Ferrite bead vs common mode choke

Question

^{simulate this circuit – Schematic created using CircuitLab}

I inherited the top circuit from a previous designer on my robotics team. The circuit uses two ferrite beads, a zener, a TVS and a capacitor to filter incoming power. The incoming power comes from batteries. Along with the digital circuitry the batteries have large motors connected to them making for a very noisy environment. My understanding is that with the help of the ferrite beads the zener and TVS suppress any spikes. Then the large capacitor holds up any droops. This circuit has worked well so far.

My question is whether replacing the ferrite beads with a common mode choke improve the filtering or if it's not broken don't fix it?

(I just used generic components to give the general circuit layout, the top is the current circuit and the bottom is my proposed change)

Additional info The circuitry is going into a robot. The robot is made out of extruded aluminum (not grounded) and the whole thing is clad in clear acrylic. The whole thing is powered by a 24V 8 cell lithium iron phosphate 20Ah 10C battery. The digital circuitry draws about 1A. The motors are two wheel chair motors. The motors are rated at 60A max but they are never driven that hard, usually around 50% or less. The motors are driven by Vex Victor H bridge motor controllers.

Answer 1

Even though this questions looks like very specific, it can be treated indeed as a much more general case filtering question: "How can one filter out electrical noise coming from power electric motors?".

The first information data we need to gather in advance is the type of noise our circuit is exposed to. Sometimes it is really difficult to get this data in advance, sometimes it is even harder to measure the noise without prior experience and high-end laboratory equipment.

In general, we can assess our noise sources in terms of:

• Intrinsic or extrinsic. I.e.: does the noise comes/is generated inside our own system? Or does it comes outside of our system?
• Coupling mechanism: capacitive coupling, inductive coupling, ground loops, EM radiation...
• Characteristics of the noise: switched, thermal (gaussian), shot, flicker...
• Frequency band and Q. How narrow or wide band is our noise? Does it fall/dissapears abruptly outside that band (quality factor)?

The above is a partial list, incomplete, which may serve as a starting point only.

Then, there are a lot of techniques, I mean literally hundreds of tricks and broader approaches depending on the case.

Delving into the specifics of the original question, this is my best guess on the sort of noise that may be originated by the system,

1. The noise is coming mainly from system itself, power motors and driver circuits. 30A of peak switching current is high enouch to generate pulses which can easily couple to the rest of the circuit.
2. Capacitive coupling, inductive coupling and ground loops can be all source of trouble here, due to the high current pulses of the drivers.
3. The noise is switched, I guess in the sub 1MHz region, however, armonics in the 1-10MHz range could be easily generated/radiated.

Some practical hints and techniques for dealing with the noise in the system above:

• If possible, separate physically the motors and drivers from the rest of the circuits. This is obviously not possible in all cases, for instance, if you have a single board for all the electronics. However, if you can afford having two separate boards, one for driving the motors, another one for the rest of the system, it is helpful doing so.
• Avoid ground problems and loop coupling of the noise by using a carefully thought star ground connection for all your circuits, including power drivers, batteries and chassis.
• Do not let any chassis or big metallic part floating, as this will interact with the EM fields generated by the motors and power drivers, reflecting, propagating and/or re-emitting the EM fields as additional noise.
• Regarding the motors themselves, and depending on the type of motor, you can certainly apply noise filters near/attached to your motors. For DC motors, which may not be your case, it is wise to solder small ceramic capacitors across each phase, as near as possible to the motor. Rugged (high voltage) 0.1uF capacitors are a good rule of thumb to start with. Depending on the application, you could also add another pair of ceramic capacitors from each of the phase leads to the chassis. Beware of checking the exact motor type and driver before going this route.
• The cabling connecting the drivers and motors should be as close as possible and be twisted.
• Decoupling/bypass capacitors should be generously added to your driver power lines, in two flavours: bulk capacitors (maybe in the hundreds of uF, for low frequency filtering) and high frequency capacitors (typically 0.1uF).

Returning to the circuit you posted, my initial approach would be:

• Not using a common-mode choke, as it is more indicated for capacitive coupling noises generated from outside of your system.
• Applying dual LC filtering for both lines (power and GND return) or even better, a dual L pi filter. This is the most effective filter for KHz to low MHz noise. A big inductor (in the mH range) in series with each of the battery terminals will improve dramatically the noise entering the digital part of your circuit. Ferrite beads, on the contrary, are dissipative by its own nature and best suited for higher (dozens of MHz frequencies).
• Substituting the standard zener & unidirectional TVS for a bidirectional rugged (high energy) TVS. The zener in your circuit can be kept, however, if your input regulator cannot withstand small peaks of overvoltage.
• Adding a pair of small ceramic capacitors in parallel with the bulk capacitor: for instance 1uF and 0.1uF MLCCs, rated conservatively (>100V). This will increase your filter effectiveness for higher frequencies (>1MHz).

