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Offline studiovape

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Suggestions for mosfet hard short protection.
« on: May 04, 2015, 09:46:30 AM »
Hi guys, I'm going to be making some mosfet boxes using single 18650, I want to incorporate some protection and am planing on installing the MHP-TA15-9-77 fuses (some info here : http://www.x-on.com.au/keywordSearching.aspx?kword=MHP-TA15-9-77 ), this will likedly protect the battery from venting (IMRs only for these), but what of the mosfet under a hard short at the atty or 510 connection will it survive the battery's high amp dump before the fuse trips? the mosfets will be PSMN1R1-30PL,127 data sheet here; http://www.x-on.com.au/DetailsPage.aspx?MFRN=PSMN1R1-30PL%2c127&MFR=NXP+Semiconductors&ID=etLDO4YegMtDz01lxThzUA%3d%3d
And if not , what are your suggestions to protect it in this hard short event?Many thanks in advance.

Offline david4500

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« Last Edit: May 05, 2015, 03:07:57 AM by david4500 »

Offline dc99

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Re: Suggestions for mosfet hard short protection.
« Reply #2 on: May 04, 2015, 07:45:42 PM »
Isnt that a 4.5v mosfet?

Offline studiovape

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Re: Suggestions for mosfet hard short protection.
« Reply #3 on: May 04, 2015, 08:37:03 PM »
Have you  looked at the data sheet DC99?
« Last Edit: May 04, 2015, 08:41:41 PM by studiovape »

Offline CraigHB

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Re: Suggestions for mosfet hard short protection.
« Reply #4 on: May 04, 2015, 08:37:17 PM »
Yes it is, but it has a low gate-source threshold for a 4.5V rated MOSFET, it kind of slips in just under the wire.  Most 4.5V rated MOSFETs don't fare that well on a single cell or parallel cells.  They'll work, but they waste a lot of power not being in the flat part of the curve when battery voltage drops down. 

A 4.5V rated MOSFET does become a bigger issue with single or parallel cells in terms of a short circuit condition.  Voltage drops quite a bit at the battery.  In turn that drops gate-source voltage down as well (unless a driver or doubler is used).  In that case you get pretty far left into the curve so the MOSFET is not only carrying much more current than normal, it's also into the active region where on-state resistance is much higher.  It's like a double whammy, much greater currents and much greater on-state resistance.

You really can't assume your MOSFET will handle a short okay once a fuse is placed inline.  It's something you have to check at the risk of burning some perfectly good MOSFETs.


« Last Edit: May 04, 2015, 08:41:58 PM by CraigHB »

Offline studiovape

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Re: Suggestions for mosfet hard short protection.
« Reply #5 on: May 04, 2015, 08:40:57 PM »
Yes it is, but it has a low gate-source threshold for a 4.5V rated MOSFET, it kind of slips in just under the wire.  Most 4.5V rated MOSFETs don't fare to well on a single cell or parallel cells.  They'll work, but they waste a lot of power not being in the flat part of the curve when battery voltage drops down. 

A 4.5V rated MOSFET does become a bigger issue with single or parallel cells in terms of a short circuit condition.  Voltages drops quite a bit at the battery during a short circuit condition.  In turn that drops gate-source voltage down as well (unless a driver or doubler is used).  In that case you get pretty far left into the curve so the MOSFET is not only carrying much more current than normal, it's also into the active region where on-state resistance is much higher.  It's like a double whammy, much greater currents and much greater on-state resistance.

You really can't assume your MOSFET will handle a short okay once a fuse is placed inline.  It's something you have to check at the risk of burning some perfectly good MOSFETs.
Thanks for your feedback Craig.
Can you suggest a better mosfet for single cell and better characteristics under hard short conditions?

Offline CraigHB

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Re: Suggestions for mosfet hard short protection.
« Reply #6 on: May 04, 2015, 08:49:08 PM »
I use Vishay MOSFETs a lot.  I like them, they seem to have the best specs for the things I look for.  That's both for high speed stuff like in switching power supplies or for user power switches such as a trigger switch in a mod.  After Vishay it would be TI.  They don't have much in the way of P-channels, but they have really nice N-channels.

For a high side user power switch using single or parallel cells, my go-to transistor is the Vishay Si7157DP.  For low side switching with single or parallel cells it would be the Vishay SiR404DP.

Offline studiovape

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Re: Suggestions for mosfet hard short protection.
« Reply #7 on: May 04, 2015, 09:03:03 PM »
I use Vishay MOSFETs a lot.  I like them, they seem to have the best specs for the things I look for.  That's both for high speed stuff like in switching power supplies or for user power switches such as a trigger switch in a mod.  After Vishay it would be TI.  They don't have much in the way of P-channels, but they have really nice N-channels.

For a high side user power switch using single or parallel cells, my go-to transistor is the Vishay Si7157DP.  For low side switching with single or parallel cells it would be the Vishay SiR404DP.

Please forgive my ignorance, but the difference between high and low side switching escapes me.
also I notice the two mosfets you listed are without leads and appear surface mount, this creates some significant problems for the design i have in mind, can you suggest a leaded mosfet with useful characteristics?

