Best non inverting driver circuit for linear mosfet

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DBTech7

Joined Apr 21, 2022
30
I'm building a remote starter for an old diesel vehicle and I've settled on using a mosfet for switching the starter/ignition on and off mostly due to pricing and availability. I've spent all morning searching for a suitable circuit example to drive the selected mosfet (CRMICRO CRSS052N08N) but I cant decide on the best and safest way to switch the mosfet.

I have 12v power from the vehicle battery available and the microcontroller is an esp32 which uses 3.3v HIGH signals (esp32 datasheet specifies appx 2.6-2.7v for HIGH output voltage)

Since this will be controlling a vehicle starter motor I would prefer it to be non inverting for some added safety so using another low level bjt/mosfet isn't viable

These will be manufactured in limited batches on a regular basis so keeping circuit as simple and cheap as possible is preferable

I'm leaning on the opamp side which seems to be how large mosfet drivers work if I'm correctly understanding what I've read about them, but the issue I have here is Im not sure which datasheet values Im looking for to select an appropriate opamp for the selected mosfet (CRMICRO CRSS052N08N)

Any help or even general advice is appreciated!
 

crutschow

Joined Mar 14, 2008
29,785
Since you are just switching the MOSFET on and off very infrequently, you don't need an elaborate driver.
I would use a NPN transistor driving a high-side P-MOSFET.
You can't readily use an N-MOSFET to drive a plus voltage to the ignition/starter.

How much current does the starter/ignition require?

Don't you need one MOSFET for the ignition and one for the starter?
 

Thread Starter

DBTech7

Joined Apr 21, 2022
30
Since you are just switching the MOSFET on and off very infrequently, you don't need an elaborate driver.
I would use a NPN transistor driving a high-side P-MOSFET.
You can't readily use an N-MOSFET to drive a plus voltage to the ignition/starter.

How much current does the starter/ignition require?

Don't you need one MOSFET for the ignition and one for the starter?
My thoughts exactly, and I actually just realized that I should be using a P-MOSFET instead of an N-MOSFET as I was looking at the starter diagram for the vehicle and found the starter engages on the high side of the solenoid.

Its a strange old school system where the ignition switch is what powers the starter solenoid which draws about 40A, pre OBD2 gm vehicles were notorious for burning ignition switches because they didn't add a separate relay to control the starter until 96. The lack of a starter relay is why I need to switch such a high current for short bursts.

The ignition draws around 40A as well on a separate wire and fuse from the starter kicker wire

They will both be on separate MOSFETS but the circuit and operation will be identical other than STARTER getting max 20 sec engagement for maybe 10-20 times max each day and the IGN will be engaged for about 15-20 mins at a time for 10-20 times max each day.

I've attached the diagram of the starter wiring and how it operatesUntitled.png
 

Thread Starter

DBTech7

Joined Apr 21, 2022
30
Here is the ignition switch diagram as well, the PINK wires will all receive power from the IGN mosfet and the PURPLE wire from the START mosfetUntitled.png
 

Thread Starter

DBTech7

Joined Apr 21, 2022
30
Since you are just switching the MOSFET on and off very infrequently, you don't need an elaborate driver.
I would use a NPN transistor driving a high-side P-MOSFET.
You can't readily use an N-MOSFET to drive a plus voltage to the ignition/starter.

How much current does the starter/ignition require?

Don't you need one MOSFET for the ignition and one for the starter?
This is the circuit I came up with based on similar examples, my only addition is the led to indicate on or off status. I dont see anything obviously wrong with it but a 2nd opinion is always helpful!1.png
 

LowQCab

Joined Nov 6, 2012
2,045
I don't think that You recognize the brutality of the Physical-Environment or the Electrical-Environment.
A TO-266 package just "ain't-gonna-cut-it", regardless of the very optimistic manufacture Specifications.

Something in a TO-247-Package might be doable, with a large Heat-Sink.
But something like this would be more appropriate .........
https://www.digikey.com/en/products...3586402?s=N4IgTCBcDaIJIA0AqA5MBGADABS0kAugL5A
Then, to insure that it is never in any state other than "On" or "Off",
it will need a proper driver with a Logic-Level-Input,
such as this ....... ( available in inverting or non-inverting configurations ).
https://www.digikey.com/en/products/detail/ixys-integrated-circuits-division/IXDD630MCI/2623250
.
.
.
 

crutschow

Joined Mar 14, 2008
29,785
Doesn't D1 1N4007W have to be able to carry the starter current when Q2 turns off?
Yes, it would have to carry the current for a short period until the inductive energy has dissipated.
The 1N4007 has a 30A surge rating for 8.3ms, so likely is marginal for the task.
 

