6
\$\begingroup\$

What are the design rules that should be applied when designing a PCB layout ? I am currently using Kicad, because it is free and it is simple to use. I made a layout for a L200 power supply. I wand to know what are the constraints for this type of PCB (power supply) and what rules I need to follow when (generally speaking) I make a layout ?

This is the layout that I made:

enter image description here

And this is the schematic:

enter image description here

\$\endgroup\$
6
  • 2
    \$\begingroup\$ The rules will come from the people who're going to make your board, they will tell you their capabilities. It's on you to make sure you use traces big enough for current carrying and to handle impedance matching and the like. \$\endgroup\$
    – Colin
    Commented Oct 16, 2018 at 15:47
  • 3
    \$\begingroup\$ Your board is missing mount holes. \$\endgroup\$
    – Janka
    Commented Oct 16, 2018 at 15:48
  • \$\begingroup\$ Are there any traces that needs to match a certain impedance ? @Colin \$\endgroup\$
    – beard999
    Commented Oct 16, 2018 at 15:54
  • \$\begingroup\$ I can add later the mounting holes. Probably the board will stay on the heatsink sustained by the transistor and by the regulator @Janka \$\endgroup\$
    – beard999
    Commented Oct 16, 2018 at 15:55
  • 4
    \$\begingroup\$ You have the potentiometer on the right. This creates a torque on the pins of your regulator and transistor. You need at least one mount hole on the other side of the pot. The usual way to design a board is starting with all the mechanical peculiarities, then do part placement and then work around them with the tracks. Because re-routing tracks is most simple. Think about it. \$\endgroup\$
    – Janka
    Commented Oct 16, 2018 at 15:58

1 Answer 1

Reset to default
3
\$\begingroup\$

The rules are this:

Realize that every trace you make adds parasitic resistance and inductance.

Resistance can be calculated the wider you make traces the lower resistance they will be. Resistance is usually in the 10mΩ range or less for most traces. If the resistance of the trace is too high for the current it will heat up. If it heats too much it will burn up. A PCB trace resistance calculator will usually help you determine the temperature of the trace with the max current you think will run through it.

Inductance is also related to trace size and is usually negligible if your circuit can be affected by 100nH or less of inductance, then look into how much inductance the trace has.

You can draw in the parasitic resistance and inductance from traces in on your schematic (usually I do this on paper) to help me visualize if it will be a problem.

Another thing to consider is placement of components, with a power supply I'd also consider ways that the components could short out or heat being dissipated in components.

\$\endgroup\$
7
  • \$\begingroup\$ Thanks for the reply. When you said "draw in the parasitic resistance and inductance" how can I make this ? How can I calculate if the inductance or resistance will affect my PCB ? @laptop2d \$\endgroup\$
    – beard999
    Commented Oct 16, 2018 at 16:31
  • \$\begingroup\$ Go through each trace between nodes and find out the resistance on the PCB trace calculator. For example, between the two caps it looks like there is a 30 mil trace that is 100mil long (you have the actual lengths), plug that in to the calculator (I get 0.00162Ω) That's probably not going to make a difference because there is little current between the caps, but I could draw it in. \$\endgroup\$
    – Voltage Spike ♦
    Commented Oct 16, 2018 at 16:36
  • \$\begingroup\$ meta.stackexchange.com/questions/126180/… \$\endgroup\$
    – Voltage Spike ♦
    Commented Oct 16, 2018 at 16:36
  • \$\begingroup\$ What is the meaning of "draw it in" ? @laptop2d \$\endgroup\$
    – beard999
    Commented Oct 16, 2018 at 16:46
  • 1
    \$\begingroup\$ Due to your basic and correct answer I decided not to delete this post, with the understanding that you could have wrote chapters on this subject alone +1. \$\endgroup\$
    – user105652
    Commented Oct 20, 2018 at 3:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.