# Can I use a mosfet to switch roughly 30A @ 12-15V

Status
Not open for further replies.

#### PedroDaGr8

##### New Member
OK, I am trying to figure this one out. Partly, it is giving myself a project to learn new things (I dunno why but I suck at learning with out a focus), partly to figure out an alternative to mechanical automotive relays (yeah I know Hella and others make solid state relays but they tend to be reallly reallly pricey at this amperage and others seem to be more for DC switched AC). I know a good amount about relays and designing odd schematics using them (i.e. a circuit that flashes the halogens but not the bix when the headlgihts are off, or one that flashes halogens when xen are off but bix when they are on) etc. I know none of that is impressive to you all, but I was happy with myself when I came up with it.

So anyways, the idea is to design a circuit for driving halogen and/or HID lamps, that gets rid of the electromechanical relay (meh relaibility and off-roaders have a problem with them getting knocked loose in rough terrain). The current requirements are technically 20-30A but I figure it's easier to design large and scale down if needed, versus design small and scale up. If it matters, HID ballasts start at a larger current (usually for a few µSec) then settle down after around 40sec or so to the constant 3A.

For those that want a clearer description, I want something to act as a solid state switch, that provides power when the signal is on, turns off when the signal is off. The signal as of now will be 12-15V as well, though this can of course be reduced as needed.

If anyone can point me in the right direction to start my reading I would greatly appreciate it. Right now I am getting a bit overwhelmed and am unsure what all of these different things mean and how they would apply to me. I am just trying to gain my footing in this and most importantly learn more. I don't want anyone to babystep me through this (then again babysteps to me and babysteps to you maybe two different things). I just want assistance and I do the rest.

I hope that I have been clear enough and don't get flammed for being too much of a noob.

#### Speakerguy

##### Active Member
There are many many mosfets which will do that. You want an N-channel MOSFEt with a Vds of at least 20V (if not 30 or 40) and a current rating of 50A or more. This should not be all that expensive and probably won't even require a heatsink, but if it gets warm you can put one on. This one is fully insulated so you can just bolt it on to any piece of metal to keep it cool. Apply signal to gate, connect drain to ballast (other side of ballast to voltage supply), and connect source to ground. If you are looking at it from the front the pins are Gate - Drain - Source in that order.

Digi-Key - IRLI3705NPBF-ND (International Rectifier - IRLI3705NPBF)

FWIW the 12-15V signal you are providing is just about ideal for driving the gate of a standard MOSFET. Makes this whole thing a lot easier.

Last edited:

#### bountyhunter

##### Well-Known Member
If you can give a more detailed description of the load profile it would be easier to recommend a circuit. Voltage, current, pulse width, switching time, etc.

I thought that HID lights needed a special high voltage power supply to operate them. Halogens do not, they can run off 12V.

If you want to use N-FETs as series switches in the power line you will need to generate a floating rail which is about 15V higher than the system's 12V line to drive the gates from. You could use a charge pump circuit, the gates require almost no current to enable but you do need the higher voltage..

Last edited:

#### RCinFLA

##### Well-Known Member
The primary attibutes for MOSFET's you want to look at is Vds breakdown voltage, and Vgs versus drain-source on-resistance.

The on-resistance at the current you want to pass will determine how much heat sink you may need and how much voltage drop there will be to light.

If this is a 12v auto use, and if you can switch the ground side of the light, you can use and N-ch. If you have to switch the positive leg, then use a P-ch. Having 10-12 volts of Vgs drive makes it pretty easy to find either N-ch or P-ch with a good low R-on. A 50 or 60 Vds breakdown spec'd part should be fine for switching 12v line.

Focus on Rs-on, for 30 amps you should be looking for something less then 5 milli-ohms. That resistance would still give you almost 5 watts of heat which is a pretty good sized heatsink.

You can also parallel up devices to lower Rs.

Last edited:

#### Speakerguy

##### Active Member
Focus on Rs-on, for 30 amps you should be looking for something less then 5 milli-ohms. That resistance would still give you almost 5 watts of heat which is a pretty good sized heatsink.
Correct me if I am wrong, but I think the 30A was initial current surge. The way I read the OP the continuous current was only 3A.

thought that HID lights needed a special high voltage power supply to operate them. Halogens do not, they can run off 12V.
I think he is powering the ballast that generates the high voltage AC for the HID lamps.

If you want to use N-FETs as series switches in the power line you will need to generate a floating rail which is about 15V higher than the system's 12V line to drive the gates from. You could use a charge pump circuit, the gates require almost no current to enable but you do need the higher voltage..
True, but unless there is a reason to have the ballasts ground terminal at vehicle ground at all times, a low side switch should work fine. I'm not an expert on car electrical systems though, does using a low side switch cause problems?

Last edited:

#### PedroDaGr8

##### New Member
Haha, this took a while to type as I had to keep rereading to make sure I was even remotely making sense.

I REALLLY REALLLY appreciate your help. Especially on what to look for, I was getting so confused looking at datasheets and not being sure what was important and what wasn't. Also, I am not LIMITED to mosfets, if there is something you think would be more ideal please mention it. They were just the first thing that really jumped out at me as maybe being able to do what I want. Also, if it helps anything, it wouldn't be too hard to put a LDO 12V Vreg (ie. the sharp PQ12RD2) on the signal line.

First, bountyhunter, this is going to power a ballast (when I said HID I meant the ballast which powers the bulb).

Awesome...thanks so much for the information from ALL of you. Yeah I can ground switch these or power switch these no problem. Is one technologically easier to do than the other?

