Status: Active

Owner: Zen & Chris, 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 75 150 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

Based on the above analysis, using an INA228 IC seems to be a better option for simpler implementation and integration. To minimize the losses with the shunt resistor, a resistance of 0.0005 ohms will be used (the current power module also uses this value). At max current of 75A, the resistor will dissipate 2.8W. To prevent overheating, a nonstandard resistor with a resistance of 0.0005 ohms and a power rating of 8W will be used, such as the one below.

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

IC Selection

While the SIC462ED is the best option for price and efficiency, the beast of a chip has 30 pins. Based on simplicity and efficiency, the TPS54560B is chosen instead.

Datasheet

Typical Application Circuit

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• V_{in_max} = 55V, V_out = 5V, I_out= 5A, K_ind = ΔI_L/I_out = 0.3 (recommended by datasheet), and f_sw= 400kHz

• Using the equation above, L_min = 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

Output Capacitor

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Datasheet recommends C_out > 62.5uF to account for transient load response

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

Justification for new part: smaller size. (10.90mm x 10.00mm) vs (7.90mm x 7.60mm). New part has higher DCR and lower current rating.

Output Capacitor

• Datasheet recommends C_out > 62.5uF to account for transient load response

• Use 3x 47uF 10V caps to get C_out = 90uF at 5V DC bias, which is safely above the minimum 62.5uF

• https://www.digikey.ca/en/products/detail/murata-electronics/GRM32EC81A476KE19L/2548451

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

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3.3V LDO

Requirements

• Steps down 18-55V to 12VThe 12V rail is only used output from 5V buck to 3.3V to power the vtxINA228, which has a 500mA current draw. Ideally, the current limit of the buck converter should be closer to 750mA to be safe

IC Selection

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LT8631IFE#PBF

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LV2862XLVDDCR

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LMR38020SDDAR

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Voltage input range (V)

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3-100

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4-60

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3.8-80

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Voltage output range (V)

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0.8-60

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0.76-58

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1-75

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Output current limit (A)

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1

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0.6

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2

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Efficiency @ 600mA load (%)

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81

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92

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90.5

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Price ($)

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17.65

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0.93

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2.68

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Link

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https://www.mouser.ca/ProductDetail/Analog-Devices/LT8631IFEPBF?qs=oahfZPh6IALt9hCBxhtB7A%3D%3D

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https://www.mouser.ca/ProductDetail/Texas-Instruments/LV2862XLVDDCR?qs=XJu%252BLGjWfSCoT1RiKkHrOA%3D%3D

• draws 640uA of current

Selected IC

https://www.digikey.ca/en/products/detail/texas-instruments/TLV73333PDBVR/5022378

• Steps down input of up to 5.5V to fixed 3.3V

• Supplies up to 300mA of current

Typical Application Circuit

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Capacitors

• 1uF X7R filtering capacitors on input and output

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

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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

IC Selection

Based on price, current limit and efficiency, the LMR38020 is chosen.

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• V_in = 55V, V_out = 12V, f_sw= 400kHz, I_out = 2A

• K = ΔI_L/I_out, choose k=0.4

• Using the above equation, L = 29.3uH

• Choose closest standard value, 33uH

• Inductor saturation current should be >= I_sc , 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

Output Capacitor

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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

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

INA228 Current Sensing IC

Typical Application Circuit

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Shunt Resistor

(Previous research based on 75A max current)

• Resistance of 0.0005 ohms to minimize losses

• At max current of 75A, the resistor will dissipate 2.8W. To prevent overheating, a nonstandard resistor with a resistance of 0.0005 ohms and a power rating of 8W is selected

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

Pullup Resistors x3

https://www.digikey.ca/en/products/detail/yageo/RC0603JR-1010KL/13694233

Decoupling Capacitor

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

Output Connector

• Output connector to Pixhawk with I2C current data from INA228 & 5V power from buck converter

https://www.digikey.caGiven the high current requirement of 150A, consider using multiple shunt resistors in parallel to spread out the power dissipation across multiple resistors.

The selected shunt resistor is 0.0002 ohms and rated for 15W. At max current, the power dissipated is 150² * 0.0002 = 4.5W. The safety margin is over 3x, so it should be safe to use only a single shunt resistor.

https://www.digikey.ca/en/products/detail/molex/5024430670/2380429eaton-electronics-division/CHSA5930R0002F/16712930

NOTE: The INA228 has two options for the differential input voltage range, ±163.84 mV and ±40.96 mV. Since we are using a very small resistance, the max V_shunt is expected to be 150 * 0.0002 = 30mV. To obtain a higher resolution, the differential input voltage range should be set to ±40.96 mV. This is done by setting the ADCRANGE bit in the CONFIG register to 1.

Pullup Resistors x3

https://www.digikey.ca/en/products/detail/yageo/RC0603JR-1010KL/13694233

Decoupling Capacitor

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

Output Connector

• Output connector to Pixhawk with I2C current data from INA228 & 5V power from buck converter

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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The footprint of the power pads can be derived the known XT90 dimensions. The pitch of the XT90 power pads can vary, but shall not exceed 11.00mm in pitch. The recommended wire gauge for XT90 is 10AWG (https://media.digikey.com/pdf/Data Sheets/DFRobot PDFs/FIT0588_Web.pdf), which is 2.588mm in diameter.

Using measurements from a COTS power module, the pad was around 5mm. The measured pitch was 5.6mm.

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. 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

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