Power Distribution Architecture Options Give Me a Major Headache

Sometimes we can have too much of a good thing. That’s how I feel when it comes to the basic topic of how to structure the creation and distribution of the many DC rails needed by a typical system or even a high-function IC. Please note: this is not the same as the also bewildering subject of the many possible DC/DC regulator (converter) architectures, such as SEPIC, PWM, CC/CV, or others.

Typically, the rails may have different nominal values, tolerances, and current requirements. Determining the “best” way to provide them while meeting demands on overall efficiency, total footprint, flexibility in siting, and cost can be a problem. Even the simplest case reveals the dilemma: try to provide two DC rails, one at 1.2 volts and one at 3.3 volts, using a 24 volt DC source. You could use an intermediate bus converter (IBC) such as the TDK-Lambda IQL24021A120V-009-R to drop the 24 volts DC down to 12 volts DC, then have local 12 volt/3 volt and 12 volt/1.2 volt regulators (Figure 1). A good example is the Linear Technology/Analog Devices LTM8054IY. Or is it better to go directly from the 24 volt DC bus down to the end rail voltages (Figure 2)?

Figure 1: One obvious approach to producing the 1.2 and 3.3 volt rails from a 24 volt source is to use an intermediate bus converter, followed by individual local regulators. (Image source: Bill Schweber)

Figure 2: Another possible potion is to go directly from the 24 V source down to the final rail voltages, using suitable regulators. (Image source: Bill Schweber)

It gets more complicated: what if one of the rails needs to be distributed to multiple ICs located across a printed circuit board? Longer DC runs may be susceptible to poor regulation, oscillation, IR drop, and noise pickup. Maybe it would be better to go with several small, local regulators (Figure 3). What about considering using single output versus multiple output units? It all gets overwhelming pretty quickly!

Figure 3: If one of the final rails must support physically separated ICs, it may be better to take the 24 or 12 volt DC rail and then use many smaller regulators, in a highly distributed, localized approach. (Image source: Bill Schweber)

So, what do you do?

The complicated reality is that many factors affect defining the system level supply distribution map; some can be assessed by hard numbers while others are less conducive to hard analysis:

  • The source DC voltage, of course
  • The specific rails needed, with their voltages, currents, tolerances, transient performance, and other parameters
  • Pros and cons of a more centralized approach versus a highly distributed one with many smaller regulators
  • Aggregate footprint of all the regulators—and where they can be located.
  • Efficiency, both in the aggregate, as well as localized
  • System fault tolerance and performance if a single regulator fails

But there’s the deeper source of engineering frustration: EDA tools can help, but really can’t handle the multidimensional and somewhat fuzzy nature of the problem. As a result, many engineers revert to spreadsheets which attempt to organize the hard and soft answers for each possible option. Efficiency, for example, can be quantified, while noise performance and layout flexibility can only be marked with a “low/medium/high” tag.

Even when this spreadsheet is done, the decision on which architecture is “best” is difficult. It’s a dilemma we face in many decisions, where “best” can only be defined with respect to priorities. A low-end consumer product might put cost as #1 by a wide margin, while an aerospace design might put efficiency and weight as top concerns.

All these options mixed with multiple priorities gives the design team serious headaches. In the end, much of the decision comes down to engineering experience and soft analysis of the tradeoffs.

The challenge doesn’t stop there, and often continues to the design review. The team brings charts, analysis, and rationales to explain options considered, and finally calls out the one (or several) approaches they feel are most appropriate. Then, inevitably, one of the meeting attendees pipes up and chirps, “Umm, didn’t you consider this other possible approach such as XYZ?” Since there are a nearly infinite number of possibilities, of course there’s another one which was not considered but which can be asked about, thus making the questioner looked oh-so-thoughtful and insightful!

The entire issue of architecting, assessing, and recommending a “best” power distribution scheme can be so multidimensional and overwhelming that it can make the designer feel like this fellow (Figure 4).

Figure 4: The enormous number of permutations for a power distribution architecture along with the many conflicting priorities, can make a design engineer feel like this fellow depicted by Max Ernst in his work, “Young Man Puzzled by the Flight of a non-Euclidean Fly” (Image source: Pinterest)

Still, the most critical point to remember that there is no such thing as an “optimum” solution until you first answer two related questions: “Optimized with respect to which factor(s)?”, and “How much is that “optimum” worth to you, anyway?”

작성자 정보

Image of Bill Schweber

Bill Schweber는 전자 엔지니어로서 전자 통신 시스템에 관한 세 권의 교과서를 집필하고 수백 건의 기술 자료, 의견 칼럼 및 제품 특집 기사를 기고해 왔습니다. 이전에는 EE Times의 다양한 주제별 사이트 관련 기술 웹 사이트 관리자와 EDN의 편집장 및 아날로그 편집자를 역임한 바 있습니다.

Analog Devices, Inc.(아날로그 및 혼합 신호 IC 업계를 선도하는 판매업체)에서는 마케팅 통신(홍보 관련)을 담당했습니다. 결과적으로 Bill은 미디어에 회사 제품, 사례, 메시지를 제공하는 기술적 PR 역할과 이러한 내용을 받는 미디어 역할 모두를 경험했습니다.

Analog의 마케팅 통신을 담당하기 전에는 평판 있는 기술 저널에서 편집장을 역임했으며 제품 마케팅 및 응용 엔지니어링 그룹에서도 근무했습니다. 그 이전에는 Instron Corp.에서 아날로그 및 전력 회로 설계와 재료 시험 기계 제어를 위한 시스템 통합 실무를 담당했습니다.

Bill은 MSEE(메사추세츠 주립대학교) 및 BSEE(컬럼비아 대학교) 학위를 취득한 공인 전문 엔지니어이자 어드밴스드 클래스 아마추어 무선 통신 면허를 보유하고 있습니다. 또한 MOSFET 기본 사항, ADC 선택, LED 구동을 비롯한 다양한 엔지니어링 주제에 관한 온라인 과정을 계획 및 작성하여 제공하고 있습니다.

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