As for the load profile, it will be automotive voltage for both the voltage source and the signal. The amperage varies some: the start-up amperage can get really high. Case in point: A person on the HIDplanet forums used his labs oscilliscope a while back to test out ballasts and their startup current profiles. He tested a Hella Gen III ballast plus a few others IIRC and measured 60-100A for roughly 200µS at startup. After which, it drops to around 10-20A or so and decreases gradually from there for the next 30-40 seconds, where it reaches its steady-state amperage of roughly 3-4A. Supposedly, newer ballasts limit this initial startup amperage (at the expense of a bit longer warmup time), but I want to design for the worst case, not the best. I don't know if that super high start up amperage is something to worry about or not, I do know that a 30A mechanical relay fused at 30A handles two ballasts starting up just fine.

Does the steady-state current of a combined 4A change the Ron recommendation (though I will definately keep it in mind as I may design part of the circuit to run halogen bulbs or bixenon solenoids).

As for the wattage, did you just use the current (30A) and Ron to calculate that?

Lastly, good idea on searching for one that could be mounted.. It's very common to mount electromechanical relays to the cars body, which should serve as a pretty good heatsink. Though, not needing one of course is more ideal, but that's definately lower down on the priority list. I guess the only problem with mounting it to the cars body would be waterproofing. Though that's something I am sure I could come up with if I had to.

#### PedroDaGr8

##### New Member
Correct me if I am wrong, but I think the 30A was initial current surge. The way I read the OP the continuous current was only 3A.
I address this in my previous post. I use 30 to 40A bosch style relays right now, that being said there are situation where I plan on powering halogen.

I think he is powering the ballast that generates the high voltage AC for the HID lamps.
correct

True, but unless there is a reason to have the ballasts ground terminal at vehicle ground at all times, a low side switch should work fine. I'm not an expert on car electrical systems though, does using a low side switch cause problems?
As long as it results in an either on or off state it doesn't matter that much, though there are some ballasts that ground through the body as well as the ground wire so I don't know if this matters. The only other time problems can arise is when the voltage is too low or too high (ie 8V or 20V+V)

#### bountyhunter

##### Well-Known Member
Does the steady-state current of a combined 4A change the Ron recommendation .
The steady state current will define the continuous power dissipation as given by I (sq) R where R is the ON resistance of the FET. However, if the load needs a high surge current, the FET will starve the load since the ON resistance won't allow a high peak to flow through it. In most designs where a load needs a high peak, you set it up so you charge a very large capacitor near the load point before the enable switch turns on the load.

#### bountyhunter

##### Well-Known Member
I address this in my previous post. I use 30 to 40A bosch style relays right now, that being said there are situation where I plan on powering halogen.

correct

As long as it results in an either on or off state it doesn't matter that much, though there are some ballasts that ground through the body as well as the ground wire so I don't know if this matters. The only other time problems can arise is when the voltage is too low or too high (ie 8V or 20V+V)
I thought the ballast grounded through the case for safety reasons.

#### PedroDaGr8

##### New Member
Not all do, though I think ALL do ground the chasis, in some, the ballast will actually turn on if you give power and touch the chasis of the ballast to the cars body.

EDIT: I guess I should account for this.
EDIT2: Somehow I get the feeling this makes things much more complex.

Last edited:

#### Speakerguy

##### Active Member
You can use a P channel MOSFET as a high side switch (on the +12V power line) and ground the ballast if it is required. You will have to invert the on/off signal though (a low voltage will turn the pfet ON and a high voltage signal will turn it OFF). You can use a very cheap N channel fet and a resistor to do the inversion of the drive signal. The P channel FET should be rated to handle whatever surge current there is at startup.

Last edited:

#### PedroDaGr8

##### New Member
OK, sounds good, I'll look around and see what I can come up with. Then post the parts and schematic here before trying it out.

Thanks again for the help everyone. You have helped out greatly.

#### RCinFLA

##### Well-Known Member
On the HID, is the 3A the 12v current or higher voltage current?

#### Speakerguy

##### Active Member
Clearly you can use a P-Fet, but here's a hard lesson in design engineering:
True, P types generally have lower performance than N channel types, but if you look hard enough you can usually find what you want/need. In any case, I was wanting to keep it simple since the OP is just now learning about transistors.

#### PedroDaGr8

##### New Member
Here's a good P channel FET.

IPP100P03P3L-04

30V, 100A, .004ohm on resistance. Tab is not isolated though. \$2.64
Haha, I just came on here to ask about that EXACT fet.

Yeah, I noticed that. The P-channels are more expensive by about double.

I am curious, about why it can't be used on the high side versus the low side. Is it because there needs to be a voltage difference between the signal and the current? EDIT: I should look this up for myself as well because its likely something basic that I am not grasping. So don't bother answering this, I will research it for myself.

Thanks so much again, yeah keeping it simple is a good idea . Though you better believe after I get this one working that's the VERY NEXT thing I am going to look into. Just follow this path to where ever it leads me and hopefully most importantly to loads of knowledge.

Now to look at how to use a FET to invert current. Don't post an answer to this, I want to see if I can figure this out on my own from what I know so far. I'll post my idea up as soon as I get it out of my head.

Last edited:

#### PedroDaGr8

##### New Member
OK, one question that I am trying to noodle out. I think I have it but here it goes: I keep seeing mentions of negative voltages. Do I treat ground as 0V in some situations and say as -12V in other situations? Also, when dealing with voltages in the transistor, I only deal with them relative to each other correct (I guess that's a way of rewording my above question)? Sorry, the more I read the less I feel like I knew.

Status
Not open for further replies.