A PCB to power them all!
Currently most of the materials that you need to build a drone are cheaply available on the internet so the idea of making a self-developed PCB is not worth it at all except for a few cases where you want to make a weird and powerful drone. In that case you better be resourceful or have a Instructables tutorial about it... ;)
Step 1: Objectives
The objectives of this PCB (and reasons why it can't be found on the internet) are:
1.- It must power the Pixhawk 4 with the current measure, the voltage measure and the same connector.
2.- It must have the I/O and the FMU connectors directed to the pins, the CAP&ADC is not needed in my case.
3.- It must be able to power 5 motors with a combined maximum current of 200A, Yep, 0,2 KiloAmperes!
Note: It is still useful for designs with less motors or less current. This is just my case.
Step 2: Schematics and Choosing Components
OK, now we know what we want to do. To proceed we will design the schematics.
If you don't want to understand the electronics behind this board just copy the schematics and go to the next step.
The schematics can be divided in two main parts, the DCDC to power the pixhawk and the power distribution of the motors.
With the DCDC the easiest way would be using a Traco Power DCDC and avoid having to design it but since I don't like the easy way I will be using a LM5576MH from Texas Instruments. This integrated is a DCDC that can manage an output up to 3A and its datasheet tells you all the information about the connections and components needed and it gives the formulas to get the desired specs of the DCDC modifying the components used.
With this the design of the DCDC for the Pixhawk, in my case, ends like seen in the picture.
On the other side the power distribution consists of the sensing of the current and voltage and the distribution itself that will be considered in the next step.
The voltage sensing will simply be a voltage divider that at its maximum voltage of 60 V (maximum voltage supported by the DCDC) it gives out a 3.3V signal.
The current sensing is a bit more complex even we will still be using the Ohm's law. To sense the current we will use shunt resistors. To maximize the amount of current they can handle, 10W resistors will be used. With that power, the smallest SMD shunt resistors I could find where of 0.5mohm.
Combining the previous data and the power formula, W = I² × R, the maximum current is 141A, which is not enough. That's why two shunt resistors in parallel will be used so that the equivalent resistance is of 0.25 mohm and then the maximum current the desired 200A. This resistors will be connected to a INA169 also from Texas instruments and, as in the DCDC, its design will be made following the datasheet.
Finally the connectors used are from the GHS series from JST connectors and the pinout from pixhawk 4 is followed to make the right connection.
Note: I didn't have the INA169 component in Altium so just used a voltage regulator with the same footprint.
Note 2: Notice that some components are placed but the value says NO, that means that they won't be used unless something in the design works wrong.
Step 3: Design of the PCB With Altium Designer
In this step the routing of the pcb will be done.
First what has to be done is placing the components and defining the board shape. In this case two different areas will be made, the DCDC and connectors, and the power zone.
In the power zone the pads are out of the board so that some heat-shrink tube can be used after soldering and the connection remains well protected.
Once that is done, next is the routing of the components, to do that the two layers are used efficiently and bigger traces are used in the power connections. And remember, no right angles in the traces!
Once the routing is done and not before, the polygons are applied, here there will be a GND polygon on the bottom layer and another one on the top layer but just covering the DCDC and connectors zone. The power zone of the top layer will be used for the voltage input as shown in the third image.
Finally, this board couldn't handle the 200A it's designed for, so some zones of the polygon will be exposed without silkscreen, as seen in the last two images, so that some uncovered wire is soldered there and then the amount of current that can go through the board is more than enough to fulfill our requirements.
Step 4: Creating the Gerber Files for JLCPCB
Once the design has finished it has to become a reality. To do that the best manufacturer I have worked with is JLCPCB, they check your board even before you pay for it so that if they find any error with it you can fix it without losing money, and trust me, this is a true lifesaver.
Since this board is a two layer board and is less than 10x10 cm, 10 units costs just 2$ + the shipping, obviously a better option than doing it yourself because for a low price you get perfect quality.
Finally this files just have to be uploaded to their quote website.
Step 5: End
And that's it!
When the PCB's arrive come the cool part, soldering and testing. And of course! I will upload more photos!
During the following week I will be soldering my prototype and testing it, so if you want to do this project wait until both of the next status mark OK. With this I will avoid you any botched job or resistance replacement.
Solder: NOT YET
Test: NOT YET
Notice that this is SMD soldering, if it's your first time soldering or don't have a nice soldering iron consider doing another project since it can be a source of trouble.
If anyone has any doubts about the process don't doubt contacting me.
Also if you do it, please, I would love know and see it!