INTERNALLY GROUNDED EMI FILTER CAPACITOR FOR HUMAN IMPLANT APPLICATIONS

摘要

Novel feedthrough capacitors designed with an internally grounded electrode plate eliminate the necessity for an outside diameter (OD) or perimeter termination. The capacitor ground electrode plate is internally attached to one or more lead wires, which can pass all the way through the device or be a grounded stud. The RF attenuation characteristics of the internally grounded feedthrough capacitor are maintained by symmetrical electrode grounding and minimization of internal inductance. The internal ground eliminates the OD termination of the feedthrough capacitor and also eliminates the need for an electrical/mechanical connection between the shielded case or housing and the capacitor OD (or perimeter in the case of a rectangular feedthrough). In an implantable medical device such as a cardiac pacemaker or defibrillator, elimination of the capacitor OD connection greatly improves capacitor reliability. The reason for this is that the capacitor is no longer mechanically connected to a material such as titanium that has a different coefficient of thermal expansion. Such EMI filter capacitors are designed to be attached to a hermetic terminal and subsequently laser welded into the titanium housing of the implantable medical device. Elimination of the OD attachment to the capacitor also greatly reduces the amount of heat and thermal shock to which the relatively brittle monolithic ceramic capacitor is exposed. Internally grounded ceramic feedthrough filter capacitors can also greatly reduce the cost and improve the reliability of EMI filters for military, space and other applications. Such filters, which include those described in MIL-F-28861, typically are designed with a conventional ceramic feedthrough capacitor whose diameter is electrically and mechanically attached to a metal housing or tubular can. As for implantable medical hermetic terminals, mechanical forces are exerted against the ceramic capacitor in these designs, which can cause cracking and latent failures. By eliminating all attachments to the OD of the feedthrough capacitor, the overall reliability and the ability to withstand thermal and mechanical shocks, are greatly improved.