Status: Active

Owner: Zen & Chris

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 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

• 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

5V Buck

IC Selection

Ratings to keep in mind: Pixie ratings → Power Distribution Architecture

Buck used to power Pixie from 12S.

V_in = ~55V

I = ~3A (Pixie draws around 3A max)

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

12V Buck

IC Selection

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

Datasheet

Typical Application Circuit

Pinout & Functions

Switching Frequency

• Higher switching frequency allows for the use of smaller inductors and capacitors as well as lower output ripple, but it comes at the cost of increased switching losses

• Since this application involves a relatively high voltage input, the selected switching frequency is 400kHz, which should provide a reasonably low ripple current while minimizing switching losses

Frequency Setting Resistor

From the table in the datasheet, for a 400kHz switching frequency, R=64.9K

https://www.digikey.ca/en/products/detail/yageo/RC0603FR-0764K9L/727344

Adjustable Output Resistor

V_ref is given as 1V and V_out is 12V. Choose R_FBT to be 100K, then R_FBB=9.09K

https://www.digikey.ca/en/products/detail/yageo/RC0603FR-079K09L/727418

https://www.digikey.ca/en/products/detail/yageo/RC0603FR-07100KL/726889

Output Inductor

• Choose k=0.4 for current ripple

• Expected current draw from the load is 500mA, design for 750mA to be safe

• This gives L=67.1uH

• Choose closest standard value, 68uH

• 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

https://www.digikey.ca/en/products/detail/würth-elektronik/7447709680/1638652

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

• Use the datasheet recommended value of 22uF for a 12V output

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

Input Capacitor

• At least 4.7uF X7R rated at twice the input voltage is required to reduce ripple and maintain input voltage during load transients

• Option 1 ($4.29): https://www.digikey.ca/en/products/detail/murata-electronics/KCM55LR72A475KH01K/2785887

• Option 2 ($2.67): use 3x 2.2uF 100V in parallel

https://www.digikey.ca/en/products/detail/yageo/CC1206KKX7R0BB225/5884563

• 100nF X7R rated at twice the input voltage is required for high frequency bypass

https://www.digikey.ca/en/products/detail/murata-electronics/GCJ21BR72A104KA01K/11618560

Bootstrap Capacitor

• 100nF 16V capacitor required to store energy for powering the mosfet gate drivers

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

INA228 Current Sensing IC

Typical Application Circuit

Shunt Resistor

• 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/RC0805JR-0710KL/728241

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/5024430670/2380429

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

High Level overview of Implementation:

1. Voltage fed through XT90 connector

2. Hall effect sensor measures the current passing through and outputs an analog voltage signal

3. Analog voltage signal is converted into digital signal.

4. Digital signal fed into MCU or application specific IC to be transmitted onto an I2C/SPI/UART bus

5. Input voltage broken down into 12V and 5V filtered clean rails

1. Hall Effect Sensor

Operation: Outputs an analog signal upon detection of a magnetic field generated by flow of electrons (current). A greater current flow means a stronger magnetic field and higher flux density. Current flow through the hall effect VCC and ground pins. The introduction of an external magnetic field will influence the magnetic field of the moving current. The electrons deflected to one side which creates a charge imbalance which results in a potential difference across the sensor (Hall voltage). The level at which the electrons are deflected will allow for an analog output regarding the strength of the magnetic field. There are amplifiers within the sensor to amplify the input signals.

Output voltage is poproptional to product of current in the conductor and

Linear IC’s take continuous range input and outputs and outputs are mostly proportional to the inputs whereas digital IC’s output only low or high. Since I will need to take in and output a contiguous range of voltages, I will need a linear IC to get an output voltage that is directly proportional to the magnetic field that interacts with the sensor.

Hall Effect Sensor Considerations:

• Linear: Output voltage can only be up to the saturation voltage that is determined by the power supply. Moving a magnet sideways across a hall effect sensor will give you an increasing analog signal as the pole becomes more and more in aligned with the sensor (doppler effect). An example of this setup is on opposite sides of motors to detect different operating characteristics (RPM, positioning, switches, proximity sensors )

• Bipolar Vs Unipolar : Unipolar turns on in the presence of one pole and off when removed whereas in bipolar, it stays on until other pole is introduced.

  Advantages of hall effect

• No moving parts

• Non contact

• Low mainteance

• Not affected by vibration, dust and water.

Example of current sense breakout board for ardunio:

https://www.sparkfun.com/datasheets/BreakoutBoards/0712.pdf

2. Current Sense Transformers:

Current sensing used for control, protection and information.

2. Analog to Digital Converters (ADC)

Measures ratio of analog input value to a reference value to express in the form of a digital value. The range is divided into n equally sized intervals and each interval is assigned to a certain value.

ADC with 8 unique values gives a resolution of 3 bit (2^N = 8 where N=3 ). Resolution defines the smallest change in input value that the converter can distinguish.

Offset and gain error can be compensated so that the end and start point of the curve are the same. However, there could be a non-linearity error

Since this project has stopped active development I’m just posting some random idea links

https://www.digikey.ca/en/products/detail/allegro-microsystems/ACS37612LLUATR-015B5/12093264