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ToDo:

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ToDo:

  • What to do with SYNCOUT and SYNCIN pins?

  • What to do with PGOOD output?

  • Look into common mode choke (optional)

  • Look into potential Vsense feature (optional)

...

  • 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.

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

    • 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

      • 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

        • 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

  • Load flyback diode (separate to the one used in the buck converter)

    • For inductive loads like servo

    • 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. 1 is enough.

  • There might need to be different TVS diodes for the different output voltages.

Sense Option A)

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Sense Option B)

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

    • Option: GRJ31CZ72A475KE01L

      • image-20240407-181512.pngImage Added

      • 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).

        • image-20240326-030331.png

      • 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:

      image-20240326-030435.png

  • Ceramic cap.s have low ESR. 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

...

  • VEN is 1.2V and IHYS is 10uA

  • It would be more ideal to have different output (5.1V, 8V, or 12V) demand a different minimum VIN(on), rather than having a 6V minimum that would be too low to be applicable to the 8V and 12V outputs. The strategy for doing this is choosing a constant VIN(on) to VIN(off) difference, in which case RUV1 will be constant.

    • Choose a difference of 0.5V. Calculating gives RUV1 = 49.9kR, 1/16W

    • Then, calculating for 3 different RUV2 resistors, which will be selected through the same jumper as RFB2:

      • For 5.1V, VIN(off) = 5.5V, VIN(on) = 6V: RUV2_5V = 12.4kR, 1/16W

        • 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 12.4kR instead to get VIN(off) = 5.6V, VIN(on) = 6.1V.

      • For 8V, VIN(off) = 8.5V, VIN(on) = 9V: RUV2_8V = 7.68kR, 1/16W

      • For 12V, VIN(off) = 12~12.5V, VIN(on) = 13V~13V: RUV2_12V = 5.11kR, 1/16W

ILIM

...

  • Going to use shunt sensing option to lower complexity of considering a changing Q2 RDS value.

  • BEC will support 3A continuous current. Anything past 4A to 4.3A will trigger ILIM protection.

    • (Maybe consider increasing the ILIM current. But, high inductor saturation current and DC current rating is hard to find and the DCR is bad)

  • Rs = 2mR, 1/4W8W.

  • RILIM = 72R71.5R, 1/4W8W

  • Per the TI design calculation tool, for these values:

    • Result in 4A limit at 5.1V output.

    • Result in 4.3A limit at 12V output.

    • It is recommended that the inductor saturation current accounts for the limit current + ripple: 4.3 + (4.3 * max 50% ripple)/2 <= 5.375A. So the inductor chosen should saturate at > 5.375A.

...

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

...

EMI Filter Design

SYNCOUT and SYNCIN

  • TBD…

Other Components

Input Polyfuse

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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

...

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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