Offline studiovape

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Re: Suggestions for mosfet hard short protection.
« Reply #8 on: May 04, 2015, 09:22:46 PM »
Ok High and low side refer to N or P chan   , still interested in any advice a on a good leaded N chan that might fare better under the hard short with the stated fuse.
« Last Edit: May 04, 2015, 09:29:36 PM by studiovape »

Offline CraigHB

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Re: Suggestions for mosfet hard short protection.
« Reply #9 on: May 04, 2015, 09:28:39 PM »
I understand that leaded parts are most convenient for free-hanging stuff, but unfortunately leaded parts are becoming obsolete.  At some point you just won't see them anymore.

The solution for free hanging stuff and SMD parts is to build a small host board.  You'll find it's a pretty quick operation to etch one out and either reflow or hand solder an SMD MOSFET and wires to it.  Granted it's more work than using a leaded part, but if you want to step into the 21st century and use the best parts available, that's what you have to do.

Offline studiovape

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Re: Suggestions for mosfet hard short protection.
« Reply #10 on: May 04, 2015, 09:34:42 PM »
I understand that leaded parts are most convenient for free-hanging stuff, but unfortunately leaded parts are becoming obsolete.  At some point you just won't see them anymore.

The solution for free hanging stuff and SMD parts is to build a small host board.  You'll find it's a pretty quick operation to etch one out and either reflow or hand solder an SMD MOSFET and wires to it.  Granted it's more work than using a leaded part, but if you want to step into the 21st century and use the best parts available, that's what you have to do.

I have maybe 20-24 units planned, so the extra work involved seems daunting, can you suggest a link to info on etching and the parts required?
Just for interest, the 3034 mosfet that all the box modders seem to use, would that fare any better than the mosfet I linked to?

Offline CraigHB

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Re: Suggestions for mosfet hard short protection.
« Reply #11 on: May 04, 2015, 09:49:42 PM »
Well, if making host boards is a problem, you could send them out to Osh Park.  For something that small it would be like a dollar for three of them, $8 would get you 24 of them.  I doubt your time is that cheap, is it?

Otherwise, there are no 2.5V leaded MOSFETs that I've seen.  Inexpensive 2.5V rated MOSFETs came out after through hole components were already well on the path to obsolescence.  No makers offer them in a leaded package, at least for stock on store shelves.

Offline dc99

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Re: Suggestions for mosfet hard short protection.
« Reply #12 on: May 04, 2015, 10:32:44 PM »
Yes I did and while I am no genius on the subject it looked really close considering you said one battery. Forgive me for trying to help.

Offline studiovape

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Re: Suggestions for mosfet hard short protection.
« Reply #13 on: May 04, 2015, 11:17:29 PM »
Yes I did and while I am no genius on the subject it looked really close considering you said one battery. Forgive me for trying to help.
Please don't be offended by my response, I was not meaning any offence, it was just a question, you may have spotted somthing I had missed. As you can see I am a million miles from being a genius on the subject. Your answer did prompt me to do some extra research and I am grateful to you for that.

Offline studiovape

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Re: Suggestions for mosfet hard short protection.
« Reply #14 on: May 04, 2015, 11:19:07 PM »
Well, if making host boards is a problem, you could send them out to Osh Park.  For something that small it would be like a dollar for three of them, $8 would get you 24 of them.  I doubt your time is that cheap, is it?

Otherwise, there are no 2.5V leaded MOSFETs that I've seen.  Inexpensive 2.5V rated MOSFETs came out after through hole components were already well on the path to obsolescence.  No makers offer them in a leaded package, at least for stock on store shelves.
Thanks Craig.

Offline Ian444

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Re: Suggestions for mosfet hard short protection.
« Reply #15 on: May 05, 2015, 05:05:54 AM »
The PSMN1R1-30PL doesn't look too shabby though:



How do builders of single battery MOSFET boxes deal with short circuits, do they just avoid them at all costs?

Offline CraigHB

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Re: Suggestions for mosfet hard short protection.
« Reply #16 on: May 05, 2015, 04:53:19 PM »
Yes that one is almost a 2.5V rated MOSFET though it's only rated at 4.5V in the data sheet.  I don't know for sure that MOSFET would not be able to handle a short with a higher rated PTC fuse.  If you use a fuse with a lower rating, it will trip before the MOSFET can heat up enough to burn out.  It's all about heat, the MOSFET has to overheat before it will burn out.

There's other ways to handle short circuit detection with a mod that utilizes an MCU.  In that case it's not necessary to rely on a fuse.  It's possible to detect a fault and open the circuit quickly, as in a few thousandths of a second.  Even with a worst case, it takes at least a couple seconds for the MOSFET to heat up enough to burn out.  A higher rated PTC fuse during a softer short can take longer than that to react.

