Smoke Stopper

• todo

  • cheaper than COTS efuse board for initial power up

• use case

  • ground validation

• why

  • maybe cheaper than COTS

  • fun

• who

  • unassigned

• timeline

  • no efs effort

  • deprioritized for now

OTS Smokestopper:

Open

Open

Features (both present in OTS and this project):

• Customizable current threshold (1A, 2A and 3A - not in OTS)

• Customizable current shut-off delay (not really clear if these are exact or maximum delays in the OTS device)

• Fault indicator LEDs

• Manual ON/OFF/RESET switches

• $12.99 VIFLY, >$40 on Amazon

Block Diagram:

Current Switch:

• Relay? Power Relays, Over 2 Amps | Electronic Components Distributor DigiKey. (10x12mm).

  • Coil current consumption is pretty high (~30mA), which would cause LDO to dissipate lots of power. Also, relay contact switch time is ≥3ms.

• MOSFETs with ~10mOhms maxRDS are cheap. This can result in up to 3x that value in voltage drop (chosen by current limit selection; 3A max), which seems negligible.

  • Query: Single FETs, MOSFETs | FETs, MOSFETs | Transistors | Electronic Components Distributor DigiKey

  • BUK9Y4R8-60E,115 Nexperia USA Inc. | Discrete Semiconductor Products | DigiKey

  • BUK9Y8R5-80EX Nexperia USA Inc. | Discrete Semiconductor Products | DigiKey

    • The two MOSFETs above have very low Rds on (4.1mOhm and 8mOhm), which isn’t needed considering that the drone is designed to work with varying battery voltage (voltage drop across MOSFETs is unimportant).

    • They cost ~$2.7, can cut this down with a higher Rds MOSFET

  • BUK9M24-60EX Nexperia USA Inc. | Discrete Semiconductor Products | DigiKey

    • 21mOhm RdsON; $1.17

  • BUK9M42-60EX Nexperia USA Inc. | Discrete Semiconductor Products | DigiKey

    • 37mOhm RdsON; $0.92

Current Measurement/Sense:

• High-side current sense IC (needs to be supplied by battery; max common mode input is VCC), 360uA supply current. $3.77 MPQ8112AGJ-AEC1-Z Monolithic Power Systems Inc. | Integrated Circuits (ICs) | DigiKey

• But we can also use low-side current sense. A flaw with low-side sense is that ground faults cannot be detected. However, in our configuration, they seem highly unlikely. The smoke-stopper’s current switch will be in between drone circuitry and the battery’s ground, so for a ground fault to occur, the circuitry will need to bypass the entire smokestopper, which is unlikely:

• Additionally, in this architecture, adding a small resistance between the load’s ground and supply ground has no worrisome effect (unlike with a load on a PCB, where adding a resistance between its ground pins and the ground plane can cause EMI problems and/or issues with how components, that are connected directly to supply ground, interface with this load - because their grounds have a potential difference). This doc mentions these problems of low-side sense, which aren’t present in this scenario.

• Current Sense IC, 20x Gain, Built-in comparator, latching output functionality, 10us fault response, $0.89 INA381A1IDSGR Texas Instruments | Integrated Circuits (ICs) | DigiKey

  • Because its common mode voltage rating is maxed out at 26V, in the case that the load is a complete short, it’s likely beneficial to add a zener diode to clamp the voltage at the sense pins to prevent damage. This zener diode will also protect the shunt resistor.

  • Latching output:

• 50mOhm shunt resistor WSLP1206R0500FEA Vishay Dale | Resistors | DigiKey

  • Worst case, will dissipate 0.45W, so I chose 1W; high availability; nice value of 50mOhms

  • Current sense amp will ouput 1V/1A, which works great if we power the circuit with 5V.

Reverse Voltage Protection:

• PMOS

  • SQJ479EP-T1_GE3 Vishay Siliconix | Discrete Semiconductor Products | DigiKey (query)

    • $1.70; Probably too expensive

    • 33mOhm @10A (Overkill?);

    • 80V max VDS

    • +-20V VGS; 10V min RdsON → choose a ~10-15V zener diode

  • GSFD500P10 Good-Ark Semiconductor | Discrete Semiconductor Products | DigiKey

    • $0.99

    • 50mOhm @15A

    • 100V max VDS

    • +/-20V VGSmax;

    • This results in a maximum voltage drop of 105mV across the PMOS (when conducting 3A)

• Zener diode (query)

  • MM5Z12VT1G onsemi | Discrete Semiconductor Products | DigiKey

    • Rated watts 0.5W; Rated current ~41.67mA @12V

    • Specs

• Pull-down resistor

  • Size it to result in <41.67mA (rated zener current) of current going through the zener, whilst being >1mA (the reverse current of the zener)

    • Worst-case supply voltage is 4.2*12=50.4V, so resistance needs to be at least (50.4-11.4)/0.04167=936Ohms.

