I'm sorry, I said I wasn't going to post again, but this is too irrational for me to ignore.
So you care about a few tens of milliohms of MOSFET Ron and it's effect on efficiency, but you don't care about potentially hundreds of milliohms of inductor resistance in series with your load?
Please, don't just assume I'm wrong or out to get you. Think it through carefully and you will see the inconsistency in your argument.
So you care about a few tens of milliohms of MOSFET Ron and it's effect on efficiency, but you don't care about potentially hundreds of milliohms of inductor resistance in series with your load?
Please, don't just assume I'm wrong or out to get you. Think it through carefully and you will see the inconsistency in your argument.
I'm sorry, but I think some of you don't have detailed electronic design experience.
Thats OK. But look, lighten up.
MOSFETs definitely have higher resistance.
The resistance in these devices is what causes the heat in them, and that is what limits how much power an amp can be designed for.
I'm not concerned about this resistance because of over all power efficiency. I'm concerned about it because I want to design an amp with lots of power, and this resistance is what gives the output transistors a thermally based power limit.
Thats OK. But look, lighten up.
MOSFETs definitely have higher resistance.
The resistance in these devices is what causes the heat in them, and that is what limits how much power an amp can be designed for.
I'm not concerned about this resistance because of over all power efficiency. I'm concerned about it because I want to design an amp with lots of power, and this resistance is what gives the output transistors a thermally based power limit.
Hmmm, I grow confused, ZenMasterBrian.
The world is telling you you're in error. You're not listening. The world can't be right, so, you must know something the world doesn't?
How about a comprehensive pro /con list to further your points on the matter? That way we can pick it apart one by one for you on a point form basis all neat and tidy. Then again who's to say if you did make the list you'd still post it?
The world is telling you you're in error. You're not listening. The world can't be right, so, you must know something the world doesn't?
How about a comprehensive pro /con list to further your points on the matter? That way we can pick it apart one by one for you on a point form basis all neat and tidy. Then again who's to say if you did make the list you'd still post it?
zenmasterbrian said:
In my textbook entitled "Power Electronics" authored by Mohan, Undeland, and Robbins is a lot of good information about power electronic devices as well as converters, applications, and design information.
Some good bits of info about power BJTs intended for power swith-mode applications:
"Nearly all the power dissipated in the switch-mode operation of a BJT occurs when the transistor is in the on state. In this circumstance the power dissipation is given by (ignoring base current losses) Pon=Ic*Vcesat. The collector-emitter saturation voltage increases with increasing collector current."
"Power BJTs have low current gain, especially at larger breakdown voltage ratings."
"The SOAs of the BJT are limited by second breakdown."
Thus, for BJTs with large voltage breakdown ratings (like you would use in your amp, Brian) beta is low. This means for large collector currents you will also need large base currents. This means you have lot more power dissipation in the base drive circuitry. Further, the negative temperature coefficient of BJTs means they are not as easily paralleled.
Some good bits of info about power MOSFETs intended for power switch-mode applications:
"Except at higher switching frequencies, nearly all of the power dissipated in a MOSFET in a switch-mode power application occurs when the device is in the on state. The instantaneous power dissipation in the on state of the MOSFET is given by Pon=Id*Id*Rdson."
"The on state resistance increases significantly with increasing junction temperature."
"On state losses in a MOSFET rise much more rapidly with blocking voltage rating than do those in a BJT."
"The SOA of a MOSFET for switch-mode application is large (rectangular) because it is not subject to second breakdown."
Power MOSFETs usually have higher Vgs ratings and require a larger gate-source voltage differential to fully turn them on and reduce Rdson. However, a large sustained current is not required to keep the MOSFET turned on. The gate capacitances need to be charged once and held at that voltage to keep the MOSFET on. This means reduced power dissipation in the gate drive circuitry. Further, the positive temperature coefficient of MOSFETs means they can be easily paralleled.
Both devices have their appeals and turn-offs, but I don't see how you can overlook the use of multiple MOSFETs instead of a single or even multiple BJTs even for low switching frequencies.
