Project whitepaper:

https://warg.365.altium.com/designs/BA1886A1-87A3-459C-B25E-4C3D181A3101

ToDo:

• Add 0 ohm resistors on FET gates

  • https://www.youtube.com/watch?app=desktop&v=wY6eGoBea9Y&ab_channel=FoolishEngineer

  • https://www.youtube.com/watch?v=nMIOkXnY-n4&ab_channel=Aviprink

• Address review comments

• Decide what to do with ground and vias under inductor

  • https://resources.altium.com/p/should-ground-be-placed-below-inductors-switching-regulators

  • https://youtu.be/6ind9vopKZs?si=SL5V51QdNREyreG8

• 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

Requirements/Needs Assessment

• 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

    • 2024 System Architecture - SysInt - WARG (atlassian.net)

  • What is the characteristic of the input coming from the PDB to the BEC?

    • PDB500[X] — Advanced Power Drives

    • 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

  • https://electronics.stackexchange.com/questions/24256/principles-of-dc-dc-converter-w-jumper-selectable-outputs

    • 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

    • 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/

    • e.g. https://www.ti.com/product/LM5148

    • 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)

Open

Sense Option B)

Open

Buck IC options

• Caters Sense Option A

  • (full on converter chips)

  • https://www.ti.com/power-management/non-isolated-dc-dc-switching-regulators/step-down-buck/buck-converter-integrated-switch/products.html#1241=Converter&2954=SOIC%3BSOT-23%3BSOT-23-THN%3BSOT-5X3%3BTO-220%3BTO-263%3BHSOIC%3BHTSSOP%3BHVSSOP&451max=4%3B40&238max=50%3B450&-4=ACTIVE&

  • 1) https://www.ti.com/product/TPS54560B

    • 4.5-60V Vin

    • 0.8-58.8 Vout

    • 5A Iout max

    • Frequency synchronization, Light Load Efficiency, Over Current Protection,

    • $7.80 Texas Instruments TPS54560BDDA

  • 2) https://www.digikey.ca/en/products/detail/analog-devices-inc/LT1170HVCT-PBF/891558

    • 3-60V Vin

    • 1.244-75V Vout

    • 5A Iout max

    • Fancy features

    • $31.20

  • 3) https://www.monolithicpower.com/en/mp4575.html

    • 4.5-55V Vin

    • 1-49.5V Vout

    • 5A Iout max

    • $4.84 https://www.mouser.ca/ProductDetail/Monolithic-Power-Systems-MPS/MP4575GF-Z?qs=nmF%252BdLtsfhVhZ%252Bd5JWMQ%2Fw%3D%3D

• Caters Sense Option B:

  • (just controller chips, needs external FETs. More design flexibility):

  • https://www.ti.com/power-management/non-isolated-dc-dc-switching-regulators/step-down-buck/buck-controllers-external-switch/products.html#1241=Controller&2192=Adjustable%20current%20limit&238min=1%3B5&238max=52%3B700&451max=10%3B765&

  • 4)https://www.ti.com/product/LM5148

    • $6.63 DigiKey

  • 5) https://www.ti.com/product/LM5149

    • $6.75 DigiKey

  • 6)https://www.ti.com/product/LM5088

    • $5.82 DigiKey

  • 7)https://www.ti.com/product/LM5085

    • $4.49 DigiKey

  • 8)https://www.monolithicpower.com/en/mp2908a.html

    • $4.29 DigiKey

  • 9) https://www.monolithicpower.com/en/mp9928.html

    • $4.14 DigiKey

  • 10) https://www.ti.com/product/LM5146?keyMatch=LM5146&tisearch=search-everything&usecase=GPN-ALT

Component Selection

Buck Converter Components

Latest TI Design Calculation Tool plug and chug excel file (ignore the file name):

• ^This is the tool I used to verify/finalize all values

Buck controller

• LM5146:

  • https://www.ti.com/product/LM5146?keyMatch=LM5146&tisearch=search-everything&usecase=GPN-ALT

