EMC Design tips for Battery-Powered Devices

Think about your favorite mobile device or battery-powered device. It needs to comply with EMI requirements just like any other electronic device, which means a requirement for low noise and low conducted emissions. It could also experience ESD at some point, and as much as we might try to implement ESD protection, there are instances where insufficient ESD suppression will lead to system failure. Another point to consider is where ESD goes once it arrives in the system. How can a battery-powered system manage these multiple aspects of EMC and safety, especially when there is no safety ground?

While there may not always be a chassis or safety ground in a battery-powered system, a simple grounding strategy and system layout can help ensure your battery-powered device passes EMC. Some changes in components and connections can help a device pass EMC testing without failure, as well as comply with EMC requirements on ESD.

EMC in Battery-Powered Systems Starts With Grounding

Systems that use a battery for power have at least one ground connection, and possibly two or more connections if there is an isolated power converter, analog interface, or chassis. Battery-powered systems are mobile and do not have an earth connection, and yet a chassis might be metalized to ensure the device is very rugged.

The ground connections in a battery-powered system could include:

• The negative battery terminal

• Metalized elements of the chassis

• Main system ground for components

• Isolated grounds where required

Two or more of these grounds could be connected together in various ways. For example, it is generally desirable to leave a chassis floating, so it would need to be connected back to one of the other grounds in the system. As another example, isolated power converter grounds and battery-powered sensors should at least be tied back to the main system ground using a capacitor in order to control radiated emissions. In some cases, connectors in mobile devices have shielding and that shielding needs to be connected somewhere in order to provide protection for ESD.

How you route these ground connections and use them to control currents can ensure successful EMC testing, let's look at the grounding strategy for these devices from the emissions and safety perspectives.

Grounding to Control Emissions

Battery-powered systems can emit electromagnetic radiation, just like any other electronic device. The cause is most commonly relating to one of the following:

• Lack of a return path for signals

• Large metal structures emitting radiation

• Signal reflection at fast rise time

• Common mode currents

These are among the most basic EMI problems that can arise in battery-powered systems. Because battery-powered systems are seen to be more basic than other devices, and they usually target a low price point, some designers may try to fit everything onto a two layer PCB. When this happens, it's easy to ignore the one guideline that is proven to reduce radiated emissions: use a ground plane. This might mean use of a four-layer PCB stack up, even for low priced products.

Using a ground plane for the system ground is great for preventing radiated noise from signals, but it does not address the issue of a floating enclosure or safety, which is important for protecting against ESD. The second of these two items relates to a human interaction with connectors, particularly shielded connectors that could receive an ESD voltage and current spike.

Large Metal Structures Acting Like Antennas

How do we deal with large structures acting like antennas in a battery-powered device? These designs require that the excess charge on metal structures, which can oscillate and then emit radiation, be eliminated or offset with an appropriate image charge. This problem can arise in designs with isolated regulators, isolated ground islands, large heat sinks, and large mechanical pieces made of metal.

This is why we need to ground all of the metal in an electronic assembly. By grounding all of the metal bits, they do not have a chance to accumulate oscillating charge and they won't radiate. You don't need to focus on grounding every tiny piece of metal, the large radiating elements like heat sinks and enclosures are the most important. For grounding, you have two options: the PCB system ground or directly to the battery negative terminal. Going directly to the negative battery terminal bypasses the PCB system ground, but they are set to the same potential reference zero volts. This is good enough to ground everything in the system.

ESD and Batteries

Battery-powered devices can receive damage from ESD just like any other electronic device. When an ESD pulse occurs near a battery-powered device, it needs to go somewhere and it will do so by passing through a conductor. Most commonly, ESD will appear near the exposed metal on the outside of the device. That means it tends to happen near switches and connectors with some exposed metal.

When ESD occurs on exposed metal, and that metal is connected to the system ground, that pulse will flow through the board and could damage the PCB. A better option is to connect the shielding on connectors and HMI elements back to the negative battery terminal. By using a large region of copper in the PCB that is separate from the system ground, it will avoid the part of the board with important signal routing. This is essentially the same as using a large guard ring around the edge of a PCB to divert ESD.

The guard ring shown here can be used in a battery-powered PCB and tied back to the negative battery terminal.

In summary, the negative battery terminal acts very similar to an earth connection and a chassis connection all at once. By providing a large metal region to sink current, it also can perform important safety functions that would normally require a large chassis or earth connection.

Any method you want to take to ensure EMC and safety in your design requires the best PCB design features in OrCAD from Cadence. If you’re ready to take even more control over net logic and board layout, you can graduate to Allegro PCB Designer for a more advanced toolset and additional simulation options for systems analysis. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity.

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