Feature: Automotive
Monolithic EMI filters from Johanson Dielectrics
Johanson Dielectrics use two separate electrical paths within a single device, attached to four external connections. It should be noted, however, that a
monolithic EMI filter is not a traditional feed-through capacitor. Although they look identical (same package and external look), their design is very different, and they are not connected in the same way. Like other EMI filters, monolithic
EMI filters attenuate all energy above a specified cutoff frequency, and only let through required signals whilst diverting others – including noise – to ground. The key difference, however, is the
very low inductance and matched impedance. With monolithic EMI filters, the terminations connect internally to a common reference (shield) electrode within the device, and the plates are separated by a reference electrode. Electrostatically, the three electrical
There are alternatives to common-mode chokes in the form of monolithic EMI filters, which provide superior rejection of common-mode noise
winding. Unfortunately, common-mode chokes are also large, heavy, expensive and subject to vibration-induced failure. Still, an ideal common-mode choke with perfect matching and coupling between the windings is completely transparent to differential signals, and offers very high impedance to common-mode noise. Another disadvantage of common-
mode chokes is their limited frequency range due to parasitic capacitance. For a given core material, the higher the inductance used to obtain lower- frequency filtering, the greater the number of turns required and consequent parasitic capacitance that defeats high-frequency filtering. Mismatch between windings from
mechanical manufacturing tolerances can also cause mode conversion, where a percentage of the signal energy converts to common-mode noise and vice versa, giving rise to electromagnetic
compatibility and immunity issues. Mismatches also reduce the effective inductance in each leg. Common-mode chokes do have a
major advantage over other options when differential signals (to let through) operate in the same frequency range as the common-mode noise that must be suppressed. With a common-mode choke, the signal passband can extend into the common-mode reject band.
Monolithic EMI filters There are alternatives to common-mode chokes in the form of monolithic EMI filters. When properly laid out, those multilayer ceramic components provide superior rejection of common-mode noise. They combine two balanced shunt capacitors in a single package, with mutual inductance cancellation and shielding. These filters from
28 September/October 2020
www.electronicsworld.co.uk
nodes are formed by two capacitive halves that share common reference electrodes, all contained in a single ceramic body. “Being very well balanced, a
monolithic EMI filter introduces almost no conversion of common- mode noise to differential signals, or vice versa. Furthermore, having a very low inductance makes it particularly effective at high frequencies,” said Cambrelin. Te balance between capacitor halves
also means piezo-electric effects are equal and opposite, canceling out. “Compared to the common mode choke
solution, this device provides significantly more RFI suppression in a substantially smaller package. It also rejects a much wider frequency band,” said Cambrelin. Te downside to monolithic EMI
filters is that they can’t be used where the common-mode noise is at the same frequency as the differential signal. In those cases, the common-mode choke is a better solution. And finally, although monolithic EMI filters initially cost more than equivalent ordinary capacitors, their cost is a fraction of that of a common- mode choke.
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
