Feature: Automotive
The value of a multi-layer ceramic capacitor with X7R dielectric varies significantly with time, bias voltage and temperature
meets regulatory standards, which limit the amount of noise that can be emitted.
EMI filters EMI filters normally consist of passive components, such as capacitors and inductors. “The inductors allow DC or low-
frequency currents to pass through, whilst blocking harmful, unwanted high-frequency currents. The capacitors provide a low impedance path to divert high-frequency noise away from the filter’s input, either back into the power supply or to ground,” explains Christophe Cambrelin of Johanson Dielectrics, a company that makes a variety of multi-layer ceramic capacitors and EMI filters. Traditional common-mode filtering approaches include low-pass filters
comprising capacitors that pass signals with a frequency below a selected cutoff frequency, and attenuate signals with frequencies higher than the cutoff frequency. A common starting point is to apply
a pair of capacitors in a differential configuration, with one capacitor between each trace and ground of the differential input. The capacitive filter in each leg diverts EMI/RFI to ground above a specified cutoff frequency. Because this configuration involves sending an opposite-phase signal through two wires, the signal-to-noise ratio is improved with unwanted noise sent to ground. “Unfortunately, the capacitance value
of a multi-layer ceramic capacitor with X7R dielectric (typically used for
this function) varies significantly with time, bias voltage and temperature,” said Cambrelin. “So, even if the two capacitors are tightly matched at room temperature, with a low voltage, at a given time it’s very likely they end up with a very different value once time, voltage or temperature have changed. This mismatch between the two lines will cause the response near the filter cutoff to be unequal and therefore it converts common mode noise to differential noise.” Another solution is to bridge a
large value X capacitor across the two Y capacitors. The X capacitor shunt delivers the desired effect of common-mode balancing – but, with the undesired side effect of differential signal filtering.
Common-mode chokes Perhaps the most common solution and an alternative to low-pass filters is the common-mode choke. This is a 1:1 transformer where both windings act as both primary and secondary. In this approach, current through one winding induces an opposing current in the other
Modern vehicles are full of components that can affect each other’s signals
www.electronicsworld.co.uk September/October 2020 27
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68
