Introduction
We’ve covered why you should always source the latest and greatest parts for your design, but when it comes to certain ubiquitous components, there’s more to consider if you want to keep your project on target and meet EMC requirements.
My next few posts will explore the impact of four common circuit components on EMC performance:
- Inductors
- Capacitors
- Ferrites
- Resistors
First up, let’s take a look at inductors.
What does an Inductor do?
An Inductor is a common component that’s often simply just a coil of wire. Inductors temporarily store energy in a magnetic field, creating a potential (also called an electro-motive force) proportional to the rate of change of current. They can be used for power supply and other functional circuit needs. They’re also efficient at coupling in magnetic fields.
Inductors can be your friend. For example, they act as low pass filters when used in a series circuit by offering increased impedance (“reactance”) to higher frequency signals, protecting the rest of the circuit from damage by blocking current flow of higher frequencies.
However, inductors can also be your foe. If the current flowing through the inductor is interrupted for any reason, it can cause an undesirable voltage spike, significant enough to damage sensitive circuitry.
Inductors as an EMC Mitigation Technique
Inductors can be added to a circuit as an EMC filtering component later in the design cycle. This is one technique to help mitigate EMC issues that have arisen along the way, but it’s not a good one! The better bet is to forestall those issues by planning ahead for them at the outset. If your design does not require inductors for functional reasons, it’s best to avoid them altogether.
Designing with Inductors
If your design does require inductors, justify them and be very aware of other sources of inductance in your circuit. The inductance associated with even long traces can tip you over the line, from an EMC perspective. Inductor use and placement can improve your EMC performance or make it worse.
The secret to designing with inductors lies in the intimate knowledge and careful management of inductance within your product. Knowing your product’s protection and exposure requirements allows you to tailor the inductance throughout your system while keeping it as low as possible.
This necessary balance has driven the market for inductors to cover an immense array of sizes, values, current tolerances, insulation properties, temperature ranges, quality factors, etc. As you explore the options available, here are three guiding principles to keep in mind.
Understand your Needs
Allow me to reiterate: If you can avoid using an inductor, do. They’re an added cost and can be bulky or difficult to place. Skipping the inductor will make your life easier, and your customer will appreciate it too. Justify every single nano-Henry of inductance in your design.
However, if your design must include an inductor for functional reasons, try to factor in trace/device inductance wherever possible. This reduces the likelihood of picking up stray magnetic fields, this is what we mean by “immunity.”
Help your Vendors Help You
When shopping with different vendors, share your pain: Offer as many details as possible about the needs you defined in step 1, and do so with multiple vendors. This allows vendors to come back to you with tailored recommendations.
Why is this valuable? Because vendors can assess your design objectively and identify questions you should be considering. They might even offer alternative module suggestions/approaches that can help optimize both your design and your process.
Vendors offer component selection tools, design and simulation tools, and experts to chat with. Leverage them early to give them the best opportunity to help.
Stay Flexible
Keeping in mind your target footprint, layout components using standard sizes so you have a wide range of values to choose from. This way, should you find yourself with a soldering iron in some EMC lab, “almost passing,” your flexible footprint may enable you to adjust the design on the fly and walk out of that lab with the certification you need.
Conclusion
I’ve said it before, and I’ll say it again: It is always smartest to design for EMC upfront rather than compensate later in the design cycle. Anticipating potential EMC issues at the outset and implementing EMC best practices along the way keeps your project on target with respect to cost, complexity, and time to market.
The best EMC practice is to avoid inductors whenever you can. When that’s not possible, I hope that this post can shed some light on considerations to make while designing around inductors so you can keep your inductance as low as possible and satisfy EMC regulations.
Come back next month to learn about capacitors, a part found in every circuit and the #1 component affecting EMC performance.
