Common Mode Choke – Derating vs Current Analysis

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.

For the circuit you have drawn, yes, the voltage spike has nothing to do with the value of the inductor. It is given entirely by the value of the current in the inductor, and the resistive load R2 across it.

What does that mean if R2 is a higher value, or even absent? In theory, with what you have drawn, if R2 was open circuit, then the voltage spike would be infinite. As you can guess, that doesn't happen in real life.

In practice, there are two things omitted from your drawing.

a) the stray capacitance across the inductor, and due to any wires from the inductor terminal to ground
b) any breakdown mechanism for your switch

capacitance

As the switch opens, the current will start to charge the stray capacitance, which will limit the rate of rise of the voltage. For large value inductors, with many turns in close proximity, this capacitance can be surprisingly large.

Sometimes an external capacitor is added to the inductor deliberately to reduce the rate of voltage rise.

No matter whether your switch is mechanical one with opening contacts, or a semi-conductor one like a MOSFET, it will not support an infinite voltage.

the switch

Mechanical switches are especially poor at breaking the current flow, as at the first break, the contact separation is very small, and an arc needs very little voltage to form. This arc will keep the current flowing, and damage the contacts. It is responsible for switch and relay failure, unless controlled.

In the old-style contact breaker car ignition system, the 'points' that connected and disconnected the coil to the battery could be subject to excess erosion from arcing. Often, the first sign that your 'condenser' (capacitor) had failed would be excessive wear at the points. The capacitor, fitted across the points, slows the rate of voltage rise so that the points are sufficiently far apart before the voltage gets high enough to create an arc.

The specifications for a MOSFET will typically give a breakdown voltage figure. Good ones will also give an energy they can withstand when the breakdown voltage is exceeded. As long as the stored energy in the inductor is less than that figure, a MOSFET can switch current off to a coil, limit the open circuit voltage to its breakdown voltage figure, and survive.