    • https://www.digikey.ca/en/products/detail/texas-instruments/LM5146RGYR/14544372

  • https://www.ti.com/tool/LM5146DESIGN-CALC

Open

Frequency setting resistor

• Open

  

• R_RT = 45.3kR, 1/16W

  • Option: https://www.digikey.ca/en/products/detail/yageo/RC0603FR-0745K3L/727245

• Yields ~220KHz switching frequency. Lower frequency means less switching loss. Can’t go too low or too large inductor and output capacitors will be needed to account for the higher ripple.

Inductor

Open

• L_F = 27uH, (saturation at > 5.375A, > 4.3A max DC current. See ILIM section)

  • Selected: https://www.mouser.ca/ProductDetail/Wurth-Elektronik/7447709270?qs=XJfXErqHgA5ctcmxsImNAA%3D%3D

    • Open

      

    • Open

      

• Alternative: https://www.digikey.ca/en/products/detail/sumida-america-inc/125CDMCCDS-470MC/9490421

  • Has a pretty bad DCR of 97.2mOhm. Need to find/consider an inductor with <= 40mOhm DCR to save about 3% efficiency at nominal I/O conditions.

• https://www.desmos.com/calculator/cmd60zlxtp

  • At 220KHz, 3A peak load, 27uH gets ripple between 25%-50% over the entire output range.

Output Capacitors

Open

Open

• C_OUT = 3 x 22uF, 25V, ceramic, X7R, 1206 or 1210 size

  • https://www.digikey.ca/en/products/detail/murata-electronics/GRM21BR61E226ME44K/4905534

  • ESR of each capacitor = 3mR at 220KHz according to SimSurfing. For 3 in parallel Z_Total = (Z₁^-1 + Z₃^-1 + Z₃^-1)^-1 = 0.001 - j0.10961. That is, 3mR becomes 1mR for 3 capacitors.

• https://www.desmos.com/calculator/iwoiula6yw

  • Lower capacitance value (30.838uF) is needed to achieve an optimal s <= 1% ripple. But, for 12s nominal input to 5.1V output, TI design calculation tool recommends de-rating to 47uF. Using 3 x 22uF for low ESR.

  • Here input voltage is X axis and capacitance is Y axis:

    Open

    

• Ceramic cap.s have low ESR. Higer ESR increases capacitance needed.

Open

GRM21BR61E226ME44K ESR Characteristic

Input Capacitors

Open

Open

• C_IN = 6 x 4.7uF, 100V, ceramic, X7R, >2.52 RMS current

  • Option: GRJ31CZ72A475KE01L

    • Open

      

    • 10mR ESR per capacitor. Approximate ESR for 6 in parallel is 10mR/6 = 1.67mR

• https://www.desmos.com/calculator/uovqhzl57b

  • To achieve an optimal 2% ripple:

    • At 5.1V out, 21.98uF minimum capacitance is needed (at 7.5V in).

      • Open

        

    • At 8V out, 13.471uF minimum capacitance is needed (at 11.8V in).

    • At 12V out, 8.772uF minimum capacitance is needed (at 17.5V in).

• TI design calculation tool recommends de-rating to 6 x 4.7uF

• https://www.desmos.com/calculator/x1iysyhtb4

  • 2.52 RMS rating computed from a de-rated 5A output (1.8 RMS rating if de-rating to 3.5A)

  • Here D = 0.5 gives the maximum value:

    Open

    

• Per the TI design tool recommendation:

  • Max ESR for 2% ripple at 5V is 15mR

  • Max ESR for 2% ripple at 8V is 25mR

  • Max ESR for 2% ripple at 12V is 39mR (7mR for 1%, which parallel capacitors can achieve)

Additional Input Capacitor

• From LM5146 datasheet Section 10:

  • Open

    

  • I think the C_D chosen in the EMI Filter section should be enough to deal with this.

