Mosfet Matching - V and R

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captJackSparrow

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Joined 2015
#1
hi,

I have a 12V/5A brick(don't have 15V regulated PSU)

V(output measured with voltmeter) - 12.31 V

Is attached resistance value fine for matching IRFP240PBF?

For resistance four 200 Ohm 3 W in parallel

I won't be using any heat sink, turn on power - taking the first reading on voltmeter - turn off power. I'll be fine, right?

Thankyou
 

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#2
It should be, but ideally you want to measure your devices at the same voltage and current levels that they will see in your completed circuit. Your set up will match them for 12v and bias of 0.25A which is a bit low if you are building a class-A amp with them. You'll be fine measuring these, this voltage and current level won't produce much heat. When you test at higher levels (~20v and ~1A), you'll be good for 20-30 seconds without a sink. Any more time and you'll burn your fingers on the devices.

I wrote up something about this a while ago - see this link: Aleph-X 100w Amplifier Construction Notes
 
#4
Thankyou Eric

I understand my reading wont be as precise as taken at operating voltage but at least it should give me rough range to consider a pair as match?

Zen,

I am getting reading ranging between 3.91-4.0 for both N and P channel without heat sink, power on - consider first reading on meter - power off. MOSFET not even warm.

Why you said heat sink is a must, are you also favouring reading at higher voltage(operating) with heat sink?
 
#5
if you heatsink it , you can try holding it in place little more (20 or so secs) and observe trend of Vgs change vs. temperature

you'll see what I'm talking about

so - for general and not so critical case matching , 20 sec per device is enough , but heatsink is a must , possibly either heated to exact temp or forced fan cooled ..... to ensure same condition for all specimens
 
#6
If you want precision ,and I guess you would , you should let the device warm and stabilize at least a minute on heatsink , you' ll have nice stabilized reading , reading just after the power on it's a simple mess !!!! .

forget about the time spent, precision is your goal .

why on heatsink ? , because the power p : volt x amps , and because the device will work that way , you want your devices matched like the working conditions



.
 
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#9
hi,

I am now matching MOSFET using heatsinks,

Resistor - 15.1R 50Watt
V - 19.5 V Brick

Yesterday i measured Vgs - 4.594 after running it for 1/2hour
Today measured again and Vgs is showing - 10.1-11 V

I have checked MOSFET Drain to Source using Diode function on Multimeter, it is ON only when Gate is charged. So is MOSFET kaput?
 
#16
Thankyou

I have tested with multimeter and mosfet looks good.

Took readings of 50 mosfet at same temperature. I had a dmm meaduring temperature and other Vgs, weird thing was, every time I powered up mosfet Vgs would start with 8-10Volt and every time I had to disconnect leads(gate, drain and dmm Vgs) and reconnect to 15R resistor and voltage would come down to 4.XX which is what is expected.
I didn't understand why that happened for all 50 mosfet, I also had a temp. probe attached to heatsink(k type) , every time I powered the setup dmm showing temp started showing random numbers and once I diconnected/reconnected(gate, drain and dmm Vgs) it started showing correct temperature.

This issue wasn't there for IRFP9610. Anyways I am done with readings.

For IRFP240 I took long time for each mosfet, but for IRFP9610 I gave few seconds to each mosfet, just letting reading stablize. Initially I tested one IRFP9610 for half hour and reading didn't change at all, so for remaining IRFP9610 I took readings very fast. Do we need to give time for IRFP9610 if surrounding temperature is stable. Also I measure at 10.80 milli ampere, hope that is fine.
 
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