Integrated dv/dt Filters for Motor Drives

Some motor and drive systems require output filters to protect the motor from high dv/dt or for EMI compliance. However, these filters are often physically large, sometimes exceeding the size and weight of the drive itself. In order to make these filters smaller, lighter, and cheaper, integrated filters have been proposed [1].

These integrated filters combine the magnetic field of an inductor, the electric field of a capacitor, and damping into a single device. The integrated topology is also simple to manufacture using industry-standard techniques. These integrated filters can be used for output filtering of a motor and drive system.

Integrated Filter Concept

The integrated filter combines the fields of a capacitor and inductor into a single core volume. Figure 1 shows this integrated filter conceptually. Integration may decrease the magnetic field energy density or electric field energy density compared to discrete components. However, at the system level, the integrated filter may be smaller, lighter, and/or cheaper to manufacture.

Integrated Filter Schematic
Figure 1 – Schematic view of the electric field in a capacitor, a magnetic field in an inductor (top). Both fields are located in the same volume in the integrated device (bottom).
Integrated Output Filters for Motor Drives

This integrated filter concept can be combined into the output bus bars of a motor drive, in order to realize a simple, compact dv/dt filter [2] as shown in Figure 2. Alternating layers of dielectric film and magnetically permeable foil are wrapped around the bus bar. The film and foil layers contain both an electric field and a magnetic field. These fields are orthogonal to each other and nearly independent. In addition, the parasitic resistances in the circuit provide an appropriate amount of damping.

Integrated Filters In Bus Bars
Figure 2 – Cross-section view of an integrated filter incorporated into a bus bar showing electric and magnetic fields in the film and foil winding around the bus bar.

Figure 3 shows the integrated filter connected in a low pass configuration. The phase current passes through the bus bar from L+ to L-. The capacitor terminal C+ connects to the L- side of the bus bar. The capacitor terminal C- connects in a floating wye neutral configuration for 3-phase operation.

Integrated Filter Connections
Figure 3 – Integrated dv/dt filter incorporated into a motor drive bus bar showing the different terminals.
Integrated Filter Benefits for Motor Drives

Finally, this type of integrated dv/dt filter may pair well with inverters used as motor drives. There are several benefits to using these filters for motor protection:

  1. The filter inductance is a result of the current through the bus bar. Therefore, it is effectively a single-turn inductor configuration. This means there is no inductor winding turn-to-turn capacitance. The elimination of this turn-to-turn capacitance allows the filter to handle very high dv/dt pulses from SiC drives, for example.
  2. The filter allows for very low gate resistances, effectively moving switching losses out of the silicon, allowing for higher phase currents from the same module.
  3. The integrated device has a low series impedance due to its single-turn inductor topology.  Therefore, the filter scales very well as phase current increases for larger motor and drive systems.

Note: This post is a summary of the peer-reviewed work published in [1] and [2] (see below).

[1] Andy Schroedermeier and Daniel C. Ludois, “Integrated Inductor and Capacitor With Co-Located Electric and Magnetic Fields,” IEEE Transactions on Industry Applications, vol. 53, no. 1, pp. 380–390, Jan. 2017. (IEEE Xplore Link)

[2] Andy Schroedermeier and Daniel C. Ludois, “Integrated inductors, capacitors, and damping in bus bars for dv/dt filter applications,” in 2018 IEEE Applied Power Electronics Conference and Exposition (APEC), 2018, pp. 2650–2657. (IEEE Xplore Link)


P.S. Are you having dv/dt issues? The simulation tools on this website can help determine if filtering is needed on your motor and drive system to mitigate voltage ringing and high dv/dt issues.