Chokes deployed in the automotive industry
A growing number of car manufacturers are deciding to utilise high-voltage equipment with a view to enhancing the comfort of driving or general vehicle users’ experience. However, this results in a higher demand for additional electric power supply.
This problem is not going to be solved by adding new energy sources — resulting in increasing vehicle weight and dimensions — but by a more effective use of existing solutions.
This is why EPC (electronic power control) systems used as performance adjusters have become more common. These systems are exposed to EMI regulations of OEM car manufacturers to ensure they do not interfere with other devices in the vehicle. Chokes are used to reduce this interference and attenuate the noise generated by the device. Material capabilities are key to complying with EMI regulations; given space and weight targets, research and development is underway to produce the core material for smaller and more efficient products.
The result of these efforts is the 7HT material used in KEMET’s SC series chokes and other customised products. Compared with the previous 5HT material, this is said to ensure 40% better noise attenuation with the same equipment parameters — in particular, its size. This level of performance is retained even at 150°C, which means the product can be installed close to vehicle engines and within high-power and drive train DC/DC converters.
Common and differential mode chokes
A choke comprises a ferromagnetic toroidal core around which a wire is wound. According to Faraday’s law of electromagnetic induction, current flow generates a magnetic field. Each variable magnetic field generates variable electromotive power in a circuit, ie, current flowing in the reverse direction (opposite to the primary one). Electromagnetic interference caused by switching semiconductors of higher frequency induces a magnetic field and flux into the material that converts to core losses and a heat rise of the magnetic material. EMI energy is converted into heat and is filtered from the power lines.
In differential chokes, a wire is wound only on one side, which means that noise is filtered by current converting to magnetic energy and finally heat. Although noise is absorbed in both the higher and lower frequency ranges, the supply of DC or AC voltage causes a continuous magnetic field and losses and requires materials with high saturation capabilities; usually iron power cores are used, as are other materials with high saturation.
In common mode chokes, two coils are wound instead of one, with opposite magnetic flux inducted that cancels itself out. This means that only an interference current and common mode current lead to magnetic flux and a heat rise in the material. Saturation capabilities only need to be as high as the interference current.
Depending on the interference behaviour, asymmetric interferences need differential mode chokes or Y-capacitors — and symmetric interferences need common mode chokes or X-capacitors — to attenuate and comply. The majority of KEMET chokes operate in the common mode, so they are suitable in applications where removal of noise and related electromagnetic interferences is necessary. Depending on the product, the differential mode (leakage) behaviour of a common mode choke can be used to design dual-mode chokes that combine both attenuation behaviours. Application of the 7HT material should enhance performance while keeping saturation behaviour stable and under control.
Ferrite material magnetic permeability
In order to ensure effective noise reduction, a material that is compatible with the target frequency band must be selected. Depending on its magnetic permeability, a given ferrite material will be effective within a given frequency band. Mn-Zn materials demonstrate higher magnetic permeability in lower frequency bands, whereas Ni-Zn materials with lower magnetic permeability attenuate better in higher frequency ranges.
Ambient conditions and choke operation
An advantage of 7HT common mode chokes is their ability to operate at large current loads (up to 25 A and more). Operating conditions for such chokes are defined by maximum ambient temperatures (in this case, 150°C); isolation requirements due to supply voltage and OEM requirements; and temperature rise resulting from choke power losses.
SC series models are available from KEMET as customised solutions, facilitating their broad-range application in strenuous conditions, eg, in engine systems. SC series chokes are also available in customised versions for automotive applications.
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