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Anonymous Poster

Mosfet Parallel

11/01/2007 8:35 PM

Hello to All....

We have 1 obselete MOSFET (2SK1723) which used 3 pcs. in parrallel for PFC circuit.

We tried to change to FS22SM-12A but one of them is always spoilt during throw-in.

Please give us your opinions.

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#1

Re: Mosfet Parallel

11/02/2007 7:13 AM

I hvnrt ever used these type of fet, but I think their are all voltage drived divice,

so you have to match at conduct voltage consistance.

what s their specification?

you can use one large to instead of your three. if your is 12a and 12x3=36, you can select 50A. too mych on market.

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#2

Re: Mosfet Parallel

11/02/2007 2:17 PM

I'd use 1 correctly rated Mosfet. I could be wrong but I think they are not suited to running in parallel, doesn't RDS ON decrease with temp or some such effect which makes one tend to run away rather than sharing the current? I vaugely remember something like this.

There are plenty of high current really low RDS ON Mosfets out there.

Del

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#3
In reply to #2

Re: Mosfet Parallel

11/03/2007 7:55 AM

Mosfets in parallel work very well, they tend to current share due to increased Resistance of the one which has started to carry extra current.

But they should be matched if possible otherwise the balance will not take place evenly and problems will arise if working close to there maximum ratings, obviously a common heat sink would help also.

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#5
In reply to #3

Re: Mosfet Parallel

11/03/2007 3:33 PM

Ok...I must've got the wrong end of the stick somewhere .

I do use 2 in parallel in my golf trolley speed controller, but I have a small amount of R in series with each to limit stall current and help current sharing... about 0.15 R if my memory serves... (although from previous post maybe it doesn't!)

Oh well never mind I shall go and wreak havoc elsewhere...

Del

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#4

Re: Mosfet Parallel

11/03/2007 10:52 AM

Depending on the circuit design, you may need to place a small value resistor in series with each MOSFET to force the current to more equally divide. Just make sure you get a resistor with a high enough wattage rating so that the resistors don't burn out. A good resistance value to start with would be around 10 Ohms The power rating needs to be the square of the expected current (for the sake of this discussion assume 4A but you will have to adjust for your application) so the wattage rating would be 4 X 4 X 10 or 160W (A new MOSFET would probably be cheaper...)

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#6
In reply to #4

Re: Mosfet Parallel

11/03/2007 10:25 PM

My personal experience tends me to say that 10 ohms is probably too high. You need a value generally under 1 ohm and if my memory serves me correctly, 1x to 2x the internal resistance of the device in question is a good starting point.

So if it had say 0.1 Ohm on resistance, then use a resistor of 0.1 to 0.2 Ohms as a starting value.....one of each placed in series with each device......must be identical resistances for each parallel device......matched devices will obviously help too....

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#7
In reply to #6

Re: Mosfet Parallel

11/04/2007 4:00 AM

Oooooh...you peeked at my values... I'm going to tell teacher!

Del

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#8
In reply to #7

Re: Mosfet Parallel

11/04/2007 5:35 AM

No I didn't, you had your paw on it!!!

Teacher, he is just trying to get me into trouble!!!

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#9

Re: Mosfet Parallel

11/05/2007 3:00 AM
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#10
In reply to #9

Re: Mosfet Parallel

11/06/2007 4:21 AM

Actually,we already try 2SK2699 and 2SK2615.

However,after testing with 2SK2699 and 2SK2615,the same problem occured and the MOSFET spoilt during response to brown out.

Do you have any idea?

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#11
In reply to #10

Re: Mosfet Parallel

11/06/2007 5:17 AM

I'm outside my comfort zone here: do you really need the 600V VDSS?

What about a single IR HEXFET (300V VDSS) 50 mΩ RDSON instead of 650 mΩ (and far greater current capability than your three original parts).

http://www.irf.com/product-info/datasheets/data/irfp4242pbf.pdf

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#12
In reply to #10

Re: Mosfet Parallel

11/06/2007 7:17 AM

I presume you have entered an incorrect part No [2SK2615 this is a 60V 2Amp device]

I agree with previous reply if you do not want to match the MOSFETs go for a single device and give yourself some headroom try IXYS IXFN36N100 1000V 36 Amp.

You have not stated what your circuit parameters are it would help if you did.

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#13
In reply to #12

Re: Mosfet Parallel

11/06/2007 8:05 AM

Garth, I agree entirely with your "Headroom" comment, too many designers design too close to voltage limits and then the engineer has to come along and find a device with better voltage range....seen it, done it and got the T-Shirt!

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#14

Re: Mosfet Parallel

11/07/2007 5:11 PM

For more details search google for "parellel" and MOSFET and you'll find a wealth of information, but to make a loong story short, MOSFET is the only power device that CAN easily be parellele, while following some simple rules.

Rdon (MOSFET's resistance when on) INCREASES as the junction temperature goes up, and that's oposite to any other semiconductor switch (take this with a grain of salt, but for this discussion it's sufficient enough that all NPN/PNP/IGBT have resistance that DECREASES with the increase in temperature).

Thanks to this physical property, if you put two or more MOSFETs in parallel, the "warmest" one will have the highest Rdon and will auto compensate by "letting" other MOSFETs draw more curent. Similar equivalent would be an additional small resistor in emiter circuit of a bipolar transistor, even though physical explanation is quite diferent (negative feedback to Ube vs. positive temperature coeficient).

Now the "simple rules" - first assumption is that one is using similar MOSFETs. Staying with the same model is quite sufficient and not difficult to achieve. In odtehr words, if you have 50AMP MOSFET and need to run 60AMP device, don't parallel with another 20AMP MOSFET as parameters probably can not be matched.

The main difficulty that's often overlooked is that putting MOSFETs in parallel you multiply their input capacitance. If your driver circuit gets double capacitance to drive, your on/off time will be affected (doubled) and that's what usually constitutes your main power disipation (MOSFET is in a linear mode during that time and disipation is MUCH higher then in steady ON state).

So, if you need to drive more juice, pair up with similar MOSFET but make sure your driver can still drive them both at the speed you need :)

Miroslav Kisacanin

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#15
In reply to #14

Re: Mosfet Parallel

11/14/2007 11:17 PM

Thanks Guy ( I mean Randall ) !.

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#16
In reply to #14

Re: Mosfet Parallel

11/15/2007 4:11 AM

Thanks Randall.

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#18
In reply to #14

Re: Mosfet Parallel

06/22/2009 11:36 AM

hi Miroslav,good to see ur answer.but as I see a dimmer they use 8.6A(STPNB50FP) and 15.6A(STU16NB50) POWER MOSFET in parallel,can u tell why they used different two in parallel? thank you1

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#17

Re: Mosfet Parallel

12/11/2007 4:11 PM

The paper from Jonathan Dodge at APT is the only good one I've ever read about paralleling MOSFETs. The subject has been hit several times at APEC conferences but the info from the APT paper holds all I've needed to know. I parallel MOSFETs usually when I have to switch current in the order of 40A or higher.

For high speed switching, it's not just Rds which will force MOSFETs to match. A large effect is due to differences in parasitics like leakage inuctance of the drain and source, especially the accidental inductance added to the pins if the layout practices are poor. After the drain voltage drops during switching, a few 10th's of a volt due to Rds mismatch appears across the drain & source inductances between the parts. If this inductance is 40 to 50 nH or so, it's going to take many u-seconds until the currents get equal. If you keep these parasitics low AND MATCHED, the currents will equal out quicker.

If you calculate these parasitics, keep them low, add anti-ringing circuits if needed, you'll have success in paralleling MOSFETs.

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