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changes.mady.by.user Taim Al-Dabbagh

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Status: Active

Owner: Zen, Taim

add link to A365 ?

Requirements

  • Passthrough 18 V - 55 V (6S lower voltage to 12S upper voltage )

  • Measure current through hall effect current sensing (without a shunt resistor to minimize losses)

  • Measures voltage with an ADC

  • I2C or UART interface

    • Preference toward I2C, but just pick one of them

  • XT90 connector for input power and output power

  • Maximum current passthrough requirements

    • Max pulsed current 200 A

    • Max continuous current 150 A

  • Steps down input voltage to clean 5V and 12V rails

  • Refer to Nathan’s current power module, photos in discord, for a reference to what this board will be replacing.

Current Sensing

Hall Effect Sensor

INA228 IC

Description

Pros

  • Does not use a shunt resistor (minimizes losses)

  • Supports Ardupilot natively

  • Simple solution

  • Can also sense voltage up to 85V

Cons

  • There needs to be a gap of 10mm on either side of the sensor where there are no traces other than the one carrying the current to be measured

  • It might be necessary to cover the sensor with a shield

  • Sensitivity and linearity may be influenced by temperature, magnetic field variations, and other external factors

  • May require calibration for accurate current measurements

  • Does not support Ardupilot natively

  • Uses a shunt resistor which would cause some power losses and heat dissipation

...

https://www.digikey.ca/en/products/detail/koa-speer-electronics-inc/PSL2NTEBL500F/1039674

Block Diagram

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  • The 12V output is left as a power pad so that a connector can be soldered onto it and a harness can be made as needed

  • The Molex connector is based on the connector standard for the Pixhawk, shown below

  • A 5V-3.3V LDO was added to provide a 3.3V rail for the SCL and SDA lines, as per the Pixhawk standard

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5V Buck

Requirements

  • Steps down 18-55V to 5V

  • Current limit should be at least 5A

...

  • Vin_max = 55V, Vout = 5V, Iout= 5A, Kind = ΔIL/Iout = 0.3 (recommended by datasheet), and fsw= 400kHz

  • Using the equation above, Lmin = 7.6uH. Choose the next standard value of 8.2uH. The saturation current of the inductor should be greater than the switch current limit of 7.5A

OLD PART: https://www.digikey.ca/en/products/detail/würth-elektronik/7443330820/2175573

NEW PART: https://www.digikey.ca/en/products/detail/codaca/CSEB0770H-8R2M/16566340

...

https://www.digikey.ca/en/products/detail/murata-electronics/GRM1885C2A510JA01D/586986

3.3V LDO

Requirements

  • Steps down output from 5V buck to 3.3V to power the INA228, which draws 640uA of current

...

https://www.digikey.ca/en/products/detail/murata-electronics/GCM188R71C105MA64D/7430540

...

12V Buck

Requirements

  • Steps down 18-55V to 12V

  • The 12V rail is only used to power the vtx, which has a 500mA current draw. Ideally, the current limit of the buck converter should be closer to 750mA to be safe

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  • Vin = 55V, Vout = 12V, fsw= 400kHz, Iout = 2A

  • K = ΔIL/Iout, choose k=0.4

  • Using the above equation, L = 29.3uH

  • Choose closest standard value, 33uH

  • Inductor saturation current should be >= Isc , the high side switch current limit of 3.2A, to avoid component damage due to high current when the inductor saturates

OLD PART: https://www.digikey.ca/en/products/detail/würth-elektronik/744770133/1638634

NEW PART: https://www.digikey.ca/en/products/detail/tdk-corporation/SPM10065VT-330M-D/12175283

Justification: Slightly smaller, cheaper, higher current and saturation current ratings.

Output Capacitor

  • Output capacitor allows for the load to be powered during the off state of the transistor and accounts for load transients

  • Voltage rating should be 2-3x the output voltage to account for voltage spikes

  • The datasheet recommends 22uF for a 12V output

  • Use 3x 22uF to account for DC bias at 12V

...

https://www.digikey.ca/en/products/detail/murata-electronics/GCJ188R71E104KA12D/7363221

INA228 Current Sensing IC

Typical Application Circuit

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https://www.digikey.ca/en/products/detail/molex/5024940670/2380433

Power Pads

Power pads will be used over XT90 connectors soldered onto board, for the reason of saving weight and space.

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Having different sources of measurements and dimensions, the WARG XT90 power pads will be 5mm square pads, with a 7mm pitch for the positive and negative leads. This allows for a balance of space to solder the wires onto, so it is neither tight or spaced too far out.

Stack-Up

The stack-up chosen is the JLC04161H-7628 Stackup, off JLCPCB.

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The planes are as dictated: PWR-SIG/GND/PWR/GND-SIG.

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Stitching Vias

Stitching vias are used to tie the power and ground planes. The stitching for the power planes allows the high current to distribute, allowing the board to handle such large amounts of current. The SaturnPCB tool was used to calculate how much amps each single via can carry, which atleast 50 vias were placed to handle 150A. Tieing the ground places with the stitching vias allows better heat dissipation.

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Addressing Issues

I2C Pull-up

The I2C lines of the IC in the schematic were shorted to 3V3 rail, with no pull-up resistors.

“I removed the pullup resistors since the Pixhawk has its own built in. This is meant to go from the output of the INA228 to the 6-pin connector for the Pixhawk, I connected them through the net names but maybe I did that incorrectly?”

Previous Research

Implementation Ideas

  • “High” voltage & current passthrough should be done with an XT90 connector.

  • Current measuring can be implemented with a smaller current transformer to be mounted on the PCB

  • A simple ADC integrated (presumably 2 channel ADC) and possibly voltage divider circuit can be used to measure both current and voltage

    • This ADC should support I2C and SPI and may be fitted with a signal buffer IC

    • Fairly standard to be able to find an ADC that can operate at 3.3V

  • A single “low voltage connector” should be used

    • This would be some relatively fine pitch connector

      • Some standard molex thing

    • Four conductors on this connector

      • GND (this will be signal ground, but should be presumed as the same potential and non-isolated from the “high voltage passthrough gnd”

      • 5V or 12 V input power (possibly a range that supports each of these and maybe more)

      • I2C or UART data lines (2 conductors for each of these protocols.

  • Alternatively, use the 12V and 5V stepped down from the input line to power all board ICs instead of plugging in external power

  • Powering the ADC

    • The ADC and buffer (if a buffer is included, just an idea) will ideally consume very miniscule current, on the order of less than 200mA which makes a simple LDO (low dropout regulator) viable

    • This LDO will take the low voltage input power and use that to power the ADC chip.

    • LDO has lower efficiency than a buck, but will save board space and will be more convenient to implment.

      • Because of the negligible total power requirements for the board a low efficiency doesnt matter as much

...