I can see where this is going................ you explain, he ignores it and tells what he's been telling since the start of this thread.
p.s: I'm not trying to start anything here. it's just what I've seen since I've been following this thread......
p.s: I'm not trying to start anything here. it's just what I've seen since I've been following this thread......
Ditto. Also if we fall pray to arguing everything arguable we just end up doing all his research for him... just looks like a fisherman to me.
I don't mind doing some research cause it helps refresh my memory as well 🙂 I never said I was gonna help design the amp! On the contrary, I have absolutely zero interest in a 1kW class d amp using BJTs.
I just figured it may sound more credible if I got the information from a textbook.
I just figured it may sound more credible if I got the information from a textbook.
Hey I'm with you. Moments ago I read 98% of that plus another 10% other (comments on bandwidth/gm/noise too), just on some website that google found virtually on its own, as one of the first three links.
Anyway he's been told all anyone can be told, bring on those designs I say 🙂
Anyway he's been told all anyone can be told, bring on those designs I say 🙂
I think that only one in this thread without "detailed electronic design experience" is YOU.zenmasterbrian said:
Have you ever designed and implemented SMPS or class-d amp?
And its NOT OK because you are damn stubborn and dont listen anyone else than your own ramblings
classd4sure, Brian ( BRWX ), djQUAN, mzzj,
I appreciate the fact that you all have different POV's. diy projects are going to be different then mass marketed products.
But there is something you need to understand. NO ONE IS REQUIRED TO POST! Meta is contagious, and it drives legitimate posters away.
I'm not going to read or respond to posts that amount to negative meta-conversation.
If someone posts topical, then I will find it and respond to it.
The rest of it I am going to ignore completely.
I'm convinced that this board is full of good people, and so I encourage everyone to ignore all meta.
Give me a little while now, and I'll try to respond to some of the topical portions.
Also, remember that this thread is titled Class D frequency. It is a question about how low the frequency can be. I'm going to try and steer it back to that.
I know that most class D designs are done for a different set of objectives.
I appreciate the fact that you all have different POV's. diy projects are going to be different then mass marketed products.
But there is something you need to understand. NO ONE IS REQUIRED TO POST! Meta is contagious, and it drives legitimate posters away.
I'm not going to read or respond to posts that amount to negative meta-conversation.
If someone posts topical, then I will find it and respond to it.
The rest of it I am going to ignore completely.
I'm convinced that this board is full of good people, and so I encourage everyone to ignore all meta.
Give me a little while now, and I'll try to respond to some of the topical portions.
Also, remember that this thread is titled Class D frequency. It is a question about how low the frequency can be. I'm going to try and steer it back to that.
I know that most class D designs are done for a different set of objectives.
Actually, it's how low it can be before you _need_ mosfets, seems to me the easy answer is build one and tell us.
Regarding Mohan, Undeland, and Robbins:
First, this sounds like a power supply book. So it is usefull, but not a prescription. Power supplys are designed for a different set of objectives than many diyaudio projects. Power supplies are usually designed for size and cost. Diyaudio can be different.
All the admonishions about how hard it is to design with BJT are true. They go back decades and decades. Thermal runaway, current hogging, and high drive currents are all difficult to contend with.
But also understand, that power and heat generated in drive circuitry does not contribute to the thermal power limit of the output devices.
<<
Both devices have their appeals and turn-offs, but I don't see how you can overlook the use of multiple MOSFETs instead of a single or even multiple BJTs even for low switching frequencies.
>>
I think this is the most important statement from this text book. Both devices have their appeals and turn-offs.
These authors do not see how you can overlook the use of multiple MOSFETs insead of a single, or even multiple BJTs, even for low switching frequencies.
This is their opinion, and it is given regarding the design of power supplies for commercial markets. DIYaudio does not have to compete in that kind of a market. Such commecial market power supplies do not tend towards low frequencies, because they want the size and cost of the magnetics to be low. So the point of these authors is more a hypothetical than something that actually occurs.
