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Breaktru Forum  |  eCigarette Forum  |  Modding  |  Topic: Mosfet musings....
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Offline sterling101

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Mosfet musings....
« on: August 06, 2012, 07:06:46 PM »
Well most of the mod is sat on the breadboard waiting for me to tidy up the main code and find the rest of my jump leads to tie it all together but I've been trying to get my head around the best MOSFET to use for switching the power to the atty.
I can't see any benefit of n channel over p channel - is there a reason to use one over the other?
I understand the need for a low RDS(on) value, but how low is low?  I've seen a few rated at 0.08ohm which to me sounds low but in MOSFET terms it might not be?

As you can tell I've not had any call to play with MOSFET's as I've always worked on isolated systems that fed other boards or relays.  So a quick online tutorial somwhere probably wouldn't go amiss either lol

Hopefully the MOSFET I need will be easy to work with and readily available as I've noticed a lot are moving straight to either bga or qfn formats which is kind of out of my ability to work with as things stand at the minute :(
Seen a few on the Fairchild site but they won't let me order samples from them and the shipping on the places I've found them is stupid - mouser uk - £0.69 for the part and £12.95 delivery!

Online Breaktru

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Re: Mosfet musings....
« Reply #1 on: August 06, 2012, 07:46:39 PM »
Being you are switching power to the atty, I believe a P-Ch would be best unless Craig can suggest another alternative.
There are a number of UK electronics distributors:

http://uk.rs-online.com/web/
http://uk.farnell.com/
http://www.astute.co.uk/

Maybe something like this: FDS6679
« Last Edit: August 06, 2012, 08:00:18 PM by Breaktru Admin »

Offline CraigHB

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Re: Mosfet musings....
« Reply #2 on: August 07, 2012, 12:32:27 AM »
P-Channels are generally more convenient for design since they switch the high side.  N-channels are better for lower cost and wider selection with the caveat you have to switch the low side.  If you're not sensing current with a low side sensor, there should be no reason not to use an N-channel if you prefer. 

Another consideration is control levels.  A high level is "on" for an N-channel.  A low level is "on" for a P-channel.  Though you can drive a P-channel with a small signal N-channel to reverse the levels.  That can be advantageous if controlling a higher voltage with a lower voltage.  As the designer, it's entirely up to you.

How low you go with RDS(on) depends on how much power you're willing to give up on the switch.  Since efficiency is so much higher with PWM as opposed to a DC-DC converter, you can afford to give up more power at the switch. 

In looking at a particular part, you can usually find a specific "on" resistance for a specific gate-source voltage using a graph of RDS(on) in the data sheet.  Once you have an "on" resistance, simply multiply the square of the load current times the 'on' resistance to calculate power loss.  Compare that to load power.  You'll find it's no more than a couple percent even with a lossy MOSFET.

If you want to maximize efficiency, select a part with a lower "on" resistance.  If you're not particularly concerned about maximizing efficiency, you can go with a more lossy part if you prefer.  Again, as the designer, it's entirely up to you.
« Last Edit: August 07, 2012, 12:48:54 AM by CraigHB »

Offline sterling101

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Re: Mosfet musings....
« Reply #3 on: August 07, 2012, 04:26:05 AM »
I'd guess my best bet is to go for an N channel to drive the gate with a high level output from the MCU as that's already coded in and have the circuit with a 0v pulldown to that pin anyway.  That's if I have the theory right on that :)

I was thinking of doing the current sensing on the high side anyway so I could operate it separately if needed at low voltage direct from the MCU.

I agree though Craig using PWM the efficiency should be less of a worry over the switching speed of the MOSFET as that will most likely dictate the frequency of the outgoing PWM signal.  Ideally I need to to be above the normal hearing range so I've got the code to run at 25khz, even though I can hear up to 22 I shouldn't get any problems with that frequency.

I'll have a dig through some of the other online suppliers too, thanks for that Dave.  I've dealt with RS a lot in the past so already have an account with them but you do have to be careful of their minimum order levels as they often show the price for one unit but will only sell in packs of 5 or 10....

