I don't think you need to re-test them. I think what Nelson was saying was to be consistent. So first, make sure that all the devices start at the same (presumably room) temperature. Then, measure the devices at a consistent time interval. So it doesn't matter if it is 1/2 second, or 1 second, or 2 seconds, or 100 seconds. As long as all the devices were measured in the same way, then they can be matched to each other.
Re: testing accuracy
Sounds like my quantitative organic chemistry prof.
+/- 10 or 20 millivolts on a lateral MOSFET is 0.4 to 0.8% at a I(d) of 1 amp., but 1.3% to 2.6% at 100ma.
The breakpoints for acuracy tolerances (let me see if I remember this correctly from grad school 30 years ago) are 30%, 3%, 0.3% etc. -- a couple decades ago I could have derived the formulas, from the logarithms, std deviations, mean-variance, etc...sorry, I have killed too many brain cells in the intervening years.
i should hook up a datalogger and graph the changes in I(d) as the device warms up, mbe something to do later this afternoon.
larryg said:When testing any device for the purpose of matching whatever is desired, don't you have to establish first the tolerance +/- of the test it self?
Sounds like my quantitative organic chemistry prof.
+/- 10 or 20 millivolts on a lateral MOSFET is 0.4 to 0.8% at a I(d) of 1 amp., but 1.3% to 2.6% at 100ma.
The breakpoints for acuracy tolerances (let me see if I remember this correctly from grad school 30 years ago) are 30%, 3%, 0.3% etc. -- a couple decades ago I could have derived the formulas, from the logarithms, std deviations, mean-variance, etc...sorry, I have killed too many brain cells in the intervening years.
i should hook up a datalogger and graph the changes in I(d) as the device warms up, mbe something to do later this afternoon.
The original poster is trying to build an amplifier for himself. He's not trying to write a scientific paper for a peer-reviewed journal.
So, no, he does not have to establish the accuracy of the test.
Anything he does will be better than randomly selecting parts. The more care he takes, the better his results will be. That is true of anything in life. That's why some things are better than others.
So, no, he does not have to establish the accuracy of the test.
Anything he does will be better than randomly selecting parts. The more care he takes, the better his results will be. That is true of anything in life. That's why some things are better than others.
Matching of MOSFETs necessary?
I am getting one stubborn doubt about this issue. There have been two types of matching discussed here:
I had this niggling doubt, then I went back and checked in the "Art of Electronics" this morning, and now my doubt is one big question which refuses to go away. So... is matching of multiple parallel power MOSFETs needed at all?
I am getting one stubborn doubt about this issue. There have been two types of matching discussed here:
- Matching between one N-channel device and its complementary P-channel device. I don't know whether this is necessary, but if you say it is, then it is.
- matching between multiple N-channel devices in cases when you have them in parallel. (Ditto, multiple P-channel devices.) This is where I have my doubt....
I had this niggling doubt, then I went back and checked in the "Art of Electronics" this morning, and now my doubt is one big question which refuses to go away. So... is matching of multiple parallel power MOSFETs needed at all?
I'm not sure who you're asking this question of, but here is my answer:
1) No, actually the opposite. According to me, there is no need to match between opposite polarities.
2) No, again. Matching is very important here. First of all, MOSFETs have a positive tempco at low currents and a negative tempco at high currents. What is "high" and what is low"? Depends on the device. Vertical MOSFETs (ie, just about anything besides a Hitachi or Magnatec) have a zero tempco point at many (ie, 5 - 20) amps. This means that in actual use, the vertical MOSFET will always have a positive tempco.
But even with a lateral MOSFET that has a negative tempco above 100 mA or so, your argument doesn't hold water. For the negative tempco to start to equalize things, there needs to be a temperature mismatch between devices. The only way there will be a temperature mismatch is because one device is doing more work.
To see the actual effect that would occur, just look at the data sheet. On the 2SK1058 operating at 500mA (a few watts into a loudspeaker), you can see that a mismatch of Vgs (threshold) of 0.15 V is equivalent to a 50 C temperature difference!
So don't be stubborn, just use a little common sense (and look at the data sheets).
1) No, actually the opposite. According to me, there is no need to match between opposite polarities.
2) No, again. Matching is very important here. First of all, MOSFETs have a positive tempco at low currents and a negative tempco at high currents. What is "high" and what is low"? Depends on the device. Vertical MOSFETs (ie, just about anything besides a Hitachi or Magnatec) have a zero tempco point at many (ie, 5 - 20) amps. This means that in actual use, the vertical MOSFET will always have a positive tempco.
But even with a lateral MOSFET that has a negative tempco above 100 mA or so, your argument doesn't hold water. For the negative tempco to start to equalize things, there needs to be a temperature mismatch between devices. The only way there will be a temperature mismatch is because one device is doing more work.
