Power Delivery

The Charge of the Bucket Brigade

An exploration of the PDN bucket brigade concept as a practical way to understand time-domain energy delivery in high-speed PCB design.
by Daniel Beeker

Why do otherwise good PCB designs still fail EMC testing, exhibit excessive jitter or require multiple redesign cycles? The answer often begins with a misunderstanding of how electromagnetic field energy moves through a power delivery network (PDN).

Every switching event in a modern electronic system creates a request for electromagnetic field energy. The question is not whether the energy exists – the question is whether the PDN can deliver it fast enough.

For decades, PCB design has often been taught from a conductor-centric perspective, where electrical current is viewed primarily as electrons moving through copper traces.

While circuit theory remains important, modern high-speed electronic systems increasingly require designers to think differently.

Electronic systems are fundamentally electromagnetic field management systems. As switching speeds continue to increase and semiconductor geometries continue to shrink, understanding how electromagnetic field energy behaves in the spaces surrounding conductors becomes critical to achieving signal integrity, power integrity, electromagnetic compatibility (EMC) and overall system reliability.

This is the foundation of what I often summarize as “All About That Space.” So far, Meghan Trainor has not objected to my repurposing of her 2014 hit song.

The Traditional View Versus the Physical Reality

Traditional circuit theory teaches that electrical energy flows inside conductors and that switches instantly cause current to flow through a loop. In reality, electromagnetic field energy propagates through the dielectric spaces between the conductors.

Switches do not simply connect wires; they create new spaces into which electromagnetic field energy must propagate, and that propagation takes time. The movement of electromagnetic field energy through space is what is observed as current flow. As signal rise times shrink into the nanosecond and picosecond range, understanding this distinction becomes increasingly important.

Figure 1. Illustration of the traditional view of power delivery. (All images courtesy of Cadence Design Systems.)

The rules of triplets. Modern electronic systems can be understood through three fundamental concepts. Electromagnetic field energy requires only three elements for containment: a conductor, a dielectric space and another conductor. Likewise, electronic systems are built from just three basic components: conductors, dielectric spaces and switches. Electromagnetic field energy can perform only three functions: it can be stored, moved or converted into kinetic energy. Together, these principles provide a simplified framework for understanding complex PCB behavior.

Figure 2. Illustration of the Rules of Triplets concept.

Signal integrity starts with power integrity. One of the most misunderstood aspects of high-speed PCB design is that signal integrity depends directly on power integrity. A poorly designed power delivery network cannot support clean signal behavior.

Figure 3. Signal integrity begins with power integrity.

Every switching event creates a demand for electromagnetic field energy. That demand propagates as a wave through the power delivery structure until energy is delivered from nearby storage structures. The critical issue is time. Field energy does not arrive instantaneously. The request for energy must travel to the storage device and return to the switching device.

This creates a hierarchy of energy delivery systems:

  • On-die capacitance
  • Package capacitance
  • PCB planar capacitance
  • Local bypass capacitors
  • Bulk capacitors
  • Power supplies

The farther away the energy source is from the switching device, the longer it takes to respond. As semiconductor switching speeds increase, the effective energy-delivery distance becomes extremely short.

At picosecond edge rates, the depletion waves requesting energy may not even travel outside the footprint of the BGA package before multiple switching cycles have already occurred. This is why low-impedance transmission line structures, tightly coupled power/ground systems, and small-package capacitors located close to the switching device become essential.

The PCB designer’s job is no longer simply routing traces. Now, the PCB designers must control electromagnetic field movement in three-dimensional spaces.

Well-defined transmission lines. Signal traces must remain one dielectric away from their return path along their entire length, including vertical transitions. This requirement applies to adjacent reference planes, coplanar structures, vias and layer transitions to maintain signal integrity and consistent electromagnetic field behavior.

Figure 4. Characteristics of well-defined transmission lines.

One of the most common design mistakes in otherwise good PCB layouts is failing to maintain proper field containment through vertical transitions. The fields do not magically find their way between layers. They fill the space between the signal via and the nearest ground via. If it is not done by design, the results can be devastating, causing SI issues and EMC failures. The electromagnetic fields continue to propagate through the surrounding spaces and must remain properly controlled.

The PDN Bucket Brigade: The Eureka Moment

Figure 5. Energy delivery from power supplies is a one-way path.

