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I’m trying to find a mosfet that will let me switch a 12-14V source from a car battery to ground using a 3.3V micro controller with minimal resistance added. I believe I understand that the gate voltage needs to be 3.3 V or less and it needs to be an n mosfet, but I’m not really sure what I need apart from that when I look at specs on component websites. Any help is appreciated. Let me know what other info is needed because I don’t really even know. My electrical skills are almost exclusively with automotive applications, so MOSFETs are pretty far out of my realm.

The goal here is to replace the ignition functions of my engine computer (which has failed) with a secondary computer. I will be putting resistors into the normal circuit to act as dummies to fool the original computer into thinking it is still hooked up. I’m then planning on driving 9 components with the mosfets: 6 fuel injectors and 3 ignition coils. If I remember correctly these injectors are around 12 ohms and coils .8 . The positive side comes from the battery through a relay and fuse goes to the injectors and coils. Then from the coils and injectors I’ll go through the mosfet to ground. I’ll try to draw a schematic of what I am going for in a bit. Mosfet seemed like the way to go but if there’s a better option let me know. I need something that switches very quickly and isn’t too expensive or likely to fail.

Honestly I may need to do two different setups: one for the injectors where the amps are gonna be decently low and one for the higher amp coil circuits. Just needs to be fast switching and would like it to be as simple as possible.example schematic

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  • \$\begingroup\$ It reads like you want to short the battery out so please explain in more detail what you are trying to do. \$\endgroup\$ – Andy aka Mar 23 at 17:15
  • \$\begingroup\$ Maybe a schematic? Auto circuits are commonly ground-switched so a N-ch MOSFET may not be the best choice, depending. \$\endgroup\$ – vir Mar 23 at 17:16
  • \$\begingroup\$ Edited to include more info about what I am trying to make. \$\endgroup\$ – Zsn0w Mar 23 at 17:52
  • \$\begingroup\$ Updated with simple example schematic \$\endgroup\$ – Zsn0w Mar 24 at 20:44
  • \$\begingroup\$ "My electrical skills are almost exclusively with automotive applications, so MOSFETs are pretty far out of my realm." Huh.. automotive applications use MOSFETs everywhere. \$\endgroup\$ – Lundin Mar 25 at 12:36
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Your question seems to be about how to drive N-MOSFETS with signals coming from a 3.3V micro-controller.

Most usually, MOSFET are driven so they’re either ON with a very little resistance between drain and source (noted Rds), or they’re OFF, with an almost infinite Rds. To be ON, a N-MOSFET needs the Gate voltage to be higher than the Source voltage. Voltage between Gate and Source is noted Vgs.

How high Vgs needs to be for a N-MOSFET to turn ON?

  • By built, each N-MOSFET has a threshold value, noted Vgsth. If Vgs is below Vgsth, then the N-MOSFET doesn’t let pass any amount of current between Drain and Source. In other words, the resistance Rds is infinite.
  • If Vgs is just above threshold, the N-MOSFET is not totally ON. It may let pass some current, but Rds can still be non negligible. In this condition the N-MOSFET would dissipate a lot of power and possibly burn.

The characteristic of Rds and Vgs is to be found in transistor’s data sheet. Often they show the Admittance rather than directly the Rds. This is an example from IXTA80N10T:

IXTA80N10T - Admittance vs Vgs

The Vgsth of this model is between 2.5V and 4.5V, and it is where the curves starts, but to really turn ON the transistor you better be close to 6V.

This poses an obvious problem when your micro-controller can only output 3.3V, but before solving this, you have to decide if your N-MOSFET is going to be in High Side or Low Side configuration:

High Side and Low Side configurations for N-MOSFET

Depending on your configuration:

  • If the N-MOSFET is placed at Low Side, then you only need to find a way of amplify the 3.3V control signal to a voltage high enough to turn on the N-MOSFET.
  • If the N-MOSFET is placed at High Side, then you need a voltage that is higher than the power source.

The circuits to solve these problems are called “Low Side Driver” and “High Side Driver”, respectively:

  • The easiest solution is to buy an integrated circuit to perform the task, particularly for the High Side Driver.
  • Some integrated circuits contain several High or Low Side Drivers in one package. Some, even, contain one High and one Low.

To correctly select the appropriate High or Low Side driver, pay attention to:

  • Maximum voltage rating - 14V is way below breaking point of most of those drivers.
  • Logical levels - most are compatible with both 3.3V and 5V.
  • Minimum voltage cut off - Some drivers will stop driving if the power source goes below a value.
  • Maximum switching frequency - compare it with your requirements.
  • Particular points for High Side Drivers:
    • Minimum switching frequency - The High Side Driver uses a charge pump to create the higher voltage, but this can only work with some switching.
    • Vgs protection - Without specific protection, a High Side Driver will typically produce a Vg that is twice the value of Vcc. In this condition, the Vgs will be as high as Vcc. The maximum rating of Vgs is usually around 20V. So, if your Vcc is above 20V, you need this kind of protection.

There are two families of drivers:

(I haven't checked if any of those examples are appropriate for your particular application)

Also, as Lundin mentions in his comments, "there's a special category referred to as "smart" high side drivers. These have all manner of protection mechanisms built-in, to make them extra rugged (undervoltage, overvoltage, overcurrent, overheat etc). And specifically designed for 12V or 24V applications".

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    \$\begingroup\$ Nice answer! I might add that for automotive in particular, there's a special category referred to as "smart" high side drivers. These have all manner of protection mechanisms built-in, to make them extra rugged (undervoltage, overvoltage, overcurrent, overheat etc). And specifically designed for 12V or 24V applications. Infineon has a very dominant market position for these, close to monopoly after the merger with IR. \$\endgroup\$ – Lundin Mar 25 at 12:51
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    \$\begingroup\$ So in this case, maybe these. It's what Infineon claims to be the latest and greatest. Big disclaimer though, Infineon is perhaps the least reliable silicon supplier in the world - they have massive supply chain problems right now and that's not an unusual state of affairs for them. \$\endgroup\$ – Lundin Mar 25 at 12:56
  • \$\begingroup\$ Jmgonet and Lundin, thank you for the information! I need to switch the circuit on the low side, so if I’m understanding this correctly, I need to put a low side driver in to change the switching voltage to 12 and then connect that to a mosfet to switch the higher amps? Or are there drivers that would work by themselves without a mosfet? \$\endgroup\$ – Zsn0w Mar 25 at 15:09
  • \$\begingroup\$ @Zsn0w I'm not quite getting if you are doing a PCB design or if you are just repairing a car. In case of the latter, you should probably just go for solid state relays, which are essentially MOSFETs in a convenient package. As for the kind of parts I linked, there's countless versions of them with varied currents. They may not necessarily be all that fast at switching though... depending on how you define "fast". \$\endgroup\$ – Lundin Mar 25 at 15:43
  • \$\begingroup\$ @Lundin I am trying to repair my car by bypassing the built in engine computer’s ignition functions (which have been damaged) with my own secondary microcontroller that will switch the injectors and coils through these components. I’ll need to be able to switch coils on and off very quickly. I’ll be working in the range of milliseconds for timing control, so I need switching in the microsecond range I think. I’m going to try to dig up all my research I did on this previously to be more specific with time requirements. \$\endgroup\$ – Zsn0w Mar 25 at 19:14

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