Some Techniques for Suppressing DC-DC Converter EMI in IoT Products
This Li ion battery is most likely connected to a switching regulator to provide stable power.
Suppressing EMI susceptibility in IoT devices from various sources is critical to ensuring your new product will work as designed. Similarly, your IoT product should limit spurious emissions if you want it to comply with EMC regulations. Among the various sources of radiated EMI from your next product, EMI within the device itself should also be controlled to prevent signal and power integrity problems.
The power source(s) in your IoT device can be a problematic source of radiated and conducted EMI, particularly switching DC-DC converters, which generally run at MHz switching frequencies. Most likely, you’ll be working with multiple DC-DC converters in your board. EMI from these converters can interfere with wireless receivers if some important steps for filtering noise and isolating receivers are not implemented. There are some basic design steps you can take during layout to reduce DC-DC converter EMI and shield other sensitive circuits from radiated and conducted EMI in your IoT PCB.
It Starts with Your Stackup
Like most signal integrity and power integrity problems, DC-DC converter EMI reduction starts with the right stackup design. Feature-packed boards for IoT devices will likely use boards with a minimum of 6 layers in order to provide sufficient space for routing, power and ground planes, and components on the board surface. The number of layers is less important than the arrangement of various layers. Newer mobile phones have gone all flex or rigid-flex as these provide extra room for a larger battery.
As your DC-DC converter circuitry will sit on the surface layer, you’ll need to include a ground plane directly below the surface layer and make it as large as possible. This will also provide a suitable reference plane with low loop inductance for other signals on the surface layer. Some datasheets for older DC-DC converters recommend cutting away a portion of the ground plane around the output trace before the output inductor. While this may be fine for older converters that use lower switching frequencies and run at higher signal levels, this is bad from an EMI perspective in newer IoT/mobile devices.
On the interior layers, place the power plane adjacent to a ground plane to provide sufficient interplane capacitance for decoupling. The arrangement, in addition to judiciously placed decoupling capacitors, will help reduce ringing on the power bus. This also allows stripline routing in the interior layers. In addition to exploiting shielding in your layer arrangement, your goal in stackup design should be to get your PDN impedance as low as possible to suppress EMI from ringing.
Isolation
Isolation comes in two forms: distance and shielding. Using a grounded shielding can to isolate a switching power supply with high current output is an obvious solution to prevent radiated EMI from inducing unintended switching in nearby digital circuits with large loop inductance. You may not need shielding cans in your IoT product if it is running on battery and uses power conservatively. Any conducted noise that is not too intense can be filtered (this is one use of the output capacitor).
Instead, you can separate important functional blocks in your board with grounded copper pour or via fences between different areas. Note that via fences are typically optimized for suppressing radiated EMI at a single wavelength (usually the frequency corresponding to the knee frequency in your switching regulator). Since your goal is to suppress radiated EMI from interfering with wireless receivers, you should place the receiver circuitry far from the converter. While the converter will produce some radiated emissions, the strength of these emissions will be lower at a receiver when it is located farther from your converter.
Shielding in a smartphone PCB
Choose the Right Components
The components in your DC-DC converter circuitry play an important role in providing EMI suppression. You should use capacitors with sufficiently high self-resonance frequency (above the knee frequency for the PWM signal in your regulator) to ensure they supply the desired capacitive impedance. Your inductors should also be of the shielded variety in order to better confine the magnetic field.
Major IC manufacturers have taken the initiative to design low EMI DC-DC converters with small form factor and reasonable cost. TI, Analog, and NXP have developed DC-DC converters that incorporate the output inductor directly into the package. You’ll also be able to place the required input and output capacitors directly next to the package, ensuring low inductance loops, or these components include these capacitors inside the IC package. You can easily include these components in your board when your design software allows you to search for manufacturer part numbers and import these components into your libraries.
You can implement all the design recommendations shown here to reduce DC-DC converter EMI when you use the powerful PCB design and analysis tools in Altium Designer®. This complete set of design tools is built on a unified rules-driven design engine, which ensures your layout will comply with basic and advanced design rules as you create your board. You’ll also have a complete set of tools for analyzing signal integrity and preparing deliverables for your manufacturer.
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