To see the actual effect that would occur, just look at the data sheet. On the 2SK1058 operating at 500mA (a few watts into a loudspeaker), you can see that a mismatch of Vgs (threshold) of 0.15 V is equivalent to a 50 C temperature difference!
So don't be stubborn, just use a little common sense (and look at the data sheets).
tcpip said:
Any device with a negative tempco can be paralleled without resistors (you meant "source", not "drain") IF you match them carefully enough.
The key parameter for any output device in this situation is its transconductance (X amps of output for Y volts of input) under the actual operating conditions. If the devices are matched, then you don't need source resistors.
You could even parallel bipolars in the same way IF you made a composite device that had a negative tempco. In other words, if each output pair had it's own temperature compensation, then you wouldn't need emitter resistors (although you would still want to match the devices).
Shall we trust Charles, or does anyone diagree?
Charles has several times said that he does not think it is
important or even good to match N-channel vs. P-channel
devices. This is actually very good news in my view, since it
simplifies the process of buying devices, and settles a question
I have brought up on several occasion before in other threads,
without getting satisfactory answers. My concern was that if
buying quantities at once, one may probably end up with
devices from the same batches (one N batch and one P batch)
so the the N devices are likely to be reasonably matched and
the P devices to be reasonably matches, but on the other hand
it would considerably decrease the chance of finding complementary
matches. The alternative then would be to buy just a few devices
at a time, spread out over a lengthish time period, which would
increase the statistical spread, thus increasing the chance of
finding complementary matches, but decrease the chance of
finding matchin N devices and matchin P devices.
So far, it seems nobody in this thread has really argued against
Charles' point of view on this, so should we take that as an
indication that all the people with experience on this agree
with him?
Charles has several times said that he does not think it is
important or even good to match N-channel vs. P-channel
devices. This is actually very good news in my view, since it
simplifies the process of buying devices, and settles a question
I have brought up on several occasion before in other threads,
without getting satisfactory answers. My concern was that if
buying quantities at once, one may probably end up with
devices from the same batches (one N batch and one P batch)
so the the N devices are likely to be reasonably matched and
the P devices to be reasonably matches, but on the other hand
it would considerably decrease the chance of finding complementary
matches. The alternative then would be to buy just a few devices
at a time, spread out over a lengthish time period, which would
increase the statistical spread, thus increasing the chance of
finding complementary matches, but decrease the chance of
finding matchin N devices and matchin P devices.
So far, it seems nobody in this thread has really argued against
Charles' point of view on this, so should we take that as an
indication that all the people with experience on this agree
with him?
The one thing that I would add is that different device types may behave differently while being tested. I bought a large batch of IRF644s and had a devil of a time getting them matched. The first minute or two the digits were whizzing by so quickly that I couldn't even read them, much less count on them to be stable. I ended up waiting about ten minutes to get a decent reading.
On the other hand, the IRFP044s I bought to build the Aleph-X prototype settled down within seconds. Pure bliss compared to the '644s.
My operating assumption is that it's a thermal phenomenon. The '644 is a TO-220, whereas the '044 is a TO-247. Why it should make such a remarkable difference, I don't know.
At any rate, yes, match your N devices to each other and your P devices to each other. Let Self, Slone, et. al. go their own way.
Grey
On the other hand, the IRFP044s I bought to build the Aleph-X prototype settled down within seconds. Pure bliss compared to the '644s.
My operating assumption is that it's a thermal phenomenon. The '644 is a TO-220, whereas the '044 is a TO-247. Why it should make such a remarkable difference, I don't know.
At any rate, yes, match your N devices to each other and your P devices to each other. Let Self, Slone, et. al. go their own way.
Grey
Okay, so there seems to be a disagreement between you and "The Art of Electronics". They say power MOSFETs can be freely paralleled without any source resistors (sorry about the drain vs source goof-up). They don't mention any need for matching. In fact they make it clear that it's the negative tempco which allows this to be done, thus (I presume) making the matching unnecessary.Charles Hansen said:
I have the book at home.... I can quote giving page number if anyone wants. I found this comment mentioned in two places, Chapters 3 and 6.
Now I'm in a jam. Who do I believe?
H&H may not be looking at the kind of heavy biasing that audio folks take for granted, particularly in class A operation. They're more likely to use parallel MOSFETs in a power supply regulator or something of that nature.
Two points:
--One is that this is DIY. If you don't want to match devices, no one is going to make you. There's no such thing as a DIY police force that will come arrest you for using unmatched devices. Music will come out, and you will be happy that you built something that plays music.
--The other is that you might mount a set of unmatched devices with really, really small Source resistors. Something like .01 ohms. Run the amp. Measure the voltage drop across the resistors. Since they're so small in value, they won't have much effect on the operation of the circuit; they're just there to give you a measuring point. Calculate the current differences between the output devices. If you feel that the current differences aren't worth worrying about, then remove the .01 ohm resistors and carry on. If you find that one device is running, say, 40% higher current than the one next to it (note that this means it's running 40% more heat dissipation), then you might want to consider increasing the resistance of the Source resistors...or using matched devices.
