Hi,
I am new here so please bear with me. I am have essentially no experience with audio amplifiers and equipment, but I need to construct a high power amplifier for research purposes and this seems to be the place with the best resources. I do have quite a bit of experience in low power DC and high power pulsed-based electronics work, but not high power DC stuff, although I've spent the past week reading up on amplifier design. The requirements my amplifier needs to meet are:
a) 0-200V, capable of sourcing ~40A at all voltages
b) DC - ~30kHz, the higher the bandwidth the better
c) reasonably small (eventually we will need 34 of them so size optimization is best)
It is to be powered from a 200V DC bus so I do not need to work about a rectifier.
Based on these requirements, I think that a class H style amplifier would be best, and I've read all of Workhorse's thread on class H amplifiers, but I did not quite understand some of the details, especially the rail-sticking. What causes rail-sticking? ilimzn made a post describing a digitally driven amplifier, but it was unclear to me what impact that had on rail-sticking. My amplifier will be FPGA/DAC driven, so if I use the FPGA to pre-switch the rails (or post-switch them on a falling edge) would that eliminate the rail-sticking, or is it caused by some inherent feature of the transistors?
Finally, all the schematics I have seen on here use almost entirely all discrete parts. What are the disadvantages to just using an op-amp and incorporating a voltage gain stage followed by a power gain stage in the feedback loop?
Thanks,
Jon
I am new here so please bear with me. I am have essentially no experience with audio amplifiers and equipment, but I need to construct a high power amplifier for research purposes and this seems to be the place with the best resources. I do have quite a bit of experience in low power DC and high power pulsed-based electronics work, but not high power DC stuff, although I've spent the past week reading up on amplifier design. The requirements my amplifier needs to meet are:
a) 0-200V, capable of sourcing ~40A at all voltages
b) DC - ~30kHz, the higher the bandwidth the better
c) reasonably small (eventually we will need 34 of them so size optimization is best)
It is to be powered from a 200V DC bus so I do not need to work about a rectifier.
Based on these requirements, I think that a class H style amplifier would be best, and I've read all of Workhorse's thread on class H amplifiers, but I did not quite understand some of the details, especially the rail-sticking. What causes rail-sticking? ilimzn made a post describing a digitally driven amplifier, but it was unclear to me what impact that had on rail-sticking. My amplifier will be FPGA/DAC driven, so if I use the FPGA to pre-switch the rails (or post-switch them on a falling edge) would that eliminate the rail-sticking, or is it caused by some inherent feature of the transistors?
Finally, all the schematics I have seen on here use almost entirely all discrete parts. What are the disadvantages to just using an op-amp and incorporating a voltage gain stage followed by a power gain stage in the feedback loop?
Thanks,
Jon
The way you mention a bunch of them, a power bus, and size, it seems like you're going to put all these in one place. I don't know what you're going to do with around 1/4 MW of audio amplifiers but the biggest one I've ever seen was a 50kW tube amplifer designed to drive a shaker table that was mounted on a 20ft cube of concrete poured into the ground. When it ran it shook the whole building anyway. By the way, that amp took up two entire 6 foot tall relay racks. It's a pretty safe bet you're not going to het a whole lot of advice on designing equipment like that here. What you're doing is going to end up looking like inverters for electric cars, except even bigger, cuz it's gonna make some Heat. The opamps will be the least of your worries.
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Hi Jon,
As Andrew mentioned, I have no idea what you plan on doing with 32 8kW amplifiers, but Class H is definitely not the solution for you. I think it might be imperative to reassess how much power you really need, and whether or not your 200V bus can supply the necessary power to run all those amplifiers.
As for an amplification solution, you'd be embarking on a 2-5 year R&D project which would ultimately cost you a small fortune. At that power level, class D is your only real option given your criteria, and you'd need to run a full bridge off your 200VDC bus with several parallel fets to deal with the 40A of current. A full bridge would minimize bus pumping, but you'd still likely end up with a fair amount of noise/EMI making its way back to your main bus.
Honestly, I don't think you're going to find a realistic solution to those requirements. Maybe if you share what it is you're trying to do, and exactly what these amplifiers will be driving, then there might be a chance that someone here would have a suggestion for a more realistic way of doing it. The only application I can think of is some kind of industrial automation, but with those power requirements, I'm really not sure.
A few questions:
1. Is that peak current/voltage output, or will the amplifier be required to deliver say 190VDC at 40A for any length of time?
2. Does distortion matter?
3. What kind of load is this thing driving? Purely resistive? Some kind of motor? A bank of batteries?
4. When you say small in size, what exactly do you mean?
5. Is the 200V bus well regulated? Reliable? Noisy?
6. Is EMI a concern?
Sounds interesting anyhow, if not a little high reaching...