Offline Ian444

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Re: Suggestions for mosfet hard short protection.
« Reply #17 on: May 05, 2015, 07:53:51 PM »
There's other ways to handle short circuit detection with a mod that utilizes an MCU.  In that case it's not necessary to rely on a fuse.  It's possible to detect a fault and open the circuit quickly, as in a few thousandths of a second.  Even with a worst case, it takes at least a couple seconds for the MOSFET to heat up enough to burn out.  A higher rated PTC fuse during a softer short can take longer than that to react.

I've been working on that recently. With a single battery I don't need a voltage regulator since the MCU will operate down to 1.8V and it has a 1.1V internal voltage reference for the ADC. I have been able to detect a short condition and turn off the MOSFET in about 1.2mS, the detection method is crude in that it relies on a 0.4V difference between battery resting voltage and loaded voltage. One problem is that the MCU resets from the low voltage and I get an oscillation as it repeatedly resets. (Well I think that is what is happening, it results in a 12% DC and I can hear it). I will try a series schottky diode feeding a 22 to 47uF cap for the MCU power supply and see if that stop the resets.

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Re: Suggestions for mosfet hard short protection.
« Reply #18 on: May 05, 2015, 08:48:21 PM »
You need something called hysteresis which is a fancy way of saying it turns on at a higher voltage level than it turns off.  Most voltage detectors have an amount of hysteresis for some it's even programmable. 

I use a two fold method to detect a fault, first is a current limit on the reading from a current sensor and the other is failure to achieve a voltage threshold within a time frame on converter startup.  Either will trigger a fault display and shut down power to the converter.  The voltage threshold sensing is faster, the current limit sensing acts as a backup for a fault occurring after normal startup.  I don't recall off-hand, but I think it takes around a milli-second, maybe less.  That's plenty fast enough.  100 Amps for 1mS every second has the same energy as 100mA continuously, not much.

Offline Ian444

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Re: Suggestions for mosfet hard short protection.
« Reply #19 on: May 06, 2015, 05:05:59 AM »
100 Amps for 1mS every second has the same energy as 100mA continuously, not much.

That's a good way of visuallizing the heat produced, thanks. I'll keep pushing ahead and see what comes out in the wash. I might try measuring the voltage drop across the mosfet to measure current and see if that shows any promise.

Sounds like you have a solid detection system for your mod, I do remember reading about it here, and seeing some pics, rather impressive to say the least. 

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Re: Suggestions for mosfet hard short protection.
« Reply #20 on: May 06, 2015, 03:17:11 PM »
You can do that.  MOSFETs have a fairly flat curve for resistance versus drain current assuming you don't get into the MOSFET's active region.  It does get exponential at the extreme, but should be good enough for short circuit detection.  It may be possible to simply monitor drain-source voltage directly with the MCU's ADC and set a threshold on the count.

Of course measurements are only valid when the MOSFET is on and if using an amp you need to be sure it tolerate the differential input voltage when the MOSFET is off.  It varies with op-amps anyway, some have rather low limits on the differential voltage they can tolerate on input pins.

Offline Ian444

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Re: Suggestions for mosfet hard short protection.
« Reply #21 on: May 08, 2015, 10:09:20 AM »
I got some pcb's in a couple of days ago. The long thin one has spare in/out at the pads marked PB1 and  PB2, which should be useful to try monitoring the voltage drop across the mosfet.  The plan is to cut off the header section after I get it programmed and running. Still learning the soldering technique. I stuffed up the solder mask at the drain of the mosfets, I will correct that next time. I got the boards made at Seeedstudio, they are cheap and good, but then so is OshPark by the looks of it. Thanks for your help Craig. I'll let you know how I go, give me a week or two.



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Re: Suggestions for mosfet hard short protection.
« Reply #22 on: May 08, 2015, 12:57:08 PM »
I've used Seeedstudio before.  The quality was good with them, but at the time the minimum was 10 prints.  They may have changed that policy since then, it was a number of years ago.  Osh Park does 3 so you don't have to buy as many.  Otherwise price per board is probably similar.

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Re: Suggestions for mosfet hard short protection.
« Reply #23 on: June 10, 2015, 07:59:41 AM »
Yep Seeedstudio still do 10 prints, they do 10 pcb's 50mm x 50mm for $10 plus a small fee for shipping. You can put up to 5 designs on each board.

I hit a snag with my project, with the mosfet connected to one of the ISP pins, the AVRISP II programmer detects it as some sort of error and won't program it.  :Thinking:

I found some 18650 high-discharge data that is interesting, quite a good read (just the one post #26).


Offline CraigHB

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Re: Suggestions for mosfet hard short protection.
« Reply #24 on: June 10, 2015, 03:01:49 PM »
You have to be careful about sharing programming pins with other stuff on the board.  Usually you're okay as long as the load is purely resistive, but in some cases pull-ups will cause a problem.  About the only thing that won't give you trouble is purely resistive load to ground.  Otherwise any voltage present on the pin or any capacitance will mess with the programmer (the gate of a MOSFET is like a capacitor).  Personally what I do is dedicate programming pins and leave them floating.  Sometimes you don't have the free pins for it so you have to arrange things so the programmer doesn't have a problem with what is connected to the pin.  An option is to use a jumper which I've done before.  A few times I've used a removable one, other times I've used a soldered one.

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