Further, I am not overlooking anything. Remember the title of this thread is Class D Frequency, not some BJT vs MOS debate. How low the frequency can be, will decide whether or not BJT can even be considered.
In the statement made by these authors I do find an implicit admission that BJT has less voltage drop and therefore lower heat generation, and therefore it can put out more power per device.
The key point for these authors would seem to be that you can parallel MOS devices easily, and get what you would have gotten from a smaller number of BJTs.
That is true. I do not contest that.
But again, they are still speaking from the realm of power supplies, where the lower switching frequencies are only a hypothetical, because of concern about cost and size of magnetics.
What I am interested in is not a power supply, it is not a broad band audio amp, and it is not intended to compete on any kind of mass market.
So the first thing to determine to see if what I want is viable, is how low can the switching frequency be?
Seems like it has to be ultrasonic. But still what is the number?
I have tentatively decided on 23.5khz, but I am still not totally sure that that is acceptable.
I believe there are precedents of people designing class D amps that ran at that rate, in decades past. I belive there is some body of knowledge about what is perceptible. I have tried looking in different places.
If 23.5khz looks like a go, I am likely to start a new thread to explore circuit toplogy and device technology issues.
If anyone has specific links to transistor data sheets, for any technology, I would be interested to see them.
All the links provided here have already been highly instructive.
First, this sounds like a power supply book. So it is usefull, but not a prescription. Power supplys are designed for a different set of objectives than many diyaudio projects. Power supplies are usually designed for size and cost. Diyaudio can be different.
All the admonishions about how hard it is to design with BJT are true. They go back decades and decades. Thermal runaway, current hogging, and high drive currents are all difficult to contend with.
But also understand, that power and heat generated in drive circuitry does not contribute to the thermal power limit of the output devices.
<<
Both devices have their appeals and turn-offs, but I don't see how you can overlook the use of multiple MOSFETs instead of a single or even multiple BJTs even for low switching frequencies.
>>
I think this is the most important statement from this text book. Both devices have their appeals and turn-offs.
These authors do not see how you can overlook the use of multiple MOSFETs insead of a single, or even multiple BJTs, even for low switching frequencies.
This is their opinion, and it is given regarding the design of power supplies for commercial markets. DIYaudio does not have to compete in that kind of a market. Such commecial market power supplies do not tend towards low frequencies, because they want the size and cost of the magnetics to be low. So the point of these authors is more a hypothetical than something that actually occurs.
Further, I am not overlooking anything. Remember the title of this thread is Class D Frequency, not some BJT vs MOS debate. How low the frequency can be, will decide whether or not BJT can even be considered.
In the statement made by these authors I do find an implicit admission that BJT has less voltage drop and therefore lower heat generation, and therefore it can put out more power per device.
The key point for these authors would seem to be that you can parallel MOS devices easily, and get what you would have gotten from a smaller number of BJTs.
That is true. I do not contest that.
But again, they are still speaking from the realm of power supplies, where the lower switching frequencies are only a hypothetical, because of concern about cost and size of magnetics.
What I am interested in is not a power supply, it is not a broad band audio amp, and it is not intended to compete on any kind of mass market.
So the first thing to determine to see if what I want is viable, is how low can the switching frequency be?
Seems like it has to be ultrasonic. But still what is the number?
I have tentatively decided on 23.5khz, but I am still not totally sure that that is acceptable.
I believe there are precedents of people designing class D amps that ran at that rate, in decades past. I belive there is some body of knowledge about what is perceptible. I have tried looking in different places.
If 23.5khz looks like a go, I am likely to start a new thread to explore circuit toplogy and device technology issues.
If anyone has specific links to transistor data sheets, for any technology, I would be interested to see them.
All the links provided here have already been highly instructive.
zenmasterbrian said:
It's a power electronics book. The subtitle is "Converters, Applications, and Design", all priniciples which can just as easily be applied to class d audio amplifier design. After all that's what an audio amplifier is - a power converter.
