Why are Some of My Mosfets Getting Super Hot?

[MonkeyTokes]

I've build quite a few boxes, and just finished 5 mosfets boxes for a couple friends and such. They are all wired exactly the same with the same brand resistor and mosfet, same wire, battery's, basically all the exact same thing. Though when I test fired them all two of them get really hot even if fired for a fraction of a second. Keep in mind I'm also using the same rda with the same build just switching the box.

I'm confused and don't really know where to start in fixing it. The boxes are ABS so there's no short on the boxes, I checked all the wiring and everything seems right and is correct. Could it be something with the resistor? I've noticed a lot of other problems that seem like a burnt out mosfet are actually due to bad resistors or bad connections with the resistor. The vape also seems a bit weaker on the ones with the hot mosfets.

I'd be glad to hear any suggestions, hopefully I don't need to rewire a new mosfet -_-

 

[Mooch]

Can you post your schematic and what MOSFET and resistor you're using?

 

[MonkeyTokes]

I don't want to post anyone else work so here's the quickest diagram I could make (sorry!). Basically pin 1 goes straight to the switch, pin 2 goes to the 510s negative and pin 3 goes to the negative battery terminal. Then from the battery's positive it goes to the 510s positive terminal then into the empty switch terminal. I'm using the typical IRLB3034PBF mosfets with 15k ohm quarter watt 5% tolerance carbon film resistors.

I decided to try and wire a new mosfet in one of them and that one isn't getting hot at all now. I guess that's good, but I'd still like to know what's causing it, if they're bunk mosfets or something else. I tested all of them with my multimeter before installing though I guess that doesn't do much for over heating as they still work they just get ridiculously hot.

 

[Mooch]

You bring up a good point...are you sure your MOSFETs are real IRLB3034's?
That could easily explain why you are getting such mixed results with your setups. Did you get them from an authorized distributor (Mouser/Digikey/Newark, etc.)?

The hot MOSFETs could be due to a very high on-state resistance. If your MOSFETs were fake then they might not be turning on all the way. This leads to a lot of MOSFET resistance, heating, and a big voltage drop that would affect your vape.

MOSFETs are static sensitive. Perhaps some were damaged. I would suspect fake FETs first though.

 

[Mooch]

Your schematic looks OK. As long as you're not shorting anything, touching the MOSFET's back and tab to anything, and all your connections are good, I would start to suspect your FETs.

Can you closely compare the appearance of the ones that aren't overheating vs the ones that do? Things like the silkscreening on the front, "dimple" size/shape on the epoxy case, leg width and the shininess of the leg finish, the finish on the tab and rear, the shape of the metal slab on the rear...all these can point to perhaps having some real ones and some counterfeits.

 

[MonkeyTokes]

I guess I can't be positive. I actually started a thread here a while back when I purchased a huge quantity of them, split between two electronics component vendors with good reputations(I don't want to seem like I'm bashing them so I won't specify who) and some people had a good point, though they looked different it was a bit odd to fake such a cheap component. I'm starting to think that may be false after seeing some clear fakes but they do have some significant differences. If you look at the back of a mosfet you'll notice a little black oval in the drain plate, one batch has them the other doesn't. The one that doesn't also doesn't have the kind of squared top of the pins where they exit the mosfet then kind of thin out - they are just thin all the way down. Oddly enough these ones came in packaging (strips) that looks incredibly authentic, while the others (with the more sturdy looking pins and oval on the back) came in basically blank strips with very little if anything labeling them with IR logos or anything else. Surprisingly the ones that are in the unmarked packaging are the ones that seem to perform better. Though they look like much more of the google image search results than the others do. I actually messaged the companies when I got them because of the variations and both companies were adamant that they were legit and the one that looks odd is just because they were made in a different plant though its only the exterior and the interior is identical. I'm guessing the ones without the oval are either Chinese mosfets cleared of their labeling and labeled as international rectifiers, or they are legit and are maybe old and have degraded over time. Today I'm going to try maybe 10 of them in a breadboard and see what happens, I guess that's the best way to see if its them or some other variable. Thanks for the help and suggestions, man!

