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

[bountyhunter]

Here's a good P channel FET.

IPP100P03P3L-04

30V, 100A, .004ohm on resistance. Tab is not isolated though. $2.64

Wow, that's a whole lot of FET for the money.

based on experience, I am a little leery of using a 30V FET in an automotive app because of the voltage transients that cruise around the so called "12V" line in a car.

 

[Hero999]

It has a huge gate capacitance though, 9.3nF maximum.

Good luck getting it to pass 100A when mounted on a PCB.

 

[PedroDaGr8]

It has a huge gate capacitance though, 9.3nF maximum.

Good luck getting it to pass 100A when mounted on a PCB.

Ugh, there is always a catch. I really should just make the design and go from there. Stop worrying so much about looking like a fool and see if

You mean their quote of 100A is functionally impossible in this package or that I would need to mount it in some particular way (i.e. use wires directly to it)? If it's the latter, if I know what the way is, that isn't a problem. Provided it isn't to get it to pass 100A you have to mount it on a Buick running 100 miles an hour backwards through a corn field in the snow.

Also, I figure I'll re-ask the question I asked earlier. WHen dealing with negative voltages for the P-type, would I treat the ground as -12V or is it zero? This is confusing me a bit.

 

[Speakerguy]

100A will toast circuit traces very quickly, unless the board has very thick copper and uses power planes. Even then... it's a huge amount of current. Not that it's not done, it's just a lot.

You could always just point-to-point wire it. No need for a board in something this simple.

 

[PedroDaGr8]

I had planned on using wire connection on perfboard, just to hold thing in place as vibration will be a problem. I hope this is acceptable.

Alright, lets see if my design is anywhere close to right. Prolly not but hey, its my first schematic. Critique away and critique hard, please also double check my designation of S,D,G on the P and N MOSFET's. I know for example I odn't have a resistor, not sure how that was supposed to be used. I just hope I didn't embarass myself TOO badly.

 

[PedroDaGr8]

Hmm, guess either everyone is busy or I've asked to many questions.

 

[bountyhunter]

I would recommend a 5.1K resistor from the gate of the P-FET to 12V to make sure it stays off until turned on by the smaller N-FET.

 

[Speakerguy]

I would change that up a bit; I'd use a 5.1k or 10k (or whatever you have) as a pull-down on the gate of the N channel fet to make sure it stays off unless the signal goes high. Then use a 100 ohm power resistor for the pull-up to 12V on the gate of the P channel FET. With a big gate capacitance, you're going to want to allow for more current to turn the P channel FET on quicker. A 3-5W 100 ohm resistor should do the job.

 

[PedroDaGr8]

OK, for the N-MOSFET how many amps should I be looking at? Judging by the wattage you selected for the resistor I am assuming max would be 0.2A.

N-MOSFET

Would that be acceptable?

Also, the 3 watt resistor, will that produce a LOT of heat? I was hoping to pot this inside of a small box (with some sort of heatsink for the P-MOSFET) to make it water proof.


Also here is the updated schematic.

 

[Hero999]

What frequency are you switching at?

You also need a pull-up resistor.

Why are you using a JugFET which needs a negative voltage to turn off?

A BJT will do.

3W is really overkill, even if a 100R reistor was placed directly acorss the 12V supply it would only dissipate 1.44W. I would just use a standard ¼W resistor but there's no harm in using a resistor with a higher rating.

 

[PedroDaGr8]

No frequency. Just turn the headlights on, the normal headlight 12V will turn this circuit on to use the P-MOSFET to switch on the power to the ballasts. Just like an automotive Bosch style relay (which I explain in the first post are have some unacceptable drawbacks in my situation). The time between turn on and turn off will be quite long. The shortest amount of time will be when one flashes the high beams ie flash-to-pass.

EDIT: What is a jugFET, I am honestly quite new to this and am trying really hard to understand the WHY of things. Is the n-MOSFET I chose the jugfet? If so I'll try to find another one. Transistors are SOOO confusing. Does anyone have a textbook or similar type of text that I could read to start understanding the practical side of transistors, not just the theoretical. It seems like the non-ideals are the most important thing.

 

[Hero999]

No frequency. Just turn the headlights on, the normal headlight 12V will turn this circuit on to use the P-MOSFET to switch on the power to the ballasts. Just like an automotive Bosch style relay (which I explain in the first post are have some unacceptable drawbacks in my situation). The time between turn on and turn off will be quite long. The shortest amount of time will be when one flashes the high beams ie flash-to-pass.

That makes things easy.

EDIT: What is a jugFET, I am honestly quite new to this and am trying really hard to understand the WHY of things. Is the n-MOSFET I chose the jugfet?

You used the symbol for a JugFET, a cheap BJT such as a PN2222A, BC337 or BC548 will do.

If so I'll try to find another one. Transistors are SOOO confusing. Does anyone have a textbook or similar type of text that I could read to start understanding the practical side of transistors, not just the theoretical. It seems like the non-ideals are the most important thing.

See Wikipedia for an explanation of how a JugFET works.
JFET - Wikipedia, the free encyclopedia

 

[Speakerguy]

Here is a schematic.

 

[Speakerguy]

You could also do away with the N channel and resistors, and just use an inverting MOSFET driver instead.

Digi-Key - MCP1406-E/P-ND (Microchip Technology - MCP1406-E/P)

 

[Hero999]

sorry I was going to post a schematic too.

 

[Speakerguy]

Hero, you forgot to edit the small error in the symbol for his pfet

Pedro, both our schematics will work fine. Mine will dissipate more power as heat but will turn on faster. If you find that's not an issue in the real system, you can change the 100ohm 3W resistor in my schematic to 1k 1/4W like Hero's.

 

[Hero999]

Corrected.

Also I've just noticed the 100R isn't required any more.

 

[bountyhunter]

I can't figure out why you would need that honking 3W pull up resistor that's going to get so hot. If the FET is being used like an ON/OFF switch, the turn on time is not critical at all. If it takes ten milliseconds to turn on or off, who cares?

 

[bountyhunter]

Corrected.

Also I've just noticed the 100R isn't required any more.

On the schematic shown, I would add a pull down resistor from the base of the NPN transistor to ground. Something like 10K to 20K would be enough. It will keep the transistor off until it is forced on.

 

[Hero999]

That depends on the signal source, if it's a microcontroller or similar digital circuit then I wouldn't bother, if it's a switch then I might depending on how noisy the environment.