Edit: This one looks like it might do the trick - http://uk.rs-online.com/web/p/mosfet-transistors/3257580/
« Last Edit: August 07, 2012, 04:29:57 AM by sterling101 »

Offline sterling101

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Re: Mosfet musings....
« Reply #4 on: August 07, 2012, 09:45:15 AM »
Just had a thought and grabbed hold of an old motherboard to see what's on it.

I've found a few Infineon 13N03LA N-Channel mosfets and a couple of Anpec APM2054N's.
Here's the datasheet for the 13N03LA - http://tinyurl.com/9ypsbv8
And the datasheet for the APM2054N - http://tinyurl.com/c8catgq

Both are TO-252 but are attached to the motherboard at present but they both look like they might be a good starting point.

Next step is to work out how to get them off the board intact - might end up using the heat gun from beneath I think!

Online Poppy26

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Re: Mosfet musings....
« Reply #5 on: August 07, 2012, 10:31:34 AM »
I have been successful using a desoldering wick/braid to remove components from boards.

Offline CraigHB

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Re: Mosfet musings....
« Reply #6 on: August 07, 2012, 03:52:39 PM »
Since you're only driving an atomizer, you could optionally run in the low audible range, like 100Hz.  That way, you can maximize your duty cycle resolution which can become an issue at higher PWM frequencies.  It depends on the MCU though, some can maintain a high resolution at higher frequencies. 

A low frequency also eliminates any potential for issues due to switch impedance.  Depending on the MOSFET, that can become significant even at frequencies near the upper audible range.  It depends on gate charge and switching speed.  MOSFETs with lower gate charge and higher switching speed have less impedance at higher frequencies, but they cost more.   I have to use relatively expensive MOSFETs for my converter because of that.

Offline sterling101

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Re: Mosfet musings....
« Reply #7 on: August 07, 2012, 05:39:35 PM »
I'll have to check the lowest frequency I can get down to.  I seem to recall at 4Mhz it's something like 145hz so that's a possibility.  I'm running at 20Mhz at the minute on the rest rig but can shift down to 4Mhz with a quick bootloader change and swap of crystal.  Lowest frequency at 20Mhz is about 1.2khz so definitely not a good choice there!

Any thoughts on those MOSFETs I found on the old motherboard Craig?  Might be worth pulling them off for a punt to see how they perform?

Offline CraigHB

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Re: Mosfet musings....
« Reply #8 on: August 07, 2012, 06:13:32 PM »
Well, I'm not a proponent of scavaging old parts.  The thing is, electronics components are inexpensive to begin with and I just don't think it's worth the potential to have problems with your circuit.  They're only designed to withstand the heat of soldering once.  When there's a problem with your circuit, it can sometimes take a while to find it.  I just don't see the sense in saving a few dollars at the risk of spending hours troubleshooting a problem introduced by a bad part. 

That said, the MOSFETs you find on old computer motherboards typically have the specs you need.  I'm sure they would work if you want to use them, assuming they survive the heat of being unsoldered and resoldered.

Offline sterling101

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Re: Mosfet musings....
« Reply #9 on: August 07, 2012, 06:21:51 PM »
That's all I was thinking really - cheap tests while I get used to the set up.

Then for the main build buy in the right part and solder it only once :)

On the plus side I've got it firing the pwm and producing the expected voltages with increments.  Just got an issue with reducing the voltage now for some reason - sure it's a code typo nothing more!

Offline CraigHB

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Re: Mosfet musings....
« Reply #10 on: August 07, 2012, 06:37:14 PM »
The coding is always the most involved for me.  The electrical design usually seems to flow smoothly, but I can get stuck on code quite a bit.

Offline sterling101

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Re: Mosfet musings....
« Reply #11 on: August 07, 2012, 06:48:43 PM »
I'm sort of the opposite - code I can do all day but actually putting stuff together I can struggle a bit sometimes.
Kind of helps I started programming when I was 8 back in 1981 and have done everything from e-commerce, time keeping apps, stock control systems, order management and contact mansgement right through to games!
Electronics, although earning a HND in it I've never really persued fully, despite building many things for fuel control and turbo boost control on a fair few cars!

Offline sterling101

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Re: Mosfet musings....
« Reply #12 on: August 09, 2012, 12:01:29 PM »
Win possibly?

