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I need a one-button on/off toggle power latch to connect/disconnect a 36V battery to two VESC-controlled 500W motors with regenerative braking.

Most of the designs I found for soft latching circuits make use of a high-side P-channel MOSFET which have a too high On-resistance for my use-case. Also, I didn't found a design which blocks the generated power from the motor on off-state, due to the latch feedback and/or body diode.

So I tried to design a circuit with the following features:

  • N-MOSFETs for low Rds_on because of high currents (around 30A)
  • Bi-directional switching to account for the load/generator states of a motor
  • Separate latching feedback
  • Fully analog, no microcontroller
  • One-button on/off with default state off

I came up with the design below. It seems to work, but I'm not sure why the current in the generative state is halved during button press when switching off. I'm also not sure about the crappy motor modeling...

Do you think this would work in reality? Any hints or tips for improvements?

Powerlatch_n-fet_lowside_bidirectional

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  • \$\begingroup\$ Personally, I would have the switching element be a relay if you can afford the coil's power draw. \$\endgroup\$ – Hearth Dec 30 '20 at 17:25
  • \$\begingroup\$ Realistically: don't. Use the enable controls of the motor drivers themselves, backed up by a fuse and a physical disconnect for when you need a safety lockout. \$\endgroup\$ – Chris Stratton Dec 30 '20 at 18:29
  • \$\begingroup\$ The problem with hard-switching are the high peak-currents caused by the motor-controllers capacitors on their supply rails. You can hear the sparks when plugging the battery via connectors (XT90). This would degrade capacitors, battery and mechanical switches lifespan. \$\endgroup\$ – Azial Dec 30 '20 at 19:50
  • \$\begingroup\$ Another problem with mechanical switches are their high on-resistance. I measured 150mΩ at a 12V, 20A rated switch. I find similar resistance ratings when searching for relays. \$\endgroup\$ – Azial Dec 30 '20 at 20:20
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So, I needed to re-design the circuit to fix two issues: The step in current as described above, and a false default on for lower voltages of V1.

enter image description here

Edit: R10 should be 560kΩ to make sure the circuit works with button presses between 0.2...2 seconds.

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    \$\begingroup\$ It's not clear if this is an answer to your own question or an update. If it's an update then you need to move the content of this post into your question and delete this post. If it is the solution then you should state that in this answer and "accept" the answer when you are allowed - usually after some hours or a day. \$\endgroup\$ – Transistor Jan 1 at 16:28
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    \$\begingroup\$ As the second post solves the issue from the initial design, I posted as as an answer. If I come up with more improvements I would edit the answer. \$\endgroup\$ – Azial Jan 1 at 16:39

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