Cheers,
Owen
As Andrew mentioned, I have no idea what you plan on doing with 32 8kW amplifiers, but Class H is definitely not the solution for you. I think it might be imperative to reassess how much power you really need, and whether or not your 200V bus can supply the necessary power to run all those amplifiers.
As for an amplification solution, you'd be embarking on a 2-5 year R&D project which would ultimately cost you a small fortune. At that power level, class D is your only real option given your criteria, and you'd need to run a full bridge off your 200VDC bus with several parallel fets to deal with the 40A of current. A full bridge would minimize bus pumping, but you'd still likely end up with a fair amount of noise/EMI making its way back to your main bus.
Honestly, I don't think you're going to find a realistic solution to those requirements. Maybe if you share what it is you're trying to do, and exactly what these amplifiers will be driving, then there might be a chance that someone here would have a suggestion for a more realistic way of doing it. The only application I can think of is some kind of industrial automation, but with those power requirements, I'm really not sure.
A few questions:
1. Is that peak current/voltage output, or will the amplifier be required to deliver say 190VDC at 40A for any length of time?
2. Does distortion matter?
3. What kind of load is this thing driving? Purely resistive? Some kind of motor? A bank of batteries?
4. When you say small in size, what exactly do you mean?
5. Is the 200V bus well regulated? Reliable? Noisy?
6. Is EMI a concern?
Sounds interesting anyhow, if not a little high reaching...
Cheers,
Owen
Our lab is wired for 250kW 480V 3 phase. The large number of them is for a much larger experiment which is slated to begin construction within the next year or so. The lab space for that has 500kW of power available. The heat issue is why we need to go with either class H or class G amps for the efficiency, and even then all of them will most likely be water cooled. Additionally, it is highly unlikely that we will be drawing a full 8kW through each amp, that is just an upper limit. My questions are not so much on how to handle power delivery but on the topology of H class amplifiers. Also, I didn't mention it above, but I am most definitely not driving any type of speakers or audio equipment with these, actually the intended purpose is to use it as a arbitrary waveform generator/programmable power supply.
Jon
Jon
Hi Jon
This sounds like some kind of industrial application, rather than audio. Does it require true AC output or just varying DC (i.e. single-rail supply with load connected to ground)?
Either way, wouldn't class D amplification be more appropriate? 40A drawn off a 200V bus means up to 8KW dissipation. That would require a huge bank of transistors and serious cooling if done with a non-switching amp.
btw: You might find this of interest, even though it's not nearly in the same power range you're looking for: http://www.diyaudio.com/forums/solid-state/154388-its-cheap-its-n-its-dirty-its-circlomos.html
Regards - Godfrey
Hi,
I work on a plasma physics experiment, and one of our research interests is electrostatic flow control, which means that the load I am driving is the plasma itself. This presents a number of problems, which is why my requirements are both vague and high. While it is possible to model the plasma in terms of circuit elements, this will basically require extensive numerical modeling on our end, as we have a very unique geometry. What this means is that I need to overdesign them so they can handle "100s of volts and 10s of amps", the reason I chose 200V and 40A is that the DC supply powering them is a 200V supply (it has plenty of current to handle my needs, and I believe it is decently regulated), and 40A is a conservatively high estimate on my part.
As I mentioned in my second post (I think because I am newly registered all my posts have to go through moderator's approval first so it hasn't shown up yet?), this is most definitely not an audio application.
The circlomos is interesting, but as you said it is definitely not powerful enough.
As a PPS, the goal is to be able to have one unit which can be set to just output a DC level, do a transient square pulse, sine waves, etc... so that we can look at the transient behavior of the plasma.
EMI is not too concerning, I would classify our lab as electrically noisy. Spectral purity would be nice, thus my interest in a class H.
My understanding is that class D amplifiers cannot reproduce a DC level, is this correct?
Thanks,
Jon
I work on a plasma physics experiment, and one of our research interests is electrostatic flow control, which means that the load I am driving is the plasma itself. This presents a number of problems, which is why my requirements are both vague and high. While it is possible to model the plasma in terms of circuit elements, this will basically require extensive numerical modeling on our end, as we have a very unique geometry. What this means is that I need to overdesign them so they can handle "100s of volts and 10s of amps", the reason I chose 200V and 40A is that the DC supply powering them is a 200V supply (it has plenty of current to handle my needs, and I believe it is decently regulated), and 40A is a conservatively high estimate on my part.
As I mentioned in my second post (I think because I am newly registered all my posts have to go through moderator's approval first so it hasn't shown up yