Author Topic: Mosfets? (Read 19560 times)

fsors · « **on:** December 12, 2011, 06:18:05 PM »

Hi Mr. Breaktru,

Would these mosfets be the right ones for making a touch switch?
http://www.ebay.com/itm/160351412604?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649#ht_500wt_949

Breaktru · « **Reply #1 on:** December 12, 2011, 07:37:07 PM »

I think that Craig is the better person to ask. He is the expert on answering the technical aspect of a Mosfet.
What I can see by the datasheet, this Mosfet is a P-Channel, 8pin SO-8 which is rather quite small at 4 x 5 mm.
It has a Gate Threshold of 1v which to me would be better suited for a Single cell mod.
Mosfets are really not that expensive so settling for this model because it's a good price doesn't make sense when you can buy exactly what you want at Digikey.

fsors · « **Reply #2 on:** December 13, 2011, 01:03:26 AM »

Thank You Mr. Breaktru.

I will be using in a single cell boosted mod. Which one do you use in your mods? ie.. Copper, 44 ect..

Craig are you out there?

If so, please chime in.

CraigHB · « **Reply #3 on:** December 13, 2011, 02:31:21 AM »

I saw this post earlier, but since you were asking Breaktru, I didn't want to butt in.

Yes, Breaktru is correct. He doesn't give himself enough credit. I think he probably has more experience with applications to various e-cig mods than anyone I've had the pleasure to read. I'm just well versed in general electronics and can give you specifics on parts in that regard.

I didn't look at the data sheet, but if the the part has a low gate-source threshold voltage as Breaktru indicated, then it should be fine to use in a single cell mod as a high side switch (switches are either high side or low side, P-channel switches are high side, N-channel switches are low side). My personal favorite P-channel for that application is the Fairchild FDS6576. You can compare data sheets between the two.

That MOSFET is in a SOIC package so it's designed to be mounted on a PCB with liberal pads to facilitate heat sinking. Though, you can probably solder wires to it. The TO-220 packages are more suited for that so if you're not using a PCB, I'd probably go with one of those instead. MOSFETs are one of the most common electronic components, right up there with resistors and capacitors, so there's no reason to settle for one that doesn't fit your application perfectly.

Once caveat with touch switches it that MOSFETs are vulnerable to ESD or static discharge, but there are ways to protect them. People don't seem to have a problem very often, but you can potentially damage the MOSFET if you shock the touch sensor with static electricity. Can happen easier than you might think.

You can protect MOSFETs externally with a Zener diode from gate to source or use ones that have protection built into the chip. The FDS6679AZ is an example of a MOSFET with built-in protection. I've burned MOSFETs with static before and they tend to start conducting through the gate when that happens. A lot of times, they'll keep working in a reduced capacity until the gate blows out completely.

fsors · « **Reply #4 on:** December 13, 2011, 02:47:12 AM »

Thank you Craig. This helps a lot. I have been searching around for a couple weeks for this kind of guidance. Hope to make the touch switch in a boosted copper soon. Have made six 3.7v coppers as well as 11 or so box mods all have used electric switches or mech switches so this will be a new adventure. Any advice you and Mr. Breaktru can give will be received with much appreciation. Mr. Breaktru and You are an inspiration.

fsors · « **Reply #5 on:** December 13, 2011, 11:39:40 AM »

Craig, I will not be using a PCB. Is this the one you like for non-PCB mount?
http://www.ebay.com/itm/MOTOROLA-MTP23P06V-TMOS-V-P-Channel-Power-MOSFET-60V-23A-TO-220-3-pin-/150675522883?pt=LH_DefaultDomain_0&hash=item2314f60543#ht_849wt_698

fsors · « **Reply #6 on:** December 13, 2011, 11:45:46 AM »

Craig, Will these work for the diode?
http://www.ebay.com/itm/5-1N5240-10-0V-500mw-Silicon-Zener-Diode-NEW-/3847530577?pt=LH_DefaultDomain_0&hash=iteme554a851#ht_500wt_949

CraigHB · « **Reply #7 on:** December 13, 2011, 07:32:54 PM »

Those diodes look good for protecting the gate on a MOSFET. Be sure to observe polarity. For Zener diodes, you want the arrow (on the schematic) or band (on the part itself) to be against the current flow. On a P-Channel, the banded end of the diode would be on the source pin of the MOSFET (batt positive) and the other end on the gate pin of the MOSFET.

You're not going to find the best selection of MOSFETs on eBay, there's just too many out there and there's no easy way to find them on eBay. The best place to go is Mouser in my opinion. Newark and Digikey are also good. Mouser has an excellent search form that makes narrowing down parts by specification a breeze.

Also, don't limit yourself to the TO-220 packages. For non-PCB stuff, Breaktru mentioned he likes the TO-251 or TO-252 packages as well. All my stuff is on PCB so I've been using SMD parts exclusively for several years now. I'm not up on the thru-hole stuff since I haven't used any of those parts for quite some time. All my part numbers have long been pidgeon holed and forgotten.

It's very time consuming to find parts, it's always been that way. When designing circuits, I spend more time looking for parts than actually designing the circuit. If you get really stuck, I'll see if I can dig up some part numbers for you.

fsors · « **Reply #8 on:** December 13, 2011, 07:46:52 PM »

Thank you Craig! So the mosfet on ebay is ok then? It says 60V/23A. Is this okay? I will also try the other places too. You are great and very kind.

Breaktru · « **Reply #9 on:** December 13, 2011, 08:59:25 PM »

Quote from: fsors on December 13, 2011, 07:46:52 PM

Thank you Craig! So the mosfet on ebay is ok then? It says 60V/23A. Is this okay? I will also try the other places too. You are great and very kind.

Just like to point out that the Mosfet you linked to has a Zener built in.

Breaktru · « **Reply #10 on:** December 13, 2011, 09:02:13 PM »

This may help. It's a TO-252 and smaller than the TO-220:
http://www.fairchildsemi.com/pf/FD/FDD6685.html

fsors · « **Reply #11 on:** December 13, 2011, 09:39:21 PM »

Thank you Mr. Breaktru! Sorry for all nooby questions. How can I tell, when looking at a mosfet, if the diode is built in? I am assuming some code in the title/numbers? I appreciate your help and the link.

fsors · « **Reply #12 on:** December 13, 2011, 11:15:58 PM »

Thank you Mr. Breaktru! The diagram is very helpful. Is there something in the numbers or letters in the name of the mosfet too? Sorry I am a rookie.

CraigHB · « **Reply #13 on:** December 14, 2011, 02:56:08 AM »

That's actually the body diode.

Any MOSFET that shows the two center bottom symbols tied together has a body diode. It's not actually an additional diode on the chip, but rather a diode formed by the materials used to construct the MOSFET. The body diode is not always shown in the schematic symbol, but if those two bottom center symbols are tied together, it has one.

The reason body diodes are there is if that one substrate is left floating, there's no body diode, but the FET can latch under certain conditions. There are MOSFETs available that do not have a body diode and run that substrate to an external pin for use as required, but they're fairly uncommon and typically not used for switching applications. Most of the time, you want the body diode.

Body diodes do have their uses. In switching applications with heavy inductive loads, body diodes can be relied upon to dissipate flyback voltage which is what happens when you shut off a motor or transformer. Other times, they can be used as synchronous rectifiers to carry the load until the MOSFET comes up to switching speed. They are handy things.

If you look at the schematic symbol in the data sheet for the FDS6679AZ, you'll see two back to back Zener didoes between the gate and source. Those are the protection diodes that are in addition to the MOSFET on the chip. If you're not using an AC input, you only need one Zener diode to protect the gate. The back to back diodes allow the application of both negative and positive gate-source voltages, though only a negative gate-source voltage actually turns on a P-Channel FET.

For any MOSFET, you need to look at the data sheet to see if it has protection diodes. It will show them in a schematic symbol and/or state it verbally in the general description. Fairchild does a handy thing where they append "AZ" to the part number of any MOSFET that's protected.

Breaktru · « **Reply #14 on:** December 14, 2011, 09:46:00 AM »

Thanks Craig, I miss-interpreted the Zener because I noticed on some datasheets the mosfet diagram shows a Zener between Drain and Source while some show it as a reg Diode. I see in your mosfet it is an additional internal Zener added between Gate and Source.
I will remove my drawing so not to confuse anyone.

fsors · « **Reply #15 on:** December 14, 2011, 11:24:56 AM »

Thank you gentlemen! So for the one I linked to on ebay:http://www.ebay.com/itm/MOTOROLA-MTP23P06V-TMOS-V-P-Channel-Power-MOSFET-60V-23A-TO-220-3-pin-/150675522883?pt=LH_DefaultDomain_0&hash=item2314f60543#ht_849wt_698

I will need to add the external z diode? Sorry, I am still a bit confused because that part number is TO-220AB. I am not sure what the "AB" represents?

Again, sorry for the nooby type questions.

CraigHB · « **Reply #16 on:** December 14, 2011, 08:08:56 PM »

That eBay part has a threshold voltage too high and an on resistance too high. It wouldn't work well with a single battery.

So, you made me do it, start looking for parts for you. Haven't looked at through-hole parts in a really long time. Wow, selection has really thinned down. They are all but obsolete these days in favor of surface mount devices you know. There's not much that looks any good that I can quickly find at my usual suppliers. It's all old stock, parts with high threshold voltages and high on resistances, like the eBay part you posted. I didn't realize that so sorry for the bad info before.

I'm afraid my suggestion would be to use the FDZ6679AZ if you want a protected part or the FDS6576 for a better performing part without built-in protection. Either solder wires to it or make a small PCB for it. That's going to be the extent of the help I can offer in finding parts for you.

Maybe Breaktru can give you some part numbers for the stuff he's been using.

fsors · « **Reply #17 on:** December 15, 2011, 12:55:27 AM »

Thank you very much Sir Craig. Sorry for your hassle. I will look at the parts that you have suggested and use the external diode on them. Your kindness and expertise is much appreciated!

Mr. Breaktru, any suggestions?

Breaktru · « **Reply #18 on:** December 15, 2011, 09:36:03 AM »

P-channel 30 V - 0.025 ohm - 24 A - DPAK / IPAK
This is the lowest RD(on) I can find that is available and is smaller than a TO-220 and bigger than an SOIC making it easier to solder to (IMO): http://www.mouser.com/ProductDetail/STMicroelectronics/STD30PF03LT4/?qs=sGAEpiMZZMvECErq9cesgMZn9ZLmPCNeMknzW%2fDRIgk%3d

Vgs = 16v
RD(on) = 0.032 Ohms (VGS= 5 V, ID= 12 A)
Vgs(th) = 1v
Package = TO-252

I've tried soldering to the SOIC package but I have broken pins off when routing the wires inside a mod. So I have switched to the TO-251 and TO-252 package. This is my preference. I am not suggesting that you not use an SOIC. Being that I do not use a PCB, this works best for ME.

fsors · « **Reply #19 on:** December 15, 2011, 04:14:39 PM »

Thank you Mr. Breaktru! I am assuming you are using the IPAK version of this? Is this right?

Breaktru · « **Reply #20 on:** December 15, 2011, 05:08:54 PM »

Quote from: fsors on December 15, 2011, 04:14:39 PM

Thank you Mr. Breaktru! I am assuming you are using the IPAK version of this? Is this right?

Either one. They are physically the same size. Leads are longer on the IPak which I cut down anyway.

fsors · « **Reply #21 on:** December 15, 2011, 07:38:42 PM »

Thank you Sir Breaktru! This will really help me in the near future.