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I have started a new project, an automatic firing system for fireworks, powered by 2 lead-acid batteries (12V). The batteries are connected in series with a common ground (GND), so the power distribution board has 3 power terminals (+12V, GND, -12V).

The firing system will include more firing channels controlled by a STM32 board, so all the MOSFETs used will be controlled by 3.3V signals. Also each channel will have 2 outputs/terminals, NEGATIVE and POSITIVE.

Because of the project requirements, the IGNITION_POS and IGNITION_NEG should be able to handle 4 different states based on the 2 switches (STM32 GPIOs):

  • Both terminals ON -> IGNITION_POS: +12V and IGNITION_NEG: -12V
  • Both terminals OFF -> IGNITION_POS: GND and IGNITION_NEG: GND
  • Positive terminal ON -> IGNITION_POS: +12V and IGNITION_NEG: GND
  • Negative terminal ON -> IGNITION_POS: GND and IGNITION_NEG: -12V

I'm unable to find a solution to switch terminals to GND when the state of the MOSFETs is OFF. Also i want to specify that the current through terminals can sometimes be more than 30A, so we can't use a simple pull-down resistor.

For simplicity i attached a schematic which includes simple ON/OFF switches, not the actual STM32 GPIOs.

Can you help me with some suggestions to achieve this behavior ?

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Just to be clear, when "INGITION_POS" [sic] is tied to ground, it is expected to be able to source the current from BATT2, which is then returned to "INGITION_NEG" through the load? \$\endgroup\$
    – Dave Tweed
    Oct 7, 2023 at 15:00
  • \$\begingroup\$ Given this is an automated firing system for fireworks, what are the Functional Safety requirements to avoid inadvertent ignition? \$\endgroup\$ Oct 7, 2023 at 15:26
  • \$\begingroup\$ @DaveTweed You are right, this is the main idea! The circuit needs to be able to deliver the 30A current in any state, except the state 2 (when both terminals are tied to GND). \$\endgroup\$ Oct 7, 2023 at 16:06
  • \$\begingroup\$ @ChesterGillon One of the hardware requirements is to tie both terminals to GND when the device is in Disarmed state. Another requirements are programmed at the software level. \$\endgroup\$ Oct 7, 2023 at 16:09

1 Answer 1

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For transferring signals from STM use 4 opto-couplers.
To pulling terminals to GND use mosfets M2 and M3.
In firmware always turn on M2 delayed after M1 turns off (also other changes of mosfet states) otherwise mosfets get shot-through. Another words use dead-times.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Thanks! This looks like exactly what kind of circuit i need. But i just need to turn the MOSFET using only 2 GPIOs. So i adapted your schematic, i removed the optocoupler for simplicity. My question is ... will be there a short circuit for short period of time because the power MOSFET is not fully OFF when the GND one turns ON ? Updated schematic: circuitlab.com/editor/#?id=wv7zeph6t784 \$\endgroup\$ Oct 7, 2023 at 22:40
  • \$\begingroup\$ A little short will occur since turning off of mosfet is more slow than opening. Maybe you can find a driver IC for such a purpose (with programmable dead-time). \$\endgroup\$ Oct 7, 2023 at 23:05
  • \$\begingroup\$ So after some tests i ended up using 4 GPIOs to control the MOSFETs, because i need 2 more possible states: State 5 -> IGNITION_POS: +12V and IGNITION_NEG: OFF, also State 6 -> IGNITION_POS: OFF and IGNITION_NEG: -12V (OFF means not GND, but both MOSFETS of one side turned OFF). The issue now is: when IGNITION_POS = +12V and negative side MOSFETs are OFF, the negative side MOSFET that has the role to tie to GND(M3) turns ON to. Updated schematic & simulation: circuitlab.com/editor/#?id=wv7zeph6t784 \$\endgroup\$ Oct 8, 2023 at 14:20
  • \$\begingroup\$ It is probably a current flowing through parasitic diode of M3 (body diode). The diode is still presented even the mosfet is off. \$\endgroup\$ Oct 8, 2023 at 15:28
  • \$\begingroup\$ Do you have any suggestion to prevent this happening ? \$\endgroup\$ Oct 8, 2023 at 15:42

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