Last but not least, devise a simple way to measure your circuit at critical points, in order to verify the effectiveness of the different approaches. Do, please, try to test under similar circumstances as the real device will operate under.

If neeeded, I can provide more references (books, articles) to the approaches aboves. If you can specify in greater detail some parts of your system, additional filtering techniques will surely apply.

Answer 2

It depends on the environment of your board. Let's call the negative pole of your supply voltage GND. For example in a car, the whole chassis is GND, but you are connected only at your supply pins, not directly on the chassis. Your board has an parasitic capacitance against chassis, so noisy HF current will flow there. If you have a case like this, the common mode choke will help, because the HF current will need through your VCC and your GND supply line.

If your board creates some kind of other HF-Noise internal, a switching regulator, or some kind of CPU or memory interface, most of the current flows from the high-speed signal to your internal GND (High speed switching). The commom mode choke won't prevent the noise from getting out of your design, because there is a current flowing in and a current flowing out the same time. In this case a Ferrite Bead would be an better choice.

I suggest you to keep the ferrites for some reasons. Common Mode problems can be eliminated, if your signals on the board have an greater capacitance to your internal GND as against the chassis or some other external devices. In addition to that, ferrites are cheaper most time. I don't know your specification, however, i work in automotive industry, i would take the ferrites.

Answer 3

The common mode choke and frrite don't necessarily contradict. Also therr are many different common mode chokes, for various currents and frequency ranges. In general, you must understand, what are you protecting from what. If you are reducing fonductive emissions caused by on board dc/dc, pick two chokes to cover the range between 0.5MHz to 50MHz and from 500MHz to 5GHz. Latter may very well appear a common mode ferrite. By the way, you may need capacitors to create effective filter around the chokes. And of course pay attention to the ground policy of your system.

Answer 4

A common mode choke is useful for reducing noise that is "common mode" - obviously, in other words - similar noise present on both lines. This may be usefull for filtering out high frequency noise like an RF signal coming from a near by radio transmitter. Systems with an ungrounded metal housing might benifit if there is suspected HF noise being induced (inductively or capacitively) to both isolated power lines, (for example if the housing had other noisey electrical systems connected to it.)

Single ferrite beads (as shown) can reduce sharp current spikes if they are sized correctly. Generally the smaller beads filter higher frequencies (though the ferrite material matters too). To filter lower freqency spikes you generally need larger (thicker beads). If the beads being used do not seem adaquate, change to a larger size, or you might use large value inductors instead, (similar large inductors are often used in power lines going to hifi audio equipment - you would also need to verify the current handling capabilities of the inductors if used).

Also, adding a small value ceramic capacitor in parellel with the large value capacitor may help filter some additional high frequency noise. Large electrolytic capacitors may not filter high frequency noise so well.

Lastly, ferrites work best when there is some relative noise current flowing. The noise currents induce magnetic fields that the ferrite material dissapates as heat.

So assuming your noise is not common mode the use of the two beads (or inductors) seems the better choice.

Answer 5

The TVS devices takes some time to turn on during which the input voltage spikes might reach the micro end. Ferrite beads can help in protecting the device in this regard while common mode choke offers only minimal impedence (leakage inductance) for the differential surge event. If your require common mode attenuation, i would suggest using a hybrid common mode choke in this case.

Answer 6

Here's a different (practical) perspective. Incoming power ferrites help for two things: 1. they attenuate noise from the power supply getting into your system. 2. They attenuate noise in your system from getting out, usually for EMC improvement. Given your robot is battery powered, which is the electrically quietest power source known, you don't need to filter noise from it. Unless this robot is a product, you also don't need to worry about EMC, so you don't need them for that reason either. Batteries are also the best electrical nose filter known, and having the ferrites limit the filtering effect of the battery on your system. So in this specific case, the ferrites do you no good, and actually prevent the good the batteries can supply so best to remove them. Same applies to a CM choke. Practically speaking, I doubt removing them will make a significant impact, but if you wanted to change them, my suggestion is to remove them.