Designing in the Negative Space Applied: Digital Control Techniques for PC Board Power Design

Getting more processing onto networking and computing devices’ printed circuit boards (PCBs) is always a “processing capacity versus power” paradox. Newer integrated circuit (IC) designs pack many times the processing power in the same space over a previous generation of devices. This increased capability enables high performance products, such as a software-defined networking (SDN) switch with additional switching capacity or an automated test equipment (ATE) device with faster throughput. Yet, higher processing capability may also mean higher power requirements, with both IC and power modules using more PCB real estate.

How can designers recover board space lost to rising power requirements fueled by the need for increased processing capability? At GE we look at solutions to these challenges with a principle we call Designing in the Negative Space.

Designing in the Negative Space (DITNS) looks at a number of engineering approaches to reduce the power module “footprint” on a printed circuit board. For example, making power modules denser while reducing their height enables designers to place them in previously unusable spaces. We see this approach used with GE’s low-height SlimLynx™ family of point-of-load (POL), or DC/DC converter, power modules that fit under a mezzanine board, or that can be placed on the back of the board, freeing up top-of-board space for taller components. (See Figure 1)

Figure 1:  Placing GE’s SlimLynx™ under a printed circuit board.

Another DITNS approach is to pack more functionality in the same power package. For example, GE’s dual-output power module DualDLynx™ (multi-output MicroDLynx™ devices) provides 2 x 6 amp or 2 x 12 amp outputs in a single package, while maintaining the individual control features for two separate outputs, all while fitting into a 25 percent smaller space than two separate modules. (See Figure 2)

Figure 2: Reclaiming “unusable” space between power modules.

A constant challenge for board designers is the increasingly demanding dynamic load requirements, or transient performance, of many modern ICs as powering voltages get lower and IC currents get higher to support increased processing capacity. GE’s patented Tunable Loop™ technology enables meeting tight target transient specifications with the least amount of output capacitance (Figure 3), again saving valuable board space.

Figure 3: Output voltage deviation due to a transient load current of 10 A vs. external capacitance for the case of a 40 A module with and without the Tunable Loop™.

Another Designing in the Negative Space approach revolves around the increased use of digital power. Over the past decade the widespread adoption of a power management bus (PMBus), has enabled board designers to get “smart” about controlling and monitoring power on their boards. Since most boards already have some kind of host controller, adding power control capability via PMBus was a straightforward extension. GE’s digital DLynx DC/DC converter family was the first mainstream POL product that supports PMBus digital features. This allows designers to move control and interface functions from hardware into software, freeing up board space normally occupied by these discrete circuit features. The DLynx series also enables designers to gracefully transition to digital power by providing both analog and digital POL modules in the same footprint, while maintaining the same basic power capabilities.

The application of digital power also gives power designers the ability to better monitor and control circuit behavior. In particular, with high-performance IC loads, high-accuracy control of the output voltage, accurate digital telemetry of current, voltage and temperature, and the ability to adjust warning and shutdown limits helps designers optimize circuits for improved performance and reduces costs by avoiding over specifying components. It also facilitates the development process by providing easy-to-use extended monitoring and recording capabilities. The ability to better understand circuit behavior in normal and fault-recovery states allows rapid development between prototypes and from one product generation to the next.

Particularly in complex power designs with multiple voltage rails, digital power can dramatically reduce development time by providing flexible power management functions such as voltage setting, margining, fault limits, sequencing, etc. In many cases, without digital controls, a power engineer might have to make component changes or even build another prototype, adding considerable time and cost.

Finally, digital control allows for tighter control of output voltage by using high resolution digital voltage trim to, for example, eliminate variations resulting from normal component tolerances during manufacturing calibration. GE uses this approach for many of our power modules, and shares this digital trim capability with other board designers and manufacturers.

Facing the growing “capacity versus power” challenges in circuit board design, power engineers can now harness a range of Designing in the Negative Space approaches, coupled with a host of new digital power design tools and technologies.

For more information on GE’s Designing in the Negative Space, view this video series: Designing in the Negative Space Introduction, Mechanical Constraints, Feature Constraints, Thermal Constraints, and Power Distribution Constraints.