Each node in the power supply should be considered a discrete electromagnetic energy domain, with field energy traveling downstream from the power source to the switching device. The farther away from the switch, the longer it takes for the request for energy to be answered. Do not ask for fast energy from a large package capacitor far away, and you will avoid disappointment. The supply system must be designed so that different portions of the electromagnetic energy spectrum are delivered from matching structures.

Imagine a network of water barrels connected by pipes of different diameters and lengths. The large barrels farther away hold substantial amounts of water, but they do not respond quickly to changes in demand. Smaller, closer barrels respond much faster but cannot supply large volumes of water.

The capacitors behave like these reservoirs distributed throughout the system. Before energy can move farther downstream, those reservoirs must first fill.

Figure 6. The overall behavior is less like the source pushing energy forward and more like the loads pulling electromagnetic field energy toward themselves.

When a switch closes, field energy moves into a newly connected space. That movement creates a localized drop in field density, which launches a depletion wave back upstream through the PDN.

Figure 7. The switch turns on.

This depletion wave travels outward requesting additional electromagnetic field energy. When the wave encounters a nearby energy storage structure, such as on-chip capacitance, package capacitance, plane capacitance or local bypass capacitors, energy begins returning toward the switching device.

Figure 8. The bucket brigade begins.

Each reflection creates a new reduction in field density at the discontinuity, launching additional waves farther upstream to request more energy. The closest energy storage structures respond first because their round-trip propagation delay is shortest. More distant structures respond later.

This process continues in a cascading hierarchy:

  • On-chip capacitance
  • Package capacitance
  • PCB planes
  • Local capacitors
  • Bulk capacitors
  • Eventually the power supply itself

The result is a distributed electromagnetic “bucket brigade” in which energy is progressively transferred from smaller, faster-switching structures and replenished by larger, slower-switching storage domains farther away. At the speeds electromagnetic waves travel, this process happens astonishingly fast. Electromagnetic waves on a standard PCB move approximately one-sixteenth of an inch in 10 picoseconds. Eight round-trip depletion and replenishment cycles may occur in only 160 picoseconds.

Figure 9. Illustration of the distributed electromagnetic “bucket brigade,” in which energy is rapidly transferred between local and remote storage structures to maintain stable power delivery.

During that time, the energy travels only one inch, and the electromagnetic field energy may not even propagate in the planar structure beyond the footprint of a typical BGA package.

Perspective

  • 1/16 inch in 10pS
  • 8 cycles in 160pS
  • Energy has moved only 1 inch
Figure 10. The energy has only traveled one inch in any direction during this time.

The power management IC still has no idea. At modern switching speeds, multiple depletion-and-replenishment cycles can occur before electromagnetic field energy has propagated beyond the footprint of a typical BGA package. The power management IC (PMIC) has no idea that the switching event has occurred. The switching device is initially supported by nearby energy storage structures rather than the distant power supply.

Figure 11. Eighth T1 cycle, and the PMIC still has no clue.

In a good design, there is enough energy available in the first two domains to supply the device with energy until the bucket brigade makes its way to the PMIC and back again. In a world where picoseconds matter, travel times in the nanosecond range and beyond make delivering energy from the power supply a critical consideration in PDN design, and the local storage capacity must be up to the challenge. This simple concept can spell the difference between success and failure.

As stated earlier, the PCB designer’s job is no longer simply routing traces. The challenge is managing and controlling electromagnetic energy movement through three-dimensional space. It’s “All About That Space.”

The New Reality

Modern PCB designers are not simply connecting circuits. We are designing three-dimensional structures that guide, contain, store and deliver electromagnetic field energy.

Or stated another way: “We are all just plumbers using very leaky water pipes.”

One final thought: if the Bucket Brigade did not exist, modern electronics simply would not work. This is a simplified perspective of the time domain, wave-based power supply behavior. There are, in fact, thousands of buckets traveling in search of energy. Because the distances depletion waves must travel across the die and package differ, each switching event creates a unique set of wave cycles. All of these requests for energy working in concert form the foundation of power supply behavior.

The conductors define the boundaries. The spaces determine the behavior. The length determines the time. This is a four-dimensional concept.

And ultimately, it’s “All About That Space.”
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Dan Beeker is the president and chief technology officer of System Solution Specialists (systemsolutionspecialists.com); danielbeeker@systemsolutionspecialists.com.