    • Lowest supply voltage is 3.2*6=19.2V (if using 6S config), and to pass at least 1mA of current through the zener, the resistance needs to be at most (19.2-12.7)/0.001=6.5kOhm

    • I will thus choose 5kOhm.

Low Voltage Power Supply:

• Will use a 5V supply, mainly due to the current sense circuitry (nominal supply voltage of the amp is 5V)

• Linear Regulator (instead of switching because low noise - for sense circuitry, and circuit consumes barely any power, will show in power analysis here) (query)
  Might cascade regulators to reduce power dissipated per regulator (if heating is an issue). Could also put diodes (or power resistor) in series with regulator input to reduce drop-out voltage, and consequently power dissipated by regulator.

  • ZXTR2005Z-13 Diodes Incorporated | Integrated Circuits (ICs) | DigiKey, 100V max, 38mA max, $0.81

    • The maximum junction to ambient thermal resistance is 112C/W (measured on 15mm x 15mm 1oz copper). If we operate on a really hot day (40C), and want the chip to rise up to no more than 70C (to be relatively safe to touch), the maximum power the regulator should dissipate is (70-40)/112 = 0.268W. In the worst case, with 12S LiPo, this means a maximum current of 0.268/(50.4-5) = 5.9mA.

  • ZXTR1005K4-13 Diodes Incorporated | Integrated Circuits (ICs) | DigiKey, 100V max, 50mA max, $1.05

Response Delay Circuitry:

• RC circuit at INA381’s current sense output line

  • C adjustable with multiple capacitors and a switch, or a varistor.

  • R fixed or a pot

  • Need to ensure amp output is capable of driving this circuit (choose proper filter resistor)

  • Since the output of the external comparator amp will only be the rail (at steady state; it will be delayed according to the time constant), the internal comparator will compare the ext. comp. with a constant voltage. I arbitrarily chose 63% of Vdd, as this will give a delay equal to the time constant.

  • MCP6541T-I/LT Microchip Technology | Integrated Circuits (ICs) | DigiKey

    • 1uA quiescent. High output current (overkill)

    • 4us propagation delay, negligible.

    • $0.60/unit

  • LM339LVPWR Texas Instruments | Integrated Circuits (ICs) | DigiKey

    • 4 comparators in one

    • 50uA max quiescent

    • 600ns propagation delay

    • $0.66/unit

  • AZV3002S-13 Diodes Incorporated | Integrated Circuits (ICs) | DigiKey

    • 2 Comparators in one

    • 9uA max quiescent current

    • 0.8us propagation delay

    • $0.86/unit

    • Push-pull

    • Can operate at output being short-circuit’ed, and output 68mA. Definitely can drive 2mA LEDs.

• Counter + Oscillator circuit (counter query, osc query) - Probably overkill, too expensive, and too power consuming for a linear regulator.

  • when certain counter value is obtained (which counts only when amp output reaches some threshold value → need an external comparator for this), comparator is tripped (after a fixed amount of time, which can be either programmed, or comparator looks at a different counter bit for a different delay: i.e. less significant bit for smaller delay, more significant for longer delay)

Status Notifier LEDs:

• Fault LED (RED)

  • Red when RST switch is closed and when overcurrent fault is detected

• ON LED (GREEN)

  • Green when RST switch is open and when no overcurrent fault is present

• Both of the LEDs above will only be on if the 5V regulator is working, so they can be used as power indicators.

• APHB1608LCGKSURKC Kingbright | Optoelectronics | DigiKey (Red and Green LEDs in one package)

• Separate red & green LEDs.

• The two-in-one comparator can drive them in the following configuration

Switches

• Is debouncing needed for each switch?

  • No, debouncing will only result in additional delay with the switch configurations I’ve planned for. Having smooth switching doesn’t seem to be necessary in this project.

• ON/RESET DPST/DPDT switch (query)

  • SLW-826947-5A-RA-D CUI Devices | Switches | DigiKey

• If delay circuit uses different capacitor options (to vary the RC time constant of the fault line output), will need a… SP3T switch?

  • CUS-13TB Nidec Components Corporation | Switches | DigiKey; Surface mount

  • SLW-1516255-6A-RA-D CUI Devices | Switches | DigiKey; Through hole

POWER ANALYSIS

As explained in the power-supply section, the 5V linear regulator should output a maximum of 5.9mA.

• INA381 (current sense chip): 450uA

• Threshold Current Dividers and pull-up resistors: max 2mA

• Comparator Quiescent Current: max 50uA

• LEDs: 5.9-(the above) = 4.4mA max

  • As only 2 LEDs are needed, and only one should be powered on at once, the 2mA LEDs should work great being powered off of the 5V regulator.

COST ANALYSIS

For buying every component (not including the majority of passives or the XT connectors) at the per-unit price, the cost is $7.22 per board. For reference, OTS is $12.99/board. This seems pretty reasonable.