Cooling Fan Electrical Noise Reduction in Your PCB

Who hasn’t opened up their PC or laptop and taken a long look at its fans and heatsinks? If you’re working with high speed components, high frequency components, or power components, then you’ll need to devise some sort of cooling strategy to remove heat from these components. Unless you want to use the nuclear option and install an evaporative cooling unit or build a water cooling system, you’ll get the best results with the smallest form factor when you use a cooling fan. It is a good idea to add the fan onto a heatsink to aid convective heat transfer.

Fan Electrical Noise and Radiated EMI

No matter which method you use for cooling your system, or if you are building a cooling system, there are some particular EMI/EMC points to consider, depending on the method used to drive your fan.

AC Driving

AC-driven fans are less-often used in compact systems as you have no speed control without frequency control, and these systems generally run at high AC voltage. Therefore, they are more likely found in industrial systems. These fans can produce significant conducted EMI (both common and differential) at the fundamental frequency and at higher order harmonics, which then propagates through the power/ground lines. This can normally be removed with common mode filtration (LC network), followed by differential filtration (another LC network), and an RC filter in series.

DC Driving

While DC fans might appear to be electrically noiseless, they do produce acoustic and electrical noise. The different types of fans will incur produce their own types of EMI, creating difficulty in passing EMC tests. Even a DC-driven motor will produce EMI thanks to the spinning magnet used to attract and repel the rotor, producing strong switching noise during commutation. EMI generated from DC fans is normally limited to conducted EMI in the fan power leads (for 2 wire DC fans). This fan electrical noise is normally injected into the common ground, where it reappears at the output of any amplifier that drives the fan.

Simple single-shaft DC cooling fan

This is not to say that a DC fan does not produce radiated EMI, but the radiated EMI will be at the same frequency as the rotation rate due to uncontained magnetic fields (UMF) from the permanent magnet and stator windings. UMF exists in virtually all fans to some degree, but the first step in dealing with UMF is the responsibility of the manufacturer. Some manufacturers will place a thin steel enclosure in their fans to suppress UMF in at least two mounting planes. This means that radiated EMI is strongly dependent on the orientation of the fan.

Radiated EMI from UMF can induce a low frequency ripple current in a nearby high inductance circuit. Larger fans generally require stronger magnetic field for driving, thus they will exhibit stronger EMI at a given rotation rate. However, even at thousands of RPM rotation rates, the frequency of this radiated EMI will only be in the range of hundreds of Hz.

PWM Driving

A PWM-driven fan provides speed control by varying the duty cycle and the PWM signal. With PWM driving, you are working with a switching MOSFET or other circuit with varying duty cycle. Note that the speed control is provided by setting the appropriate duty cycle and pulse frequency. This is actually rather important as, in extreme cases of very low pulse frequency, the fan can slow to a stop while the PWM signal is low. If the PWM signal is very fast (high frequency), you will hear some interesting noise due to aliasing effects as you try to drive the fan too fast.

In the case of PWM-driven fans, most PWM drivers produce common mode noise at high frequency reaching into the MHz range. Inductive motors driven with PWM can induce common-mode noise in nearby circuits through the power lines as conducted EMI, which may affect your EMC rating. This type of fan driving is more common in computers that require speed control. Note that this also requires the use of a temperature control and speed regulation circuit to ensure the fan maintains a steady speed, and so that the controller can increase/decrease the duty cycle as necessary.

Simple single-shaft DC cooling fan

Note that the PWM circuit itself will also produce conducted EMI due to overshoot/ringing. This should be smoothed or filtered, but you should check your fan manufacturer’s guidelines before you go adding a bypass capacitor or ferrite bead to the input of your fan. I’ve seen recommendations for addressing this problem include building an LC filter, to a bandstop filter to remove the ringing signal, to using an RC filter on the output. In any case, make sure your filtering strategy satisfies your manufacturer’s recommendations.

If the PWM signal has fast rise time, then you can have a similar problem as that seen in switched-mode power supplies, where the switching signal induces crosstalk in some nearby circuit. If you are using a high current PWM signal to drive a large fan, the switching action of the PWM signal can cause involuntary switching in nearby digital circuits. This occurs regardless of the PWM pulse train frequency or duty cycle. At this point, you should consider adding some shielding to the PWM circuit.

Choosing a Fan and Reducing Fan Electrical Noise

As conducted EMI is the primary factor to address in designing a system that uses a fan, you need to devise some way to address this noise. If you’re going to take a filtering strategy, then you should take some time to determine which frequencies you need to filter. Personally, I would take the time to order a few fans and test them out with an oscilloscope in a prototype or evaluation board for sensitive components. While you might not like spending $100 on some fans and waiting a few days for them to arrive in the mail, it’s better than overlooking a noise source and having to redesign a portion of your board.

When you need to devise a routing strategy to protect sensitive components from fan electrical noise, your design software should include a comprehensive set of routing tools, layer stack design tools, and an extensive component library. Altium Designer includes all this and much more, allowing you to implement the noise suppression mechanism that works best for your next device. These features integrate directly with your layout tools and run on top of a unified design engine, allowing you to create top quality boards for any application.

If you’re interested in learning more about Altium Designer, you can contact us or download a free trial and get access to the industry’s best layout, routing, and simulation tools. Talk to an Altium expert today to learn more.