Thread Starter

DBTech7

Joined Apr 21, 2022
30
Yes, it would have to carry the current for a short period until the inductive energy has dissipated.
The 1N4007 has a 30A surge rating for 8.3ms, so likely is marginal for the task.
I'll take a look at some other diodes and see if anything available allows for a bit more headroom just incase
 

eetech00

Joined Jun 8, 2013
3,150
This is the circuit I came up with based on similar examples, my only addition is the led to indicate on or off status. I dont see anything obviously wrong with it but a 2nd opinion is always helpful!View attachment 265626
You should move R1 to the junction of Q1/R2 and change its value to about 10k.
Also, might want to re-think the LED circuit (the mosfet isn't fully off).
See below

1650688255057.png
 

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Thread Starter

DBTech7

Joined Apr 21, 2022
30
You should move R1 to the junction of Q1/R2 and change its value to about 10k.
Also, might want to re-think the LED circuit (the mosfet isn't fully off).
See below

View attachment 265633
Wouldn't that create a voltage divider circuit, and if that's the case than would it create a problem with Q1?

Would there be a better way to implement the led? Or is flipping it like in your diagram the best way?
 

crutschow

Joined Mar 14, 2008
29,785
Below is my LTspice simulation of the circuit:
I added some 15V Zener diodes to protect the MOSFET gates from the high spike voltages that can occur in a vehicle power bus from things like the starter disengaging.
Most MOSFET gates can tolerate no more than about 20V.

1650690223017.png
 

eetech00

Joined Jun 8, 2013
3,150
Wouldn't that create a voltage divider circuit, and if that's the case than would it create a problem with Q1?
Actually, the way you have it connected creates a voltage divider. The voltage at the base of Q1 will be 3.3/2 since both resistors are of equal value. Moving the resistor to the base of Q1 and increasing its value ensures the transistor will be off.

Would there be a better way to implement the led?
You could connect the LED the way crustchow has it in post #13.
I'm not sure the LED provides any real value.

Or is flipping it like in your diagram the best way?
The circuit is functionally the same as yours, except for the move of R1. Your drawing was drawn somewhat backwards. I flipped the circuit around so it would be easier to understand and follows standard electronics drafting practice.
 
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crutschow

Joined Mar 14, 2008
29,785
In case you are not aware, a few standard electronic drafting guidelines are:
  • Single flow left to right
  • Plus supply on top
  • Negative supply on bottom
  • Ground symbols with pointed end on bottom
  • Transistors with current flow from top to bottom.
 

Thread Starter

DBTech7

Joined Apr 21, 2022
30
Below is my LTspice simulation of the circuit:
I added some 15V Zener diodes to protect the MOSFET gates from the high spike voltages that can occur in a vehicle power bus from things like the starter disengaging.
Most MOSFET gates can tolerate no more than about 20V.

View attachment 265636
The diode placement makes sense, I'm calling it a night for tonight but I'll make the adjustments in the morning and hopefully I can get onto the pcb. I appreciate your simulation and circuit diagrams, thank you!

I appreciate the guidelines as well, I cant really say what compelled me to design it backwards like this, maybe I haven't been taking enough breaks...o_O
 

Thread Starter

DBTech7

Joined Apr 21, 2022
30
I also suppose the led isnt really neccesary, Its just to have a visual indicator during testing but there really is no point in that. Im dumb XD
 

eetech00

Joined Jun 8, 2013
3,150
Here's an alternate circuit using automotive mosfets.
M2 has a max VGS(th) of 2.5v so should be OK, and will require much less drive current than an NPN.

1650733448648.png
 

eetech00

Joined Jun 8, 2013
3,150
Thank you for this, so If Im understanding correctly ltspice is an eda software that seems to incorporate some type of circuit simulation?
Yes. It is a free "spice" circuit simulator. Many members here use it as it is a very useful and reliable tool. It does have a bit of a learning curve though...
 
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