Project whitepaper:
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https://warg.365.altium.com/designs/BA1886A1-87A3-459C-B25E-4C3D181A3101
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ToDo:
Add 0 ohm resistors on FET gates
Address review comments
Decide what to do with ground and vias under inductor
Think more deeply about anti-ringing snubber values
Add a bit of an intro/context section to this Confluence
Look into common mode choke (optional)
add in a summary to this confluence page, what this project is, why
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Meets or exceeds spec of https://rotorgeeks.com/matek-bec-12s-pro
Externally visible current sense (optional),
needs ADC to interpret op amp value if doing digital comms
Ask FW team for digital (or analog) requirement. (eg. prontocol? CAN? etc…)
Questions:
How many BECs max on the drone?
Depends on the number of peripherals and current requirements
How many peripherals should one BEC connect to (one or multiple)? Would they be same or mixed voltage?
High current devices use one BEC
Some peripherals will share one BEC
Any BEC will only be configured for one voltage, not multiple voltages at once
Which peripherals being applied to, is there a list? Need to see if the spec of Matek BEC is even sufficient for them.
raspberry pi (5.1V input)
servos
video transmitters
radio
lighting
What is the characteristic of the input coming from the PDB to the BEC?
Input range from 9V to 12s (44.4v nominal, 50v fully charged)
Maybe there can be some level of digital control, instead of jumpers? I think this would make it more universal.
Better to have jumpers/switches
(but switches can cause accidents)
Options will definitely include 5(.1)V and 12V, and maybe include 8V.
->Go with 5.1V over 5V since there will be some voltage drop in the harness at higher current. Not doing 5.2V to be safe since some devices have tight 0.1V tolerance on 5V https://ca.robotshop.com/products/benewake-tfmini-s-micro-lidar-module-i2c-12m .
Maybe instead of discrete options, there could be something more continuous involving a potentiometer.
This would be more complex, and is only worthwhile if the target peripherals each have unique input voltage requirements, which is not really the case
Linear or Switching BEC?
Definitely switching since more efficient and less heat (although more noisy).
Could optocoupler serve any purpose?
Not sure, will research more
Should current sense be accessible as an analog or digital value?
Digital will probably end up using some CAN protocol, in which case the ADC output needs to be passed through a microcontroller and then to the CAN driver and connector.
Analog will be less reliable and need some digital conversion on the part of other components on the drone.
Should external current sense go before or after the buck converter (or both)?
1) Output side in series with load
Pros
Get to see what the peripheral is drawing from the BEC.
Cons:
Sense resistor on output side may introduce a small voltage drop that varies depending on the current. This may decrease and maybe somewhat destabilize output voltage (adds error).
However, I think a 2 mOhm max current sense resistor w/ amplifier will hardly have any voltage drop.
1.1) Another option: output side in series with the Buck converter inductor
Pros
If I have something like in this image https://theorycircuit.com/wp-content/uploads/2021/02/75-V-to-10-V-DC-DC-Buck-Converter-Circuit.png, but an extra sense resistor is in series with the inductor, wont the feedback mechanism just account for the error created by the sense resistor?
Cons:
This just gives the current passing through the MOSFET (& other elements) within the buck converter IC, not particularly the output or input. The usefulness of this sensing this is questionable.
Also, the inductor current will have a much wider ripple range than the load current.
2) Input side in series with supply:
Pros:
Get to see what the BEC as a whole is drawing from the battery. This might be more valuable in the context of the system.
Also, if the efficiency of the BEC is tested and proven to be in some range (e.g. 85-95%), then peripheral current draw can be estimated from BEC current draw.
Cons:
Higher current on input side, so more power consumed by current sense resistor.
Then again, the highest current drawing is probably the 3A of the RPI. Even if the input current is something extreme like 20A, (20A^2)*2mOhms = 0.8W, which compared to what the peripheral itself would be drawing, is negligible.
Current sense would be on a signal that is switching on and off. One would have to obtain an average current using a microcontroller, which introduces some complexity.
3) Both sides:
Pros:
Accurate BEC efficiency tracking. But this is not really necessary
Cons
All the cons from the input side + output side.
Deciding how to create multiple voltage output options
1) one adjustable buck using jumpers
Cheapest option, but has the technical challenge of figuring out the jumper/resistor configuration.
Makes the most sense since each BEC will only power devices of the same voltage.
1.1) one adjustable buck using a potentiometer
Cheap, involves less jumpers, and offers more flexibility in output voltage
But, it’s technically complex. Also, is the flexibility even needed?
2) multiple bucks
Expensive, but effective and technically simpler.
3) one buck with multiple LDOs connected to it
Medium price and simple/effective, but high power consumption.
Overcurrent protection and thermal shutdown
Simplest option is to just pick a buck IC that has these features built in, just as the Matek BEC does with the LM5116
Spike protection?
Depends on the characteristics of the input LIPO battery, PDB, and loads…
e.g. TVS diode (unidirectional), flyback diode, snubber circuit…
External current limiting on output
1) dedicated circuitry, e.g. https://www.youtube.com/watch?v=8uoo5pAeWZI&ab_channel=GreatScott!
