Device Matching Automation - A better way - diyAudio
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Old 26th November 2001, 01:13 AM   #1
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Location: Fairbanks, Alaska
Lets face it device matching is a pain so over the years I have come up with what I feel is the best way to handle this. It involves using a little bit of automation.

The method I like to use involves measuring the current through the MOSFET and using a servo system to drive the gate to obtain a preset selected current. Any current can be set from fractions of milliamps to amps.

All one has to do is to measure the gate drive voltage required to obtain the present current which was set by a comparitors reference voltage. Either drain or source current sensing can be used to match the intended purpose in the final application.

Such a system can be automated with some gating and sample and hold circuitry for both a low and high current limit. For power MOSFETS the low could be 50 MA and the high 300 MA. Thus each device has two matching points. The sample and hold circuit for the low and high set points feeding a seperate voltmeter.

Thus the drive voltage required to reach the low and high current setting can be read nearly simulatiously. A simple linear ramp voltage source can be used as a drive source. The sample and hold circuits hold the reading when the low and high currents are reached. The whole proces can take less than a second and does not heat up to MOSFET to any degree.

There is no nead to use methods that short leads together to match MOSFETS. This is not normally the way they will be used in the circuit so why attempt to match using such stone age methods?

With a small microprocessor that has a built in A to D converter the whole measuring circuit can be simplfied lot. A simple program could preset currents, read the drive voltages required to obtain the preset currents and remember and also display the results. Now we have a fully automatic matching system that could actually compare readings to a present standard and give the differences in percent, voltage or current. External power supplies are required to actually supply power to the device being tested.

This would be a real good project for someone to design and share with everyone who needs to do device matching.


John Fassotte
Alaskan Audio
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Old 26th November 2001, 01:56 AM   #2
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John,
I've came across something similar to this in a book by Ben Duncan or is it a GO NO GO MOSFET tester ( a much simpler version ). I'll have to search for it. Will keep you posted.
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Old 28th November 2001, 02:14 PM   #3
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John,
Apparently it's a GO NO GO MOSFet tester.I wonder will it be any use to you? Sorry for the wrong info.
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Old 28th November 2001, 10:00 PM   #4
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cp642,

I don't think that such a piece of test equipment would do me much good. When I have time I will likely design a scaled down version of what I have in mind to obtain the accuracy that I would like to have.

I have a old Sencore transistor/FET tester which is probably similar to the one you mention.

John
Alaskan Audio
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Old 28th November 2001, 10:38 PM   #5
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Default Sound card

You can use a sound card to do the matching. I have been planning to do this for a while, but ended up doing single points which was no big deal (120 devices).

When you get more than 1 point, you then have the problem of deciding how you will weigh those points ... There are many options -- minimum square sum error in region of interest might be a good one.

Petter
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Old 29th November 2001, 11:20 PM   #6
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Petter,

Your idea of using a sound card is very good. I had not thought of this.

I just installed the DigitalAudio SoundDeluxe card in one of our computers and as soon as I have it working properly I can give your idea a try. I'm having difficulty with the cards digital 24 and 16 bit modes. The analog inputs and outputs appears to be working ok but untill it all works properly I have limited faith in the measurements it is providing me.

Would a sound card that is not able to output or input a DC level to work? In other words are you out putting a constant DC voltage or depending on a specific output waveform from the card?

John
Alaskan Audio
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