The choice is yours...not mine, Charles's, or anyone else's. The ball's in your court.
Grey
Two points:
--One is that this is DIY. If you don't want to match devices, no one is going to make you. There's no such thing as a DIY police force that will come arrest you for using unmatched devices. Music will come out, and you will be happy that you built something that plays music.
--The other is that you might mount a set of unmatched devices with really, really small Source resistors. Something like .01 ohms. Run the amp. Measure the voltage drop across the resistors. Since they're so small in value, they won't have much effect on the operation of the circuit; they're just there to give you a measuring point. Calculate the current differences between the output devices. If you feel that the current differences aren't worth worrying about, then remove the .01 ohm resistors and carry on. If you find that one device is running, say, 40% higher current than the one next to it (note that this means it's running 40% more heat dissipation), then you might want to consider increasing the resistance of the Source resistors...or using matched devices.
The choice is yours...not mine, Charles's, or anyone else's. The ball's in your court.
Grey
tcpip said:
There is not much to understand. It's an expensive nuisance to match MOSFETs, but it's necessary in all the designs I've seen if you want reasonable current sharing.
For the case only of the CF configuration, there is a way not to have to do this. Rather than use a single driver for multiple parallel CF outputs, use instead multiple drivers, one per MOSFET device. It sounds very brute force, but consider that the drivers will do the matching, using the inherent 100% negative feedback in the CF configuration. And the drivers could be very cheap TO-92 devices, since each drives only a single MOSFET.
Assuming we have precisely matched source resistors, each driver will set Vgs to the "right" value to force equal current through the source resistor, so the device currents must match within the tolerances of the source resistors and driver Vbe's. I would use 1% source resistors; driver Vbe spread is not great.
Again, this is trading modest higher parts cost of the extra drivers for inventory and labor cost reduction due to not having to match output parts during manufacturing or field repair. For a DIY builder, it means you use all the output devices you have, not just the ones that match closely.
So, it's brute force and simple but has some significant benefits.
Dear all,
Thanks for the answers.
My reason for posting was because I was getting confusing inputs from different sources. By treating opinions expressed here as being on par with opinions in a classic text, I thought I was showing you all how important I considered your opinions.. I am not certain some of you understood this. Anyway, let's focus on MOSFETs, not misunderstandings.
I find an opinion expressed in a very highly-regarded textbook to be something I can't dismiss lightly. That was the reason for my re-visiting the issue. If some of you feel that such sources can be dismissed, maybe you could have said so, and explained why. That would have helped me look at opinions from that book in a new way, and would have helped me learn faster. For instance, I really appreciate your comment about the context of the book's comments (switching) may be different from that of audio. That's the kind of pointers a learner needs.
The success and usefulness of something like diyaudio depends on
Have fun.
Thanks for the answers.
Yes, this seems to be something the book didn't cover. I guess I'll now have to graduate to datasheets.millwood said:
Thank you.Charles Hansen said:
Yes, this is my suspicion too. The book seems to be discussing only on-off binary applications like motor switching, not linear applications, when they discuss power MOSFETs.GRollins said:
I already knew this.
My reason for posting was because I was getting confusing inputs from different sources. By treating opinions expressed here as being on par with opinions in a classic text, I thought I was showing you all how important I considered your opinions.. I am not certain some of you understood this. Anyway, let's focus on MOSFETs, not misunderstandings.Yes, this is interesting. I think I'll try this when I build my first MOSFET amp. Where can I get 2W 1% 0.01Ohm resistors?
Yes, I know. I'd think this is the norm in DIY. However, when many DIYers get together, it's quite normal for one DIYer to ask questions to others to help clear his confusion. I was under the impression that that's what I was doing, but it appears that I've tested the patience of some members on this thread severely. I can't figure out why.
I find an opinion expressed in a very highly-regarded textbook to be something I can't dismiss lightly. That was the reason for my re-visiting the issue. If some of you feel that such sources can be dismissed, maybe you could have said so, and explained why. That would have helped me look at opinions from that book in a new way, and would have helped me learn faster. For instance, I really appreciate your comment about the context of the book's comments (switching) may be different from that of audio. That's the kind of pointers a learner needs.
The success and usefulness of something like diyaudio depends on
- all of us carefully differentiating genuine questions/confusion from "argument for the same of argument"
- being patient with those of us who know less than we do
- giving pointers to help others learn, instead of making remarks which don't help. I've had to receive remarks like: "So don't be stubborn, just use a little common sense..." Etc. Most of us are already trying to use whatever common sense we have; let's just take that on trust.
Have fun.
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