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Is this an acceptable way to drive a mosfet? In this case, I'm trying to turn on a high current load, but in case the load is changing it's resistance/impedance, or in the possibility it's disconnected altogether, I still want the mosfet to turn fully on and not have Vgs be dependent on the load having a proper ground connection.

In this circuit, I'm using a 1K Ohm resistor in parallel with my load to ensure the mosfet when receiving gate drive voltage is always turned on, and outputting a voltage.

For context, this is going to be part of an automotive circuit where it's not uncommon that during production, damage can occur resulting in a disconnected load or short to ground. My end goal is to make this MOSFET perform nearly exactly like a relay. Link to the circuit below.

Apologies if this answer is facepalm levels of easy. I've never seen this method done anywhere else on the sitefor using High-Side N-Channel controlling, I'm not even really sure what I would search to find a circuit like this. MOSFET N-Channel High-Side Bypass Resistor?

Circuit

Picture of circuit

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  • \$\begingroup\$ I think you underestimate the difficulty of driving Ciss hard with a 1mOhm switch @ 40kHz \$\endgroup\$ Mar 1 '21 at 17:49
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 see my comment at the very bottom. Meant to post it up here. Primarily these will not be pwm driven, just one-off. \$\endgroup\$ Mar 1 '21 at 18:40
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Is this an acceptable way to drive a mosfet?

Quite possibly not acceptable - you have to ensure that the gate drive voltage can fully turn on the MOSFET and, because the MOSFET source is expected to reach the same voltage as the +14.5 volt supply, the gate drive voltage needs to be several volts above that (anything between 4 volts and 10 volts higher).

So you might need a gate drive voltage of around 20 volts. Then that comes with a bit of a price because if the source load is shorted out, you might exceed to gate-source voltage of the MOSFET. A lot are specified to be +/- 20 volts but, that's right at the limit.

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  • \$\begingroup\$ Absolutely. I intend to use a gate driver to get the gate voltage well above Vds. I intended to just have a 24V rail for this purpose and use Optoisolators, but I've realized that I can't do any kind of short protection through that method, so a gate driver is necessary. But yeah Vgs will be well above the Vds of the fet \$\endgroup\$ Mar 1 '21 at 15:40
  • \$\begingroup\$ @ColbyJohnson you'll have to choose a MOSFET with high enough Vgs voltage if the output becomes shorted even momentarily. There are not that many above 20 volts (as mentioned in my answer). \$\endgroup\$
    – Andy aka
    Mar 1 '21 at 15:52
  • \$\begingroup\$ Theres some mosfet drivers out there that have short detection which I'd hope would be able to protect against overloading the gate. Do you see any problems with the bypass resistor? I've seen a ton of methods of driving High-Side N-Channels before but never anything as simple as just giving the source a secondary path to ground. I can't think of any reason it wouldnt work. \$\endgroup\$ Mar 1 '21 at 16:04
  • \$\begingroup\$ I don't see the resistor as a problem. The gate-source junction can be destroyed with over-voltage in nano seconds. \$\endgroup\$
    – Andy aka
    Mar 1 '21 at 16:05
  • \$\begingroup\$ Gotcha, I'll have to make sure that that can't happen. Thank you for the answer! \$\endgroup\$ Mar 1 '21 at 16:06
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These parts require a high side switch with a RdsOn < 1% of the load or 5mOhm which does exist. The load power is ~300W steady state so if it is inductive the DCR may be much lower (50mOhm) (3kW) so suitable heatsink may be necessary to dissipate 1% and be within the SOA (A-s limit).

  • a high side switch or Pch requires 0V for gate voltage, so you would not use an Nch unless you have a charge pump for 2.5x Vgs(th) bias.
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this is going to be part of an automotive circuit where it's not uncommon that during production, damage can occur resulting in a disconnected load or short to ground.

If you look into the "high side load switch" category on mouser or digikey you will find very useful ICs which contain the MOSFET and the charge pump driver, plus all the protection circuits it needs to survive short-circuits and other unfortunate events. These protections are quite necessary and not trivial to design in a way that is cheap and robust.

Some also have a feedback pin to tell your microcontroller if the load is actually connected and drawing current.

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  • \$\begingroup\$ I will check these out. Thank you Bob! The end goal is to set up a bunch of these circuits and leave them open to different types of loads. A fuse panel but with mosfets as the relays. \$\endgroup\$ Mar 1 '21 at 16:19
  • \$\begingroup\$ The 40 khz was more just to show a good demonstration of flow while connecting and disconnecting the chassis load (0.5ohm resistor). These will be primarily driven with no PWM and a simple on off signal. I'll have to figure out some way to drive them with a gate driver so I can take advantage of their built in short circuit protection. \$\endgroup\$ Mar 1 '21 at 18:38
  • \$\begingroup\$ If you get a high side load switch, the gate driver is already included in the chip. These take a simple logic input and do all the work. \$\endgroup\$
    – bobflux
    Mar 1 '21 at 18:42
  • \$\begingroup\$ Found a very nice one, BTS700151ESPXUMA1. Suitable for up to 27.6A nominally. I'll have to do some back and forth and decide what to do, but I had no idea these were even a thing. Thank you for spreading the knowledge around! \$\endgroup\$ Mar 1 '21 at 19:06
  • \$\begingroup\$ If you have lots of amps, using low RdsON switch saves a heatsink: digikey.ca/en/products/detail/stmicroelectronics/VN7004CHTR/… \$\endgroup\$
    – bobflux
    Mar 1 '21 at 19:38

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