What’s the difference between power and energy density, and why is the former so vital to power-supply design?

Steve Taranovich

APR 09, 2021  Related To: Electronic Design

To design robust power supplies, designers need to understand some basics and pitfalls.

Is power density the same as energy density?

Power density and energy density are quite different.

Energy density (W/kg) can be described as the amount of energy stored per unit with mass (kg), area (cm²), or volume (L). Power density (W/kg) is the amount of energy flow per unit (mass, area, volume) per unit time (s).  A system is said to have a high energy density when it’s able to store a large quantity of energy in a small amount of mass.

Regarding batteries: Energy density relates to the amount of energy that can be stored per battery unit, whereas power density relates to the maximum amount of energy that can be discharged or delivered per battery unit.

By comparison, capacitors offer a very high power density but low energy density. Capacitors are good for short burst of very high current and batteries excel at providing lower currents for extended periods of time.

Why is power density important?

Power density is important in a power supply, especially in a space-restricted area. A good example here are data centers—designers need to efficiently fill the spaces with as much processing power as possible. Processing-power speed continues to accelerate by leaps and bounds and power-hungry processors require great amounts of power. This means higher-power-density supplies become ever-more critical.

One of the most important areas where power density is vital to humans on Planet Earth is the amount of energy we can extract from renewable sources per section of land – that is, power density in W/m².

What kind of research is being done to optimize power density?

Essentially, reducing the size of a power supply will lead to better power density (see figure).

The Power Supply Manufacturers Association (PSMA) is doing important work on power density via the development of 3D packaging at the printed-circuit-board (PCB) scale. This group also helps to develop designs to increase current density and power density in electric motors with improved thermal management.

The PSMA is involved in PCB embedding technologies in high-volume production. This, along with 3D packaging technology, will lead to a significant performance and size reduction in a power supply. In addition, high-power-density component embedding and 3D packaging of power semiconductors must overcome a “thermal barrier.” That’s because it’s more challenging to remove heat generated within the body of a 3D integrated system than from a planar surface.

How does GaN-on-SiC improve power density?

Gallium nitride on silicon carbide (GaN-on-SiC) has three times the thermal conductivity of GaN-on-Si, enabling devices to run at a much higher voltage and higher power density. Telecom and wireless industries need good thermal conductivity.

Silicon devices can’t even come close to this level of power density. Power-supply designers are now able to select a GaN transistor instead of silicon for its small form factor and high efficiency. GaN transistors also dissipate less power and offer higher thermal conductivity compared to silicon devices with higher thermal-management requirements.

How important is the physical size of a power supply to power density?

First of all, improving power-supply efficiency leads to the physical size reduction of the supply. So, power-density numbers are essentially based on the power-converter power rating and the “box volume” (length × width × height) of the power solution with all associated components included. Designers’ first efforts are usually to simply reduce the size of bulky, but important, passive energy-conversion components like capacitors, inductors, and transformers.

Modern power supplies can usually run at higher switching frequencies via the use of wide-bandgap (WBG) power-transistor components primarily from the GaN and SiC transistor variety. Drawbacks of WBG devices running at higher switching speeds are the power-transistor switching losses at high frequencies that lead to higher temperatures and higher switching losses. These problems can be addressed by innovative thermal designs to remove heat and creative gate-driver designs.

In addition, the PSMA has some very advanced techniques in integration that are leading to smaller “box volumes.” Enter 3D power packaging.

3D Power Packaging

PCB real estate is at a premium, so the PSMA has chosen to build vertically in the z-axis. Semiconductor ICs and passive components, including magnetics, are all stacked upwards.

Power devices are being integrated into a PCB inside a molded package; clever designs are being implemented with 3D stacking of die in packaging; and even substrate-embedded passives are being used to increase component density and greatly decrease interconnection lengths.

New design ideas have cleverly made use of many other technologies. They include techniques that move toward shrinking a conventional PCB to the point of ultimately eliminating it altogether while increasing power density (W/cm³).

What are some major areas requiring high power density?

Drones

Electric propulsion is one of the prime technologies for drones due to its many advantages like efficiency, reliability, reduced noise and thermal signatures, and precision control. The internal combustion engine (ICE) lacks these key features, preventing it from being used despite its very high power and energy density.

Fuel cell and battery: Drones, when powered by fuel cells as a unique power source, will have some limitations. The battery will supply the peak power needed in takeoff and climbing since it has higher power density and faster response.

A supercapacitor, when compared to a battery, has much higher specific power. Supercapacitor technology is characterized by a fast charging/discharging speed, overcharge tolerance, and the ability to extremely reduce the dc bus voltage fluctuations. Therefore, a supercapacitor will reinforce the hybrid power-supply system by allowing a higher power density and rapid power response.

Data Centers

Data centers have experienced a growing need for higher density and higher levels of redundancy.

Dramatic changes in how power is utilized in data centers have challenged a 40-year-old architecture, specifically driven by increasing power density. Looking at the power density of IT racks, the per-rack power density has increased exponentially. A modern metric in data centers is the higher a particular facility’s power density, the better it could support the needs of clients as well as end users.

Specifying data centers at traditional densities of 40 to 80 W/ft.² (430 to 861 W/m²) will result in the inability to reliably deploy the latest generation of IT equipment.

RF propagation

For you RF designers: The power of an RF transmitter, radiated from an isotropic antenna, will have a uniform power density (power/unit area) in all directions. The power density at any distance from an isotropic antenna is simply the transmitter power divided by the surface area of a sphere (4π R²) at that distance.

The surface area of the sphere increases by the square of the radius; therefore, the power density, P_D, (watts/square meter) decreases by the square of the radius.

Conclusion

Power density is a useful metric that power designers need to fully understand and utilize to achieve robust and efficient power-converter designs. Power-system designs will vary over temperature and environmental extremes, so designers must recognize the specific needs of the environment and the specific function the power supply will need to serve. Understanding power density and its importance in your design will improve the overall system efficiency and longevity for many years to come.