Patent Application
20150357812
The present disclosure relates generally to electrical systems, and more particularly to electrical systems having galvanically isolated input-output circuitry.
Traditional locomotives are known to use several on-board electrical systems that have input-output (“i/o”) circuitry to input and output signals. It is well known to have galvanic (voltage) isolation for each data port to tolerate and separate different ground potentials between remotely mounted devices. The galvanic isolation greatly reduces or eliminates potential ground loops from interfering with communication signals between the remote devices. In addition to eliminating ground loops, galvanic isolation of a devices' i/o circuitry allows the device to withstand application of high potentials (HIPOT) between the i/o circuit and the grounded device chassis. HIPOT voltage ratings commonly range from 500V to 1500V, although certain devices may have a requirement to withstand even higher HIPOT voltages, for high intended voltage application in normal use.
Another desirable aspect of i/o circuitry includes electromagnetic shielding of the data communication conductors inside the device and of the conductors contained within the external data cables that carry communication signals between the remote devices. Such shielding, implemented wisely, increases the device's immunity to impinging radiated electromagnetic interference and greatly decreases a device's radiated emissions as well.
Another desirable aspect of electronic devices is resilience to electrostatic discharge (ESD) events. ESD events may utilize voltages up to 20 kV. ESD protection may be especially important for devices that normally require human intervention during wiring installation and servicing. Common methods of ESD protection include transient protection diodes, metal oxide varistors (MOVs) and components containing spark gaps.
However, galvanic isolation of i/o circuitry presents an apparent conflict with respect to ESD protection methods. For example, HIPOT testing of circuitry that includes the above-noted ESD protection components could permanently damage the ESD protection components. Furthermore, incorporation of the ESD protection components could also defeat the galvanic isolation measures.
U.S. Publication No. 2012/0258678 (“the '678 publication”) discloses a technique to reduce a common-mode interference signal between parallel coupled cables of a power adapter apparatus and a tuner apparatus in order to provide an enhanced cable that has both a reception antenna and a conductive path for electrical power. While the '678 publication may reduce common-mode interference signals between an antenna and a power cable, the '678 publication does not provide a solution such that the benefits of electromagnetic shielding, galvanic isolation, and ESD protection can be reaped for the same device.
The presently disclosed electrical system is directed to overcoming one or more of the problems set forth above and/or other problems in the art.
In accordance with one aspect, the present disclosure is directed to an electrical system. The electrical system may include a first device having a first chassis and a first port therein. The electrical system may further include a second device having a second chassis and a second port therein. The first device and the second device may be connected by a cable through the first port and the second port. The second chassis may be conductively connected to earth ground. The pin on the second port may be connected to isolated ground in the second device through an electrostatic discharge protection device, and the isolated ground may be connected to the second chassis through a set of capacitors.
According to another aspect, the present disclosure is directed to a device. The device may include a chassis having a port therein and a circuit board. The chassis may be conductively connected to earth ground. A pin on the port may be connected to isolated ground on the circuit board through an electrostatic discharge protection device, and the isolated ground may be connected to the chassis through a set of capacitors.
While not shown in
Cable 223 may carry data between devices 201 and 202, and may contain a conductive shield that connects to the chassis 211 or 212 through the connector shrouds and the cable connectors (not shown) that couple with connectors 222, 224. Once a 360° connection is formed between the cable connectors and connectors 222, 224, electrical system 200 can be said to have been covered by a single conductive, shielding surface. In other words, the entire system 200 can be said to have a single conductive skin, which provides electromagnetic shielding.
Inside each device 201 and 202, there may be additional circuitry that operates on the data communicated by 223. For example, in device 201, there is shown an isolation circuitry 221 that is coupled to connector 222 and communicates data received or transmitted by cable 223. As an example, line CANH from isolation circuitry 221 may be connected to pin P1 of connector 222. Similarly, line CANL from isolation circuitry 221 may be connected to pin P2 of connector 222.
Isolation circuitry 221 may isolate signals between different parts of device 201. For example, the signal communicated on the TX line causes data to undergo transmission, in differential signal form (CANH-CANL), from device 201 onto the data cable 223. Similar, the RX line responds to differential data (CANH-CANL) incoming from data cable 223 and through the isolation circuitry 221. Additionally, isolation circuitry may also isolate the power and ground planes between different parts of device 201. As shown in
In
An ESD protection device 231 may be connected to pins 7 and 2 to provide ESD protection. The ESD protection device 231 exemplarily includes zener diodes in this embodiment. While zener diodes have been used in this embodiment, it will be understood that other ESD protection devices such as metal oxide varistors or a device containing a spark gap may be used in place of the zener diodes. In
Assume that an ESD event occurs during installation or wiring of cable 223 with connector 222. Specifically, assume that an ESD event occurs with respect to pin 7. If a 10 kV voltage is applied to pin 7 during the ESD event, the total charge that is transferred may be calculated by assuming the capacitance of the human operator to be 150 pF. The zener diode connected to pin 7 will conduct that charge, which ultimately charges the capacitors 232 to a voltage below 1 kV because of the higher capacitance value (greater than 1 nF) of the capacitors 232. The 1 kV or less voltage value now results in a tolerable value for the isolated circuitry. Moreover, the multiple series resistors in resistive network 233 may withstand a higher voltage than a single resistor and provide a bleed discharge path for the ESD-interception capacitors 232. Such a bleed discharge path provides resiliency to repeated ESD events, by discharging the capacitors 232 within approximately 100 milliseconds.
The disclosed electrical system and circuitry may provide a solution such that the benefits of electromagnetic shielding, galvanic isolation, and ESD protection can be reaped for the same device. The ESD interception capacitors may, for example, load down 10 kV ESD events to a tolerable level of less than 1 kV. Accordingly, the i/o circuitry tolerates HIPOT testing and ESD events.
Further, the circuit board may provide a return path for common mode currents contained between the data signal conductors (e.g., CANH and CANL) and the chassis through the capacitors 232 to isolated ground. Moreover, the entire system can be enveloped in a conductive skin to provide RF immunity and reduction of RF emissions. The above advantages may not be provided by conventional techniques. Moreover, it will be understood that the above disclosed techniques are applicable to other interface types such as RS232, RS422, RS 485, and Ethernet.
It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed techniques. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed techniques. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents.
