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Old 25th March 2004, 01:25 PM   #1
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Default Matching L-Mosfets

What are typical voltages and resistance values involved in matching Lateral Mosfets like 2sk1058 and 2sj162? Could someone post a schematic if possible?

Thanks!
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Old 25th March 2004, 04:17 PM   #2
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One relatively easy way is to connect the drain to the supply, the gate to ground, and the source to a current source that has the other end grounded. Then just measure the voltage across the current source to find Vgs.

On the one hand you want to come reasonably close to the actual operating conditions of the transistor, which will typically be 50 Vds at 100 mA or so. However, this would dissipate 5 watts in the device which would destroy it unless heatsinked. So instead pick something smaller. I would suggest 10 Vds and 25 mA. This gives a total device dissipation of 250 mW, which will be fine.

You can make a current source from a 3-pin regulator if you read the app notes.

Don't try to match the P-channel devices to the N-channel devices. Just match each polarity to itself.
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Old 25th March 2004, 04:26 PM   #3
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Also, I think there is an article on matching (vertical) MOSFETs on the passdiy.com site, but it seems to be down at the moment.
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Old 25th March 2004, 04:33 PM   #4
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Did not test this, so an 'approval' reply is needed:
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Old 25th March 2004, 04:43 PM   #5
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Charles,

What is your prefered method of matching bipolar devices?

Regards,
Jam
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Old 25th March 2004, 05:34 PM   #6
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The method lucpes posted is really only suitable for vertical devices. With this approach you will only apply Vgs (threshold) across the device. This is OK for vertical devices as you will have 3 or 4 volts Vds. But for lateral devices you will only have a few hundred millivolts, which is kind of sketchy.

However if you don't want to build the current source as I proposed in my previous post, you could just use a resistor as lucpes proposed. Please note that it should replace the current source in my posting and be connected between source and ground.

Also, please be aware that the pinouts in lucpes' diagram are for vertical MOSFETs. The lateral MOSFETs have the source and drain pins interchanged.
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Old 25th March 2004, 05:37 PM   #7
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The PassDIY site is back up now. However their matching article uses the same method as in lucpes' post and is therefore only suitable for vertical devices.
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Old 25th March 2004, 05:46 PM   #8
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Quote:
Originally posted by Charles Hansen
.... However, this would dissipate 5 watts in the device which would destroy it unless heatsinked. So instead pick something smaller. I would suggest 10 Vds and 25 mA. This gives a total device dissipation of 250 mW, which will be fine.
I'm pretty sure that IRF uses a 20uS pulse -- I haven't gotten around to making a programmable pulse generator to automate the process, i.e. to ramp the 20uS pulses in voltage to measure transconductance etc.

I think (correct me if I'm wrong) they (IRF, not Hitachi) also characterize the threshold at 100mA.

the thought of a programmable pulse generator is percolating somewhere between the front and back burners right now...
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Old 25th March 2004, 05:46 PM   #9
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If you're only matching, I beleive it will work fine with both types.
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Old 25th March 2004, 05:53 PM   #10
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Sorry, my previous instructions were not correct. They would work for a depletion mode JFET, but not for an enhancement mode MOSFET.

You will need to add a second supply as shown in the attached schematic to test lateral MOSFETs. You can either use a true current source, or just substitute a resistor. I would recommend +/- 10 volts for the supplies. Then for the 2SK1058/2SJ162 parts I would recommend either a 25 mA current source or a 350 ohm resistor. These values are not critical and can easily be varied +/- 50% or more.
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