These have arrived from TI http://www.ti.com/lit/ds/symlink/csd17312q5.pdf

Just need to attach some fly leads to one to test it on the breadboard and it'll be good to use I think :)

Rest of circuit is working fine, just got the testing with the MOSFETs to do and get the timer interrupts running and I think I'll be about ready to burn onto the actual chip.


Offline CraigHB

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Re: Mosfet musings....
« Reply #13 on: August 09, 2012, 03:53:21 PM »
Wow, that's an amazing part, amazingly low RDS(on) and gate charge.  You won't have to worry about PWM frequency with that.  I want to use that part.

Offline sterling101

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Re: Mosfet musings....
« Reply #14 on: August 09, 2012, 04:10:39 PM »
Well it would be great except I can't solder smd components!

Tried adding some fly leads and just can't hit the right temp to get them to take on such tiny pads without risking the MOSFET itself.

Guess I might need to re-think and get something cheap and large :)

Offline CraigHB

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Re: Mosfet musings....
« Reply #15 on: August 09, 2012, 04:21:10 PM »
Yes, you would have to solder mount that one to a circuit board.  You could make a little proto board for it and solder it by hand or reflow it.

Offline sterling101

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Re: Mosfet musings....
« Reply #16 on: August 09, 2012, 04:24:19 PM »
I need to buy the kit to get the pcb made up though - got my old stuff together out of the attic today and it's well past it's best so need new chemicals etc at least.
I then have the problem with flowing as I don't have anything up to scratch that'll do it well enough sadly.  Cracking MOSFET though - just a little too small for my shaky hands lol

Offline sterling101

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Re: Mosfet musings....
« Reply #17 on: August 09, 2012, 06:09:30 PM »
I'm looking at this one as it may have a decent enough switching time to cope ok - http://www.fairchildsemi.com/ds/RF/RFD14N05L.pdf

If you want these two spare TI MOSFETs I've got here Craig, shoot me a pm and I'll get them posted over to you :)

Offline CraigHB

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Re: Mosfet musings....
« Reply #18 on: August 09, 2012, 08:01:10 PM »
That part should work okay, but it's pretty lossy with 100m Ohms on resistance.  It will also get pretty hot.

I'll probably pass on those TI parts.  They are really nice, but I don't think the slightly higher efficiency is worth the design change.  The PCB design for my latest battery mod is ready to be submitted for fabrication and I don't want to change it.  Thanks for the offer though.

Offline sterling101

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Re: Mosfet musings....
« Reply #19 on: August 12, 2012, 04:56:29 AM »
Right then - having tested the circuit successfully with the 13N03LA lifted from the motherboard I think I've found a reasonable replacement that should do the trick and won't be too hard to put together.
The part is an STB60NF06LT4 (http://tinyurl.com/d4ka4ac) and should mean I can mount it onto the PCB successfully.

For now though with the super cheap lifted component it works great although it looks like a dogs dinner as I can't find my fly leads anywhere :(

Offline zotek

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Re: Mosfet musings....
« Reply #20 on: November 25, 2014, 11:01:29 AM »
does anyone has finish setup using this mosfet http://www.ti.com/product/csd17312q5/samplebuy

thanks

Offline nwooley83

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Re: Mosfet musings....
« Reply #21 on: December 27, 2014, 07:23:53 PM »
Would this mosfet work for starting out?  This is for a single 18650 unregulated box mod but I want to control the power going to the switch. I am just not sure how to read the data sheet to know if it can handle the amperage.
« Last Edit: December 28, 2014, 01:50:27 AM by nwooley83 »

Offline david4500

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Re: Mosfet musings....
« Reply #22 on: December 28, 2014, 03:41:33 AM »
Max rating ID = 4 amps

that is the current rating

Offline nwooley83

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Re: Mosfet musings....
« Reply #23 on: December 28, 2014, 10:40:41 AM »
Awesome thank you. That was the part that I was unsure of.  So I guess ordering online is the only way I am going to find the mosfet that I will need.

Offline Meanyman

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Re: Mosfet musings....
« Reply #24 on: January 07, 2015, 03:35:00 PM »
This info is great.

Offline nwooley83

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Re: Mosfet musings....
« Reply #25 on: January 25, 2015, 03:32:21 PM »
wrong post
« Last Edit: January 25, 2015, 10:49:21 PM by nwooley83 »

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