MosFet IRLB3034PbF - How Low Can U Go?

[Gummy Bare]

What's the lowest resistance coil i could fire on a box mod with a IRLB3034PbF Mosfet chip?

The box will be a dual parallel 18650 box mod. I also have some 26650 battery sleds, so i was thinking of making a dual parallel of that as well.

I'm new to making box mods; so i'm not exactly sure what the limits of the mosfet chip is or what negative effects it could have on the circuit that a mechanical mod would not have. I just have read the mosfet is "safer" and allows you to use any button you want no matter that the DC rating is.



thanks in advance for the help

 

[JohnnyDill]

That MOSFET is rated at 375W so, IMO, the limiting factor will be your batteries.......

 

[WKS01]

That MOSFET is rated at 375W so, IMO, the limiting factor will be your batteries.......

It is rated for 375 watts with an additional heat sink, not as a stand alone package, noted by the condition of T_C, sometimes written as T_mb (Mounting base temperature), equal to 25^oC. Some data sheets determine the max power dissipation when the MOSFET is attached to one square inch of copper on the board and would be noted as such. For the TO220 package I would not go much over 2 watts.

Quick quote for NXP's PDF explaining how to read and use their data sheets: (Page 6 -Understanding power MOSFET data sheet parameters)

"Ptot (total power dissipation) is the maximum allowed continuous power dissipation for a device with a mounting base at 25^oC. The power dissipation is calculated as that which would take the device to the maximum allowed junction temperature while keeping the mounting base at 25^oC"

 

[JohnnyDill]

It is rated for 375 watts with an additional heat sink, not as a stand alone package, noted by the condition of T_C, sometimes written as T_mb (Mounting base temperature), equal to 25^oC. Some data sheets determine the max power dissipation when the MOSFET is attached to one square inch of copper on the board and would be noted as such.

Thanks for the informative post. I am looking at the data sheet for this mosfet right now. http://www.irf.com/product-info/datasheets/data/irlb3034pbf.pdf

 

[Gummy Bare]

Thanks for all the info. The data sheet for the mosfet I'm using seemed to be able to handle all that my batteries could give it, but I also didn't fully understand the data sheet.

I'm gonna read through the document you posted on understanding the mosfet data sheet once I'm back to a computer.

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[WKS01]

Let me preface this by clearly stating I do not claim to have a vast knowledge of electrical analysis. There are also additional factors that can be seen in the data sheets showing how to derate the limits under various conditions. Temperature, voltage, current, along with other factors, can change characteristics.
It is worth mentioning the RDS_on will depend on the voltage. If you look at the IRLB3034PbF data sheet there are two listings for RDS_on, one with V_GS at 10V and another at 4.5V. If your design simply using the battery voltage to trigger the gate, 4.2V, you may experience higher resistance than shown. This higher resistance will lead to more heat dissipation of the MOSFET. Below shows some calculations for determining an estimate for heat dissipation.

Where I= current drain.
​

VGS = 10V	VGS = 4.5V
RDSon = 0.0017 ohm	RDSon = 0.0020 ohm
P=302 * 0.0017	P=302 * 0.0020
P=1.52W	P=1.80W
DIFFERENCE:	0.28W

Now at a 30A current drain both calculated powers fall within the 2W desired for the TO220 package without additional heat sinks. So lets up the current to say 34A.

VGS = 10V	VGS = 4.5V
RDSon = 0.0017 ohm	RDSon = 0.0020 ohm
P=342 * 0.0017	P=342 * 0.0020
P=1.96W	P=2.31W
DIFFERENCE:	0.35W

Now we see that when the MOSFET is triggered by a 4.5V source the higher RDS_on at 34A will cause the MOSFET to overheat. Of course the difference between the wattage increases dramatically as the current is squared. Here is an example at 60A, though I would not recommended running 60A even on a parallel setup. Though theoretically the available amps are doubled, realistically this is not the case.

VGS = 10V	VGS = 4.5V
RDSon = 0.0017 ohm	RDSon = 0.0020 ohm
P=602 * 0.0017	P=602 * 0.0020
P=6.12W	P=7.20W
DIFFERENCE:	1.08W

In this case additional heat sinks would need to be added due to the possibility of the MOSFET failing due to overheating. A MOSFET failing can be an issue as they tend to fail closed and would start to auto fire. This is a reason for not taxing you batteries by going beyond their continuous discharge limit. Should a failure occur, such as the MOSFET, the build will not put your batteries at a high risk of thermal runaway as they will not be exceeding their limits.

 

[Gummy Bare]

Nice read.
This was basically the wiring diagram and parts list I was going to use. I planed on putting heat shrink over the mosfet chip as well as the leads and wire soldering areas. Not sure if that will make the chip heat up easier. Maybe I need to and a heat sink to it as well? I'm super new to all of this, I just wanna make a nice box... But also insure t's safe.

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[WharfRat1976]

What Mosfet would you recommend for a single 18650 mech box? Any wiring diagram would be awesome-Thankyou.

Builds from .4 and up.

ps: Sorry for the hijack.

 

[WharfRat1976]

I just bought the 3034 and 16k resistor from Stearnsy on ebay. That guy is great at everything a seller does and is a huge help to vapors.

I am doing a single battery bottom feeder so same wiring as the posted diagram and I am going to shrink tube it all as well.

Good luck.