This would be cool but quite a technical challenge to implement.
2) simple fuse
3) Nothing external
It may be simplest to pick a buck with preexisting current limiting feature, which a lot of them have. In this sense, it may be best to wait on whether to have external output current limiting until the part selection stage
https://www.quora.com/What-happens-if-you-short-circuit-the-output-of-a-boost-converter
https://www.renesas.com/us/en/document/whp/how-protect-buck-regulators-overcurrent-damage
Load flyback diode (separate to the one used in the buck converter)
For inductive loads like servo
https://www.youtube.com/watch?v=LXGtE3X2k7Y&ab_channel=Afrotechmods
Does control circuit inside our servos deal with the back EMF? It should, but an extra layer of safety on BEC is needed
It may be the case that the diode used for the buck converter is enough of a flyback for inductive load, so a 2nd diode is not needed. To be determined during part selection.
Actually, considering some analysis the load side diode would act as a clamp, not a flyback. That means choose a TVS or Zener diode (TVS has faster response to transients). Nonetheless it will help with voltage spikes.
Snubber on Buck converter and other output side diodes
The need for this depends on the buck converter IC chosen during part selection.
Polyfuse vs normal fuse
Polyfuse is resettable and there are ones that don’t instantly trigger on overcurrent, so switching current transients won’t trigger them.
Other bonus features?
Need to consider how output to USB C for rpi will work
Consider adding useful common output terminals as an a feature.
Or, have a simple generic output terminal to minimize board weight and size. The simple terminal can be connected to the needed adapter cable.
Something useful should be done with the current sense
Option A) Current sense terminal will be useful if there is also a terminal to the enable pin of the buck converter for another board to turn it off
Option B) If a buck controller/converter with adjustable current limiting is present, current sense resistor will be useful to limiting current of the buck/
In this case, there may not be a need to provide terminals for another board to read the current sense and control the enable, it would just be a bonus item at that point
Architecture
Some notes:
Input will be 2S (around 6V lowest) min not 5V.
Using 3 output LEDs is wasteful. 0 is enough.
There might need to be different TVS diodes for the different output voltages. → Nah
Sense Option A)
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Sense Option B)
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LM5146 datasheet recommends 10kR to 100kR
RPGOOD = 20kR, 1/16W
Should the PGOOD output be available through an external port?
VCC Capacitor
TI design calculation tool recommends 2.2uF
CVCC = 2.2uF, 16V
Vcc is the output of a 7.5V BIAS regulator, 16V capacitor rating is enough
Option: GRM188R61E225KA12D
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https://www.ti.com/lit/an/slyt682/slyt682.pdf?ts=1714980093816
This is a big topic that needs more digging to find ideal snubber values, but for now I will just use the example 2.2R and 100pF
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The input TVS will be located between the EMI Filter and CIN of the buck converter.https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/911201/the-correct-order-of-tvs-filters-and-clamp-diodes#:~:text=As%20a%20general%20rule%3A,also%20add%20an%20insulation%20resistance). ← Note: They’re not using a PI EMI filter bridge.
To protect the EMI filter capacitors, TVS will come before the EMI filter and before the fuse.
Actually, Daniel mentioned a possible fail where the TVS shorts → very bad. So the TVS will go after the fuse.
Theory:
Selected: https://www.digikey.ca/en/products/detail/littelfuse-inc/SMBJ51CA/688283
$0.53
Better power rating but more expensive: https://www.digikey.ca/en/products/detail/littelfuse-inc/SMDJ51CA/2024548
$1.73
Input Common Mode Choke?
TBD. Is this necessary?
What is the comparison between this and the EMI filtering option in the LM5148 datasheet?
→ Jerry recommends going with the EMI filter option since the datasheet mentions it.
BUT, don’t completely disregard the choke option incase the EMI filtering option is not enough for common mode noise rejection from ESCs and other sources.
Input Header
XT60PW-M connector
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Current Sense Amplifier and its Header???
This design will use Sense Option B architecture option. I don’t see an advantage to current sense unless the BEC has an emergency shutoff that is controlled externally.
Shower Thought on a Vsense line:
There could be Vsense line feature added for accounting for voltage drop in the harness. Vsense+ and Vsense- come from the load device. Need to think more about this, but it would be a valuable feature to add if feasible/economical.
Brainstormed ideas:
1) Feed Vsense+ and Vsense- into a unity gain difference amplifier, that has rail-to-rail voltage swing. The amplifier output would tie directly into the FB pin of the LM5146. The amplifier will be referenced to local ground.
Con: The rail-to-rail thing becomes tricky when the load voltage is already almost the same as the BEC output voltage. The LM5146 would end up thinking the load voltage is a couple mV lower than it really is and raise it when unneeded. The op-amp should be chosen with great care to achieve an acceptably small rail-to-rail margin.
2) Feed the difference amp output into a VCR (Voltage controlled resistor) that is part of the resistor divider on the FB pin.
etc…
Layout and Manufacturing Considerations
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