how to match mosFETs inexpensively

[callum450]

I'm not sure if the mosfets are died yet but just incase they are i would like to know how to match them with inexpensive equipment.
they are IRFP240


side note are theses different from each other

IRFP240PBF(here)​

IRFP240(here)​

they both take me to the same spec sheet is it just a name change?

thx for any help

 

[ItsAllInMyHead]

Depending on how in-depth you'd like to go, here is a nice start.

https://www.firstwatt.com/pdf/art_matching.pdf

All you need is a power supply, a resistor or two, and a voltmeter.

Pb - Lead
F - Free

 

[Roppu]

PBF is lead free.
Check here

 

[callum450]

Depending on how in-depth you'd like to go, here is a nice start.

https://www.firstwatt.com/pdf/art_matching.pdf

All you need is a power supply, a resistor or two, and a voltmeter.

Pb - Lead
F - Free

thank you so much do you know of any videos show this and are there any meters that can do this like peak DCA55

i dont have a power supply on hand

 

[JMFahey]

Try the Fahey MOSFET Matcher 😉:
https://www.diyaudio.com/community/...p8n10-and-mtp8p10-mosfets.391097/post-7157977

And yes, you need a simple +/-15V power supply.
Easy peasy, simplest needs one 12V transformer, 2 1N4002 diodes and 2 2200uFx25V caps

 

[marconi118]

you need to match these mosfets at the rated current of your amp.

 

[ItsAllInMyHead]

thank you so much do you know of any videos show this and are there any meters that can do this like peak DCA55

i dont have a power supply on hand

Any time.

To the best of my knowledge there is not a ready-made device that can do power MOSFETs under the conditions of their normal use (or close to it). There may be one... I just don't know of it. You need to match under the conditions of use (or get close) or it's not terribly valid.

The other thing to consider is... what are you matching for and why... and what circuit is it going into? The article shows how to check for Vgs(th). There are a lot of other parameters that might be important.

What I'd consider is ... a suitable power supply (typically) is cheaper than something like the Peak Atlas.
Learning what parameters to match for and how to do it (even crudely) with a breadboard and a couple parts is priceless.

Maybe share with the thread what you're trying to accomplish. No videos that I know of. If you don't understand the circuits... then ... please say so, and maybe someone might put up a simple diagram. From my perspective, I can see how it might be intimidating if you've never done it.

If you're unfamiliar with how to build a basic DC PSU, my advice (if you were considering a Peak Atlas) is to put some of that money toward a decent (or even an el-cheapo) bench supply. It will likely save you some time and frustration. It might be safer for you to use also.

 

[callum450]

you need to match these mosfets at the rated current of your amp.

i would need a psu that can do 44v.

will this work with an old 12v wall wart psu?

 

[gary s]

The IRFP240 current will depend on whether the amp is running Class A or AB. If class A, the quiescent current will be around 1ampDC or so. So for the mosfet test circuit in post #5 above with a 12VDC supply, the resistor R1 will need to be something like 10 ohm 20watts, this will enable you to measure Vgs at a current of around 1 amp, provided your PSU can deliver that.

If the amp is class AB then the device current will likely be around 100mA or so - then the resistor needs to be 100 ohm at 2 watts.

You don't need a +/- supply - +12vDC and Gnd will be fine. Read the FW article linked in post #2.

 

[callum450]

thanks that's amazing.
I'm looking at that first watt page and the diagram is blurry does anyone know were i can find a better diagrams for setting it up?

just to make sure I'm on the same page, i need to pick the ones with the same or closest to each other.
do i need to use the formula or just go off by the readout in volts?

 

[gary s]

Try this link from Zen Mod. Same as the NP method.

https://www.diyaudio.com/community/threads/pass-mosfet-matching-help.396010/page-3#post-7428454

Post #42 on that page, and a lot more info in that thread for you. Measure the Vgs off the DMM.

 

[callum450]

so i have a 12v wall wart that can do up to 1.5 amps
need a few wires and a 100ohm 2watt resister.
and just got a new dmm so I'm all good to go once i got the gear?

 

[JMFahey]

i would need a psu that can do 44v.

will this work with an old 12v wall wart psu?

Current, not voltage.
Above 5V or so drain current is almost a constant, so no need to test at full rail voltage.

To each his own, but I am happy matching at or near bias current.
At higher currents, ballast resistor drop swamps any minor Vgs difference.
Think 1 to 3 V swamping a few mV difference.
Also found that MosFets matched at 11mA still track very well at 100mA, but of course to each his own 🙂
1 A idle?
Please be my guest 🙂

 

[Roppu]

Try the Fahey MOSFET Matcher 😉:
https://www.diyaudio.com/community/...p8n10-and-mtp8p10-mosfets.391097/post-7157977

And yes, you need a simple +/-15V power supply.
Easy peasy, simplest needs one 12V transformer, 2 1N4002 diodes and 2 2200uFx25V caps

I can't seem to find a schematic of how to use diodes and caps, @JMFahey . May you tell me where I can see it?

 

[JMFahey]

Raw +/- 16V supplies, good enough for MosFet matching.

Transformer: cheap 12 or 12+12 VAC, 200-500 mA. Even 1 A won't hurt.

Really, the idea is to order nothing, but kludge these using junk box stuff.

Doesn't hurt but isn't really needed either to add some regulation, down to +/-12V or +/-9 VDC.

 

[Roppu]

My bad JMFahey. I didn't realise you were talking about the PSU. I thought it was some jig for mosfet testing. I use a ready made PSU.
Anyway, thank you for your effort and I'm sure it will be useful for the OP too

 

[callum450]

its been some time but I've just got round to ordering the Mosfets and i do have 12v DC psu on hand
I've also ordered that 100ohm resister rated at 100watt (overkill)

and ordered 2 x 8ohm 100watt resisters to create a dummy load.

 

[callum450]

I've heard that in some types of amplifiers that you don't need to match mosfets is this true

 

[wahab]

Lateral fets from Hitachi dont need to be matched since they have a negative thermal coefficient past 100mA Ids.
You can put several in parralel with no source resistances, i have a power amp using 3 pairs of such devices
per channel, it worked for 20 years as PA amplifier with extremely harsh conditions and no forced cooling.

After all these years i changed the transformer to use it as low power amplifier that is always on.
Transistors are the TO3 120V 2SJ48/2SK133 wich are currently obsolete, modern parts with same characteristics are the
TOP3 2SJ160/2SK1056, 140V models are the 2SJ161/2SK1057 and the 2SJ162/2SK1058 for 160V Vds.

These transistors have much lower Vth (threshold voltage) than vertical fets and as such they are more indicated for power
amps despite a lower transconductance, dunno if they are still fabbed to this day, the datasheet i have is dated from 2003.

 

[esprit]

Would the, Peak Atlas DCA, model DCA55, (12v) be suitable for matching mosfets for use in a first watt amp.
The user guide states, "The Atlas DCA detects that drain-source conduction has started when it reaches 2.50mA."
The Gate Threshold is displayed to two decimal points as Vgs= (ie 3.47)