 

[Mooch]

Yea, it's amazing what gets counterfeited these days.
Great idea to breadboard a bunch. Good luck with the tests!

 

[TheRac25]

these days even the reputable suppliers can be get bitten by counterfeit parts and i have noticed certain suppliers fail to ship devices in static shielding bags (farnells/element14 looking at you)

 

[Mooch]

No anti-static bags?
That's practically criminal.

Gotta' love Digikey...poly anti-static inside bag, metallized outer bag, anti-static warning label, and they will even heat-seal the bag (with silica gel and moisture-indicating card) if the component is moisture sensitive. Even if you order just one!

 

[MonkeyTokes]

these days even the reputable suppliers can be get bitten by counterfeit parts and i have noticed certain suppliers fail to ship devices in static shielding bags (farnells/element14 looking at you)

Oddly enough my battery holders came individually packaged in static resistant bags(keep in mind static electricity wouldn't do anything to a battery holder) but the mosfets came in plain old plastic strips 0_o

After building a test circuit operating at 4v with a .6ohm resistance I tested 10 of the remaining mosfets in question. After testing them only 2 of the 10 tested were working properly or at what I would consider reasonable as far as heat generated vs time engaged, though two others worked without getting ridiculously hot extremely fast, but they still got significantly hotter than the other 2 in a much shorter amount of time. Next time I have a bit of time I will try and up the voltage and see if it is simply that these knock offs or whatever they are need a higher working voltage to turn on properly. I suppose we'll see, but either way I'd keep away from the mosfets with no little black spot on the back.

 

[Mooch]

Oddly enough my battery holders came individually packaged in static resistant bags(keep in mind static electricity wouldn't do anything to a battery holder) but the mosfets came in plain old plastic strips 0_o

<face palm> sheesh...

 

[Chip_]

Oddly enough my battery holders came individually packaged in static resistant bags(keep in mind static electricity wouldn't do anything to a battery holder) but the mosfets came in plain old plastic strips 0_o

After building a test circuit operating at 4v with a .6ohm resistance I tested 10 of the remaining mosfets in question. After testing them only 2 of the 10 tested were working properly or at what I would consider reasonable as far as heat generated vs time engaged, though two others worked without getting ridiculously hot extremely fast, but they still got significantly hotter than the other 2 in a much shorter amount of time. Next time I have a bit of time I will try and up the voltage and see if it is simply that these knock offs or whatever they are need a higher working voltage to turn on properly. I suppose we'll see, but either way I'd keep away from the mosfets with no little black spot on the back.

Those plastic strips are more then likely anti-static holders.

Also have you looked at the data sheet? Look on page 4 upper right corner the chart labeled "Operation in this area Limited by RDS(on)". You have not shown any heat-sinking at all. These mosfets tend to get hot when you start transferring amps with more then a couple of volts across the Drain-Source, and these amp transfers last for more then 10 miliseconds, - in your case your probably holding for a few seconds. You are supposed to heatsink these things.

 

[MonkeyTokes]

For anyone who stumbles on this thread in the future and doesn't know what I mean by the little black oval on the back here's a picture of the two different mosfets I recieved.

 

[MonkeyTokes]

Those plastic strips are more then likely anti-static holders.

Also have you looked at the data sheet? Look on page 4 upper right corner the chart labeled "Operation in this area Limited by RDS(on)". You have not shown any heat-sinking at all. These mosfets tend to get hot when you start transferring amps with more then a couple of volts across the Drain-Source, and these amp transfers last for more then 10 miliseconds, - in your case your probably holding for a few seconds. You are supposed to heatsink these things.

