Bourns has various TVS diodes to protect Ethernet ports. Most of them (for example CD143A-SR05) have GND, VCC and I/O pins. I/O pins are for Ethernet signal line. Why do these TVS diodes have VCC pin?
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\$\begingroup\$ Can you point to a diagram showing the connections you mention? VCC is typically used connected to the Ethernet transeiver isolation transformer itself, but is not typically brought out to the consumer connections. \$\endgroup\$– R DrastCommented May 20, 2015 at 10:41
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\$\begingroup\$ @R Drast, added a diagram from some Bourns document \$\endgroup\$– KonstantinCommented May 20, 2015 at 10:47
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2 Answers
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Using a VCC rail in conjunction with unidirectional TVS diodes allows the device to ensure the data lines are clamped between the supply voltage and ground. This way, they can adapt to different supply voltages without requiring a separate device for each, and start clamping as soon as the voltage is more than a diode drop outside the rails.
Another consequence of this is that if power is applied to the data line while VCC is unpowered, the TVS will conduct from the data line to VCC, just like an IC's protection diodes. You can reduce this by using series resistors to limit fault current.
answered May 20, 2015 at 10:42
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\$\begingroup\$ Nick, thank you for the reply! So if there is a voltage surge (lightning) when the device isn't powered, some spike would occur on Vcc and possibly burn down the equipment? If I have 3.3V Ethernet, wouldn't it be better to use a TVS array with no VCC pin? \$\endgroup\$ Commented May 20, 2015 at 10:52
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\$\begingroup\$ @Konstantin Note the TVS between VCC and GND in the device, too. If a large spike such as a static discharge occurs when the device is powered down, it will be conducted to ground via that TVS diode. A direct lightning strike is likely to burn out any protection equipment you can reasonably solder to a PCB regardless of what you pick! \$\endgroup\$ Commented May 20, 2015 at 10:55
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\$\begingroup\$ You mean static discharge on I/O pins or on Vcc? So it's more like a protection of Vcc of the board. I need 61000-compliant protection for induced lightning surges. I'm thinking about a GDT and TBU on the primary side and TVS array on the secondary side. \$\endgroup\$ Commented May 20, 2015 at 10:58
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\$\begingroup\$ @Konstantin Either one - a discharge on the I/O pin will conduct to VCC via the clamping diode, then to GND via the TVS. \$\endgroup\$ Commented May 20, 2015 at 11:01
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\$\begingroup\$ Nick, thank you for your help! Should I use TVS arrays without Vcc or with Vcc then for the maximum protection? Is it really that necessary to adjust clamping level for Ethernet signals? \$\endgroup\$ Commented May 20, 2015 at 11:04
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This component is used between the magnetics and the PHY, so it clamps the voltage to VCC (say, 3.3V) and GND. This way they can use shottcky diodes, which are faster then zeners, and have lower capacitance. In fact, there is just one TVS between VCC and GND, the others are shottcky.
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\$\begingroup\$ Gregory, thank you for the reply! Do you think that it's inferior protection in comparison to TVS diodes without VCC voltage? \$\endgroup\$ Commented May 20, 2015 at 10:50
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\$\begingroup\$ Personally i use SLVU2.8-4A1 between RJ45 and the transformer and TPD4E1U06DBVR between the transformer and the MCU. \$\endgroup\$– user76844Commented May 21, 2015 at 12:14
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\$\begingroup\$ By the way, i can see you ask Nick about lightnings. Those TVS protect against ESD- the charge on your fingers or static charge on equipment. Don't even try to protect the circuit from lightning, there are special people for that. Google what effects lightnings cause- it's much more than just high voltage. \$\endgroup\$– user76844Commented May 21, 2015 at 12:16