The heat dissipated in the drive circuitry doesn't contribute to the thermal power limit of the devices, but the base drive current of a BJT does. Also, overall efficiency is effected in a negative way with large base drive requirements which is an argument you used in favor of BJTs.
That last <<snip>> is not a quote from the book. I did not put it in quotes because it is my writing.
I know this isn't a debate over BJTs versus MOSFETs but output device selection is so important because you have to design the entire output stage around them. You can use MOSFETs at any frequency and expand the limits fairly easily by just adding more in parallel as long as the gate drive circuitry can handle it. You can't just do that with BJTs, but for whatever switching frequency you choose you should most certainly be able to use BJTs.
Go to the manufacturers web sites and search for BJT datasheets.
You don't need us to tell you which switching frequeny to use because 1) you won't listen and 2) it seems you've already decided on one.
BWRX said:
Let me just clarify a couple of points. A class D amp is similar to a switching power supply. But it is not necessarily designed to meet the same contraints. Small and cheap is not necessarily an objective.
Second, I am only secondarily concerned with power efficiency from the power mains to the speaker terminals. The efficiency I am really concerned with is in the output devices themselves, and that is only because that is what limits how much power I can design for.
I've got an interlibrary loan request to get something that I remember has a table, showing that BJT is still used in some industrial power switching applications.
I listen to everything. It is true that I have already decided what approach I want to explore the feasibility of. If it is not feasible, I will abandon it. But nothing I have heard so far has convinced me of that.
I don't know what the environmental issues of low ultrasonics are. If there really are problems, that would sink my initial approach to this.
Has anyone seen amps that run in that frequency range? I believe they have existed. Any links?
Any links to material written about low ultrasonics pertaining to switching. I know that in the 1970's, switching power supplies did run in the 20khz to 25khz range. They did not go lower because of audible sound.
zenmasterbrian said:
IRFB4410 is a 100v mosfet in to-220 case. 8mOhms rdson typically, 10mohms max. BUV20 is one of the best bipolars that I can find. Its a high-current NPN bipolar in bigger and more expensive TO-3 case. VCE sat 0.3v typical and 0.6v max at 25amps. 4 times cheaper mosfet in smaller package have lower voltage drop.... (to220 vs. TO-3)
TO220 bipolars dont have any change against TO220 mosfets so I didnt even take them to comparision.
I'd only make the single request that you don't go starting new thread after new thread and keep it all localized here..... please.
Since you already answered your own questions it really is time to start seeing some circuits don't you think? At least lay out your design criteria. What have you set forth so far, slowest, most inefficient class d amp ever, for bass only.
The thought occurred to me, perhaps you'd consider limiting your search timeline to everything from at least over a decade ago. That'll keep your results alot more relevant.
Does this have anything to do with a school assignment?
Since you already answered your own questions it really is time to start seeing some circuits don't you think? At least lay out your design criteria. What have you set forth so far, slowest, most inefficient class d amp ever, for bass only.
The thought occurred to me, perhaps you'd consider limiting your search timeline to everything from at least over a decade ago. That'll keep your results alot more relevant.
Does this have anything to do with a school assignment?
classd4sure and poobah
I'm not going to read or respond to posts that amount to negative meta-conversation.
If someone posts topical, then I will find it and respond to it.
No one is required to post here.
mjjz,
Do you have a like for such a data sheet. I will try to find such myself now. But it is well know that comparable sized MOS has more voltage drop than bipolars.
If you are looking at online data, posting it would help.
I'm not going to read or respond to posts that amount to negative meta-conversation.
If someone posts topical, then I will find it and respond to it.
No one is required to post here.
mjjz,
Do you have a like for such a data sheet. I will try to find such myself now. But it is well know that comparable sized MOS has more voltage drop than bipolars.
If you are looking at online data, posting it would help.
http://www.irf.com/product-info/datasheets/data/irfs4410.pdf#search="IRFB4410"
This would seem to be it? I know that International Rectifier does make some fine parts.
This would seem to be it? I know that International Rectifier does make some fine parts.
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