I can assure you they're not. Perhaps the tinted blue ones, the others are standard PET plastic. Also, your point may make sense if the other mosfets that also have absolutely no heatsinks were getting the same temperature, or even any noticeable increase in heat, but they aren't. If that were the case then I would have my answer and wouldn't have started this thread. If you look at any unregulated box using a mosfet about 99% (every single one I've ever seen) has no mosfet, maybe unless the builder made it specifically to be able to house a heatsink. Also none of the other mosfets rise above 180°f with a 2 second fire at .6ohms. I get that they are going to get hot if I go puffin' away on them or push really high current through them, that's obvious and doesn't really need to be pointed out, but when my other boxes I've built don't get too hot to hold nor do the same boxes with the mosfet replaced with the other one, its obviously more than a heatsink which none of my boxes without heat issues(all of them now since replacing the mosfets) have heatsinks. The only variable left is the mosfet, or possibly a bunch of bunk resistors that by luck I've only used with the mosfets of one exterior as I also replace them every time I replaced a mosfet with the ones I haven't had problems with.

 

[Chip_]

Does this mean you aren't going to read the datasheet?

 

[Chip_]

Also, your point may make sense if the other mosfets that also have absolutely no heatsinks were getting the same temperature, or even any noticeable increase in heat, but they aren't.

We used to order batches of semiconductors and test them out and then select the outstanding ones that performed better then the datasheets. Simply becuause we couldn't get devices that performed that well. So ya, you will sometimes get devices that perform way better then the datasheet states.

 

[Chip_]

MonkeyTokes:

Are you an electrical engineer or electronics tech?

 

[MonkeyTokes]

Does this mean you aren't going to read the datasheet?

If i seemed standoffish which has somehow given you fuel to make it your sole objective to prove me wrong, I apologize.

I just don't think you're following the point of this thread. I received mosfets from two vendors, both supposed to be the same model, IRLB3034PBF, but with clear distinguishable differences. All of one appearance don't work but a few, and those few get hot enough to melt solder in seconds. All of the other appearance do not get hot at all even when fired in excess of 20 seconds(far longer than I'd ever need, just wanted to see how long it'd take to get one as hot) - as a mosfet rated for 195A in consideration with the package should(mind you I have not picked out the good ones of this batch to create inflated results). Though I do know, like many other things, including computer components which I am very familiar with, there is always fluctuation, some specific pieces function better than others, can be overclocked higher etc. What I also know is that it's unreasonable to assume 100% from one vendor were above standard and 20% from the other were barely usable, making them "normal". As far as the data sheet, I have read it, I have it saved on every device I own to reference when needed. That doesn't change the fact of real life scenarios where essentially no one heatsinks their mods and they don't get too hot to hold nor do they melt solder. In 4 seconds the mosfet pictured was hot enough to melt solder, while in 23 seconds the mosfet of the other appearance was still not even too hot to handle. Sure its not very scientific, but if every mosfet mod that was getting hot works perfectly after I switch out the one kind of mosfet for the other, then the problem would be the one mosfet, which I now realize is more than likely some Chinese junk stamped with a popular mosfets markings to move them at higher cost, especially since the logo is not even the same as the actual IR logo. Also considering the company refunded me for every piece after one email, my guess is they know they got burnt on a batch of fake mosfets. That's pretty much it. No data sheet issues. No need for an engineering degree, no heat sinking problems, just some bad mosfets that I wanted to see if anyone had any ideas to rule out before deciding they were complete junk and tossing them.

 

[Chip_]

If i seemed standoffish which has somehow given you fuel to make it your sole objective to prove me wrong, I apologize.
.

I am not trying to prove you wrong. I was trying to establish your level of expertise, in troubleshooting electronic circuits.
I am trying to figure out if you are measuring your circuits correctly.

Blaming the manufacturer/vendor is easy. Troubleshooting a problem isn't all that easy. You should provide a schematic and indicate the locations on this schematic, your measurements taken. The schematic doesn't have to be done with a CAD/CAE package, it can be hand drawn and scanned in.

On this schematic you should identify, power source - levels, any Resistance, VDS drop, V gate values, Voltage value at the load, and what type of Volt Ohm meter you are using. When you say .6 ohms is that off the V/ohm meter? Some meters will measure .5 ohms into a dead short.

 

[WharfRat1976]

any "good" FET can go "bad."