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I am designing a two layer PCB. Two part schematic is shown in attached figures.

In the circuit, I am getting four gate signals from FPGA of 3.3V and and error signal, also of 3.3V as inputs and I need to step up the gate signals to 5V while ensuring that signals are turned off in case if error persist (ERR is active low). So I use a digital isolator/level shifter to step the input signals upto 5V and an AND gate to ensure signal turn off in case of an error.

The next part is to generate the voltage levels. I am getting 3.3V as an input from other PCBs and I need to generate 5V for the level shifter and my AND gate. To do that, I use two Traco dc-dc converter to first step down voltage from 24V( from floating lab power supply) to 15 V and another TRACO to get 5V from 15V.

Now as can be seen in schematic, I have four isolated grounds on my PCB : For 24V, 15V , 5V and two signals of 3.3V. I would want them to remain isolated. I have following queries:

  1. Do I really need to isolate grounds of 15V and 5V( although TRACO ensures it)
  2. Which ground level should I choose for ground plane
  3. Should I use different ground planes for my digital signals (eg5V)and my same voltage level power ground and connect them at one point or should I rather use one ground plane for digital and analog signal of a particular voltage level (eg : 5V power supply ground and 5V digital IC ground) PS: My gate signals are 40kHz and TRACo dc-dc converter switches at 400kHz

Thanks

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Figure 2

PCB layout with different planes for each ground

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  • \$\begingroup\$ Ground planes are necessary for supporting the shortest possible return paths. therefore according to your output you ought to have as big as possible plane to reduce the return paths. \$\endgroup\$ – BD_CE Oct 6 '18 at 19:26
  • \$\begingroup\$ So you mean, among the options of different grounds I have to form a plane from, I should go with the one which is related to my final output gate signals? \$\endgroup\$ – Autobot Oct 6 '18 at 19:40
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    \$\begingroup\$ by the way, your computer most definitely has a screenshot functionality. No need to take pictures with your camera! \$\endgroup\$ – Marcus Müller Oct 6 '18 at 22:54
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As far as I understand from your schematic and PCB, you are generating two 3.3V DC and one 5V DC as the output of your circuit and you need isolation from 15V DC and the input that you have used for generating the 15V DC.

IF all of these sources are going to feed a card or a complex of connected cards, you ought to have a single ground plane for all the sources and making separation between grounds may lead to hazardous potential difference between the separated planes that is not good. Separated grounds are OK just if the Power supply card is feeding completely separated cards and none of the cards are using more than one output of the Power supply card.

Based of this, my recommendation is 0. use as unique as possible ground plane (not separated) at the output side of the Power supply card. 1. ensure that the 4 millimeter separation on the PCB under the TRACO is supported. 4 millimeter will give you 4kV isolation. 2. It is a very good practice that near the FPGA connector, connect the screw hole directly to ground and connect the other screw hole to ground plane via low ESR capacitors. Obviously the screw hole is connected to Earth or metallic case.

  1. It seems that copper density at the top layer is high therefore during automatic assembly process it could make bad tensions on the assembled parts. normally you ought to have near the same copper density at the bottom of your PCB too to balance the tension or if the PCB is a single layer one, It is recommended to not using solid copper plane and instead of that, use the hashed copper plane.
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    \$\begingroup\$ Connecting screw holes to ground and then earth is good for reducing unwanted waves on the ground and also reducing the heat. normally, we connect one of the screw holes that is probably positioned near to main connector (high frequency, high power, ...) directly to ground and the others via capacitors for preventing from unwanted ground loops those may be generated due unwanted potential differences on the case and ground plane. the medium frequency waves will be shorted to the earth due to the capacitors but low frequency ones could not pass through caps. \$\endgroup\$ – BD_CE Oct 7 '18 at 10:24
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    \$\begingroup\$ Some manufacturers using combination of thin conductive and insulator washers and a metallic screw to make an intrinsically capacitor that connect the earth and ground. In answer to your question "what is better metal or plastic" I have to say: it depends on your application requirements. For majority of cases the best practice is using metallic screw but such firm connection will transfer all the tensions on the case to the PCB. in some applications we need it (AirbagControlUnit). no need to say that placing sensitive components near screw hole could reduce the reliability of pin connections \$\endgroup\$ – BD_CE Oct 7 '18 at 10:29
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    \$\begingroup\$ A2. Ground plane is similar to sea. high current trace and pins will make unwanted potential differences on the return paths. you need some thing to reduce these waves that a screw hole connected to chassis and then earth is one of them. in case that you have no earth or metallic case for preventing the problem, you could use the combination of hashed ground plane (has intrinsically RLC circuits) and bypass capacitors for mitigate the phenomena. You also could increase the thickness of the PCB's ground plane. \$\endgroup\$ – BD_CE Oct 7 '18 at 15:33
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    \$\begingroup\$ A3. For managing the heat, you could use more area for ground in the bottom layer and connect it to top layer ground by stitching Vias. heat pipes and heat sinks are second options and depends on your requirements. \$\endgroup\$ – BD_CE Oct 7 '18 at 15:37
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    \$\begingroup\$ A1. The output of TRACO modules are related to their own output GND pin and If you do not connect these pins together, any connection between them on the supplied circuit could lead to electrical over stress unless existing a unique reference point. when you connect two system those are ON before connecting their ground first, you will have EOS and probably damage. \$\endgroup\$ – BD_CE Oct 7 '18 at 15:42

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