Power MOSFETs

Open

• https://www.desmos.com/calculator/o9jukbktfa

  • Power loss calculations assume nominal conditions: 3A, 45V in, 5V out. (Note: Nominal is updated to 5.1V output, but the power analysis results in the below table/graphs should be effectively the same.)

  • Assuming 14ns t_dt1 and t_dt2, which is the LM5146 default.

Some example combos:

1) CSD18563Q5A and CSD18534Q5A

Open

2) BSC034N06NSATMA1 and BSC039N06NSATMA1

Open

3) CSD18563Q5A and BSC039N06NSATMA1

Open

Although Combo 3 has appx. 1% better efficiency, Combo 1 has some benefits. Both MOSFETS would be of the same brand and have similar rise/fall time. Also, Combo 1 is $1.04 cheaper overall.

• Q1 = CSD18563Q5A

• Q2 = CSD18534Q5A

Soft-Start Capacitor

Open

• C_ss = 16V, 680nF X7R

  • Option: https://www.digikey.ca/en/products/detail/murata-electronics/GCM188C71C684KA64D/2591904

  • 

• Yields 54.4ms startup time. Longer time will help lower inrush currents.

• Theory: https://www.analog.com/en/resources/analog-dialogue/articles/preventing-start-up-issues-due-to-output-inrush-in-switching-converters.html

  • It seems 54.4ms is more than good enough, even overkill, to ensure that I_CAP is small

    Open

    

Feedback Resistors

• R_FB1 = 21kR, 1/16W

  • Option: https://www.digikey.ca/en/products/detail/yageo/RC0603FR-0721KL/727048?s=N4IgTCBcDaIEoGEAMA2JBmAYnAtEg7GAIwDSAMiALoC%2BQA

• 3 different R_FB2 will be jumper selected for the different output voltage options, such that input on FB pin of LM5146 is 0.8V.

  • For 5.1V: R_{FB2_5V1} = 3.92kR, 1/16W

    • Option: https://www.digikey.ca/en/products/detail/yageo/RC0603FR-073K92L/727138?s=N4IgTCBcDaIEoGEAMA2JBmAYnAtEg7OgNICcYAMiALoC%2BQA

    • Note: There may be a case that 5.2V output is realized to be more appropriate due to power harness resistance loss. In this case, use 3.83kR instead.

  • For 8V: R_{FB2_8V} = 2.32kR, 1/16W

    • Option:https://www.digikey.ca/en/products/detail/yageo/RC0603FR-072K32L/727021

  • For 12V: R_{FB2_12V} = 1.5kR, 1/10W

    • Option: https://www.digikey.ca/en/products/detail/yageo/RC0603FR-071K5L/726864

Control Loop Compensation

Open

• Desired crossover frequency is typically chosen to be 10% to 20% of of the switching frequency, so 33kHz is a good pick.

• Zeros and poles are placed per TI design tool recommendation.

  • 2 compensator zeros are placed just before the LC double pole.

  • The 1st compensator pole is placed near the zero created by C_OUT and it’s ESR. Since the ESR of the C_OUT currently chosen is so low (1mR but assuming 2mR worst case) the first pole is at a very high frequency (not even visible on the Bode Plot).

  • The 2nd compensator pole is half the switching frequency.

  • Phase shift is between 50% to 70% at crossover frequency, for all 3 output voltages, which is what is recommended.

Open

Open

Latest Bode Plot for 45V Nominal Input, 5.1V Output

• Thus,

  • R_C1 =12.1kR, 1/16W

    • Option: https://www.digikey.ca/en/products/detail/yageo/RC0603FR-0712K1L/726915

  • C_C1 = 4700pF, 50V

    • Option:https://www.digikey.ca/en/products/detail/murata-electronics/GCM188R71H472KA37D/1765180

  • C_C2 = 120pF, 50V

    • Option: https://www.digikey.ca/en/products/detail/murata-electronics/GCM1885C2A121FA16D/11618655