keeps your planes perfect, part 2
Simulate the power-distribution network in your PCB to eliminate thermal and voltage problems.
Paul Rako, Technical Editor -- EDN, July 14, 2011
This article is a continuation of "Power-integrity simulation keeps your planes perfect, part 1."Software choices
The physical geometry of a power network is critical to its performance, so most software vendors use field-solver technology in their power-integrity tools (Reference 3). These tools should give you a fast answer and accurate results. RF-IC and system designers routinely use full-wave field solvers to solve Maxwell’s equations in 3-D. However, 3-D field solvers take a long time to achieve a result, especially if you apply them to a relatively large physical item, such as a PCB. Accordingly, power-integrity vendors design hybrid solver technology into their power-integrity tools. While solving for traces, these tools might use a 2-D solver with a fast technique employing transmission-line theory. For simulating planes, the tools can use 2-D or 2.5-D finite-element techniques. In some cases, the software can model vias using a lumped-element capacitor and inductor model. The tools apply a full-wave 3-D solver to the vias for accurate results.
You would also use a full-wave solver to simulate the effect of 3-D structures, such as connector pins and other mechanical devices in the power path. Software vendors also put thermal analysis in their tools. You can use this feature alone or export the thermal information to a specialized thermal-analysis tool, such as Mentor Graphics’ FloTherm, a CFD (computational-fluid-dynamics) 3-D simulation environment. Mentor’s HyperLynx simulation tool can do its own thermal analyses and export the results to FloTherm so that you can model the thermal performance of an entire system or an enclosure.
Agilent has re-engineered its ADS (advanced-design-system) Momentum product to provide simulations results when you have heavily perforated power and ground planes. It also accommodates designs that need a few signal traces in the planes. MOM (method of moments), the fastest simulation method for multilayer structures, solves full 3-D fields, including all of the terms in Maxwell’s equations. This full-wave approach accounts for the high-frequency effects of Faraday’s Law and the displacement current term that Maxwell added to Ampère’s Equation (Reference 4). Using MOM to simulate large planes is time-consuming, so Agilent invented algorithms that reduce the time necessary for achieving accurate results. The tool works down to dc using a tree/co-tree method, according to Colin Warwick, product owner for high-speed digital at Agilent.
You can also adapt lumped-element
analysis to planar elements. NEC’s
PIStream software models planes as
matrices of lumped elements, making it
suitable for analysis using Spice engines
and other lumped-element techniques.
For a plane, the software generates an
RLGC (resistance/inductance/conductance/capacitance) equivalent using the
PEEC (partial-element-equivalent-circuit)
technique. The software similarly
generates lumped-element models for
the vias and cavities that form between
the ground and the voltage planes
(Figure 7). The software also models a
decoupling capacitor using a series RLC
(resistance/inductance/capacitance)
model that combines the parasitic resistance
and capacitance of the capacitor
with the parasitic resistance and inductance
of the fan-out traces and vias. You
can set up a simulation run to quickly
perform single-pair analyses. When you
change settings, the software will perform
a multilayer analysis that takes into
account all relevant planes.In addition to simulating the physical configuration of finished boards, a software tool such as HyperLynx lets you perform early-stage floorplanning of your planes and decoupling structure. You can then quickly run an analysis to give you some idea of the transfer impedance and other variables. Giga Hertz Technology has developed a faster Spice engine and integrated it into NEC’s PDN (power-delivery-network) Expert. With these floorplanning tools, you can manually sketch the PCB and plane and optimize the capacitors earlier in the design. Thus, you get an idea of the plane’s shape, size, stackup, and capacitor count.
Some power-integrity-software vendors from the PC world, such as Mentor Graphics and Cadence, integrate their tools into the design flow. Although it is reassuring to have one vendor supplying all the tools, the power-integrity simulation uses a physical representation of the PCB and makes a geometric model. Ansys and Sigrity both accept inputs from Cadence’s Allegro; Mentor Graphics’ PADS; and tools from Zuken and Altium. Agilent derives its power-integrity tools from its significant expertise in RF design. In addition to working with the ADS design tool, the company’s EMPro software can import PCB data from Cadence’s Allegro. Customers often use NEC’s PIStream with Zuken’s PCB tools, but the software can accept inputs from Cadence’s Allegro and other PCB software.
Although some engineers prefer that their board flow has integrated tools, getting tools from a simulation expert, such as Ansys, has some advantages. For example, the company’s SI (signal-integrity) Wave tool is similar to Mentor Graphics’ HyperLynx, and a PIAdvisor tool helps you delve into power-integrity issues. The tools have 3-D solvers for simulating vias. You can also use the Ansys HFSS (high-frequency-simulator-system) tool for full 3-D simulations of physical problems, such as connectors and other 3-D geometry. Some customers import the output of the Ansys power- and signal-integrity tools to the same HFSS tool they use to model the enclosure. In that way, they can evaluate their product’s EMI. CST’s (Computer Simulation Technology’s) EM Studio software imports Gerber PCB files and can calculate 3-D IR (current/resistance) drop.
The software you select must have
the capabilities you need. Many companies expect that you will solve the
signal- and power-integrity problems
separately, assuming that, once you sufficiently
reduce the power impedance,
you will then look at signal integrity. The
problem with this approach is that power
and signal noise interacts. To offset that
problem, Sigrity allows you to simulate
the effect of power noise on signal integrity
(Figure 8). CST’s Microwave Studio
also lets you analyze noise propagating
from power planes in close proximity.
The price of power-integrity software often shocks inexperienced engineers. A simple dc simulator can cost $15,000, and a full-blown system with power-integrity, signal-integrity, and thermal solvers can cost as much as $75,000. This figure may seem high for software until you consider the costs of power-integrity failure. A complex board spin can cost $5000 or $10,000 in fabrication and engineering and $1 million in time-to-market expenses. Another consideration is the BOM (bill-of-materials) cost of your system. If your power-integrity software can save you 50 cents in capacitors, you could recover the cost of the power-integrity software in a few months for a high-volume product.
Ansys’ Pytel observes that three
engineers once did power-integrity, signal-integrity, and EMI analyses in isolation.
These days, although one engineer
may do EMI analysis, that person first
works with a person who performs both
power- and signal-integrity analysis, and
they often all share the same software.
Sigrity’s Brim notes that IBIS (input/output
buffer specification) 5.0 has power-ground
and signal data that allows your
simulation software to relate the noise
on the power pin of a 5.0 model to the
noise that leaks through to the output,
similar to the PSRR spec in an analog
part. All of these features combine into
one unified effort to give your company a
solid, well-designed product (Figure 9).If you understand and know how to use these expensive tools, your worth as an engineer increases. Learning the tools is not hard for engineers who embrace CAD (computer-aided-design) software. Mentor Graphics offers free workshops for HyperLynx at many of the company’s sales locations. If you are experienced with other types of simulators, you will have little problem learning power-integrity tools. You need to learn and understand the concept and lingo of the frequency domain, just as an RF designer does. By adding that knowledge to your time-domain expertise, you can take on the toughest design challenges and come out a winner.
You can reach Technical Editor Paul Rako at 1-408-745-1994 and paul.rako@ubm.com.
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