ENERGY STORAGE Four ways to improve the power density of power electronic devices

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Power density is a crucial factor in reducing the size and increasing the efficiency of power electronic systems. The reduced size and increased efficiency would result in saving the cost of power electronics projects. This article explains power density in brief and details four ways to increase it for power electronic devices.

Power and energy are two interrelated terms in physics. Power is the rate of energy transfer or conversion in an electronic/electrical system. Similarly, power density and energy density are two interrelated parameters for energy storage devices to understand their efficiency.
By definition, energy density is the ability of an energy storage device to “store the energy” and power density is the rate at which the energy storage device provides the energy. In simple words, energy density is the capacity to store energy, and power density is the measure of how fast an energy storage device supplies energy.

Energy density is related to energy storage devices such as batteries, cells, capacitors, supercapacitors, superconductor magnetic energy storage, flywheel energy storage, etc. But power density is crucial in power electronics, transmission lines, renewable energy, the automotive industry, communication systems, and many more.

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Power density for power electronic devices

The power density of a power electronics device is defined as the ability to generate power per unit mass or volume. In other words, power density is the capacity of a power electronics device to draw maximum current per unit mass or volume. Power density is categorized into two types- volumetric and gravimetric.

• The volumetric power density operates over volume and is expressed in Watts/Ltr. The power density over volume determines the ability of an electric or hybrid-electric vehicle to accelerate.

• Gravimetric power density, also known as specific power, is expressed in Watts/Kilogram. Specific power is important in power electronics to determine the size of an energy device (batteries, cells, etc) in an application.

The power density of a power electronic device should be as high as possible. A device with a high power density is capable of processing more power in a smaller space. Such a device reduces the size, weight, and cost of the power system. The power density value enables the selection of the right component for a power system application. However, at the same time, the power device should not trade off with low energy density. It is because most devices have either power or energy density high and the other one lower.

How to Improve the Power Density of Power Electronic Devices?

1. Performing high-frequency switching operation

The rate at which a power device is capable of performing an operation is known as switching frequency. High-frequency operation above kHz and in MHz tends to increase the power density of power devices. It is because the device would be able to generate more power in terms of size in a smaller time period.

Transistor technologies like SiC and GaN have higher switching frequencies compared to Si-based semiconductors. GaN has the highest switching frequency, reaching up to an order of gigahertz. Both these technologies have higher power densities for smaller sizes.

To achieve a higher operating frequency, embedding small inductors and capacitors in the power chip is the most basic step. Magnetic components like inductors and transformers contribute to losses in a power device. Choosing high-frequency magnetic materials, and choosing suitable winding configurations may result in enhanced power density. Other methods are optimization of gate circuitry, minimization of stray parameters, reduction of parasitic elements, etc.

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2. Effective thermal management

Proper thermal management of a power system is the key to obtaining efficiency and high power density. A power device’s goal is to effectively manage the heating effect of current and achieve high thermal performance. The package must dissipate more heat per unit volume for an increase in power density.

Practicing more and more miniaturization to incorporate several components on a single chip and reduce the cost of the system is a constant pressure in the power semiconductor industry. Reducing the size imposes the challenge of proper heat dissipation and the introduction of packaging techniques.

If the power device packaging fails to get the heat out, the rise of temperature supports power losses in converters and other such power electronic devices. Thermal vias, liquid cooling, direct bond copper, small outline transistors for surface mount packaging, and advanced thermal interface materials are some cutting-edge packaging technologies to manage heat.

3. Achieving more miniaturization

Embedding all the components of a power system on a single chip is an optimal solution for miniaturization. It can be done by using advanced integration techniques, reducing the size of chip components, and employing effective interconnectivity technologies. The components of a power system like switches, gate drivers, filters, current sensors, passive components, and heat sinks occupy most space.

One of the most basic ways to increase power density is to miniaturize passive components such as resistors, capacitors, inductors, etc. These components are responsible for storing and converting energy during power operations. Reducing the size of the passive components is achievable by increasing the switching frequency of the device. A high-switching frequency enables less usage of energy for switching cycles. Another method for miniaturization is using advanced modulation techniques like PWM and control algorithms that result in the enhancement of power density.

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4. Reducing the losses

Since achieving a high switching frequency is a basic solution to increase the power density, switching loss is a limiting factor. The higher on-state resistance of a semiconductor device is a reason for increased switching loss and parasitic capacitance.

Another type of loss that lowers power density is the reverse recovery loss of power MOSFETs. The internal body diode of the power MOSFET gets reverse biased and the device is “OFF” but a recovery current flows through the semiconductor. Such loss lowers the capacity of the power device to draw maximum current per unit volume.

The reverse recovery loss can be reduced by optimizing the power MOSFET design and lowering the delay between the turn-on time. Reducing most of the losses in a power device can be achieved by optimizing the PCB layout and employing an advanced circuit design. Hence, reduction in losses tend to increase power density of a power device.

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