Electronics > Beginners
Why am not able to get a grasp of PCB designing?
redgear:
@Rerouter thanks a lot for taking the time to design the layout.
I have few questions:
1) The design recommendation says the BAT+ and BAT- should be on the opposite ends of the PCB. You have them placed very close to each other.
Can I move the BAT- pad to the right side of the PCB and flip the Batter Protection circuit(they recommend the battery protection to be close to the BAT- to reduce copper impedance from BAT- to GND) to the bottom side? Can they be on different layers? Will punching 12 vias be enough? Do the traces connecting the R1 to BAT+ be fat?
2) Can I route all differential pairs(except j5), mosfet current sensing and low current traces to the back side? If it's ok to do that, I can place the mosfets near the usb ports.
3) What should be the width of traces that connect to VOUT of mosfets?
4) They recommend atleast 12 vias to be punched on VOUT, SW, B+, B-. Do vias on VOUT (A,B,C) connecting to mosfets count? Can I use free space on the board to punch vias for GND?
5) Do I need copper pour on top of the BAT+ and BAT- pads, shouldn't pads themselves be enough?
Thanks
EDIT: Added the layout by Rerouter
thinkfat:
--- Quote from: redgear on December 06, 2019, 09:36:18 am ---@Rerouter thanks a lot for taking the time to design the layout.
I have few questions:
1) The design recommendation says the BAT+ and BAT- should be on the opposite ends of the PCB. You have them placed very close to each other.
Can I move the BAT- pad to the right side of the PCB and flip the Batter Protection circuit(they recommend the battery protection to be close to the BAT- to reduce copper impedance from BAT- to GND) to the bottom side? Can they be on different layers? Will punching 12 vias be enough? Do the traces connecting the R1 to BAT+ be fat?
--- End quote ---
That is a safety measure if you plan to use bare Li-Ion or Li-Po battery cells without built-in protection circuit. Imagine the spark if the wires touch each other while you solder them to the board. Imagine you short the battery terminals with a solder blob.
What batteries were you planning to use, 18650 cells? If you use cells with built-in protection, you can put the BAT+/- where you want and you can also drop the protection circuit completely. If you plan to use bare cells, follow their advice for your own safety and of those who will use the device.
--- Quote from: redgear on December 06, 2019, 09:36:18 am ---2) Can I route all differential pairs(except j5), mosfet current sensing and low current traces to the back side? If it's ok to do that, I can place the mosfets near the usb ports.
--- End quote ---
I'd keep the differential pairs where they are and only drop the mosfet current sensing and gate traces to the back side.
--- Quote from: redgear on December 06, 2019, 09:36:18 am ---3) What should be the width of traces that connect to VOUT of mosfets?
--- End quote ---
That depends on what you want each individual port to source. The main VOUT trace should be at least 2mm (80mil) for your 4A maximum current. The traces from there to each mosfet could be weaker, depends on what the maximum current there will be. BTW, KiCAD has a built-in PCB calculator that can do trace width calculations.
--- Quote from: redgear on December 06, 2019, 09:36:18 am ---4) They recommend atleast 12 vias to be punched on VOUT, SW, B+, B-. Do vias on VOUT (A,B,C) connecting to mosfets count? Can I use free space on the board to punch vias for GND?
--- End quote ---
That depends on how much current will flow through these vias. My own rule-of-thumb says 500mA is the maximum allowance for a single via. If you want to push 1A, put two.
thinkfat:
BTW, how do you intend to charge the battery?
redgear:
--- Quote from: thinkfat on December 06, 2019, 10:31:05 am ---What batteries were you planning to use, 18650 cells? If you use cells with built-in protection, you can put the BAT+/- where you want and you can also drop the protection circuit completely. If you plan to use bare cells, follow their advice for your own safety and of those who will use the device.
--- End quote ---
Yes, I am planning to use 18650s. And they don't come with built-in protection. Can I flip the Battery Protection circuit(they recommend the battery protection to be close to the BAT- to reduce copper impedance from BAT- to GND) to the bottom side? Can they be on different layers? Will punching 12 vias be enough? Do the traces connecting the R1 to BAT+ need to be >80mils?
--- Quote ---I'd keep the differential pairs where they are and only drop the mosfet current sensing and gate traces to the back side.
--- End quote ---
Ok, but I still have to place 4 mosfets on the top layer, I got no space for them. Can I flip the mosfets to the bottom layer?
--- Quote ---That depends on what you want each individual port to source. The main VOUT trace should be at least 2mm (80mil) for your 4A maximum current. The traces from there to each mosfet could be weaker, depends on what the maximum current there will be. BTW, KiCAD has a built-in PCB calculator that can do trace width calculations.
--- End quote ---
The combined max output current will 4A either through a single port(except micro USB) or all outputs combined. Low Voltage Direct charge mode is at 5V. From their datasheet:
--- Quote ---In non low voltage direct charge mode, when output voltage is lower than 6V, its current is up to 3A; When output voltage is higher than 6V, its power output is limited to 18W; When output voltage is 9V, its current is up to 2A; When output voltage is 12V, its current is up to 1.5A. In low voltage direct charge mode, its power outputisupto22.5W.
--- End quote ---
The max charging current will be 5A. This is what their datasheet says, but I cannot understand it. Idk what do they mean by charge current in input current and battery current.
--- Quote ---The charge current in input current varies with input voltage, while battery current is up to 5A. When input voltage is 5V, input charge current is set to 2Afor Micro-B and Lightning port, and 3Afor Type-C port. When input voltage is 9V, input charge current is set to 2A. When input voltage is 12V, input charge current is set to 1.5A.
--- End quote ---
--- Quote ---That depends on how much current will flow through these vias. My own rule-of-thumb says 500mA is the maximum allowance for a single via. If you want to push 1A, put two.
--- End quote ---
Ok, Say I need 6 vias on vout, placing vias any net that connects to vout is fine,right?
--- Quote ---BTW, how do you intend to charge the battery?
--- End quote ---
The USB connectors are bi-directional, acts as a source when sink is detected or acts as a sink when source is connected.
Thanks
Rerouter:
you can put the mosfets wherever you like, they don't even need to be next to the port, they can be closer to the micro,
As per Vias, just throw down as many as your compfortable with, the 500mA per via is not a bad rule of thumb,
As per exactly how to run the traces to the ports, get creative, same with the battery connection terminals, I just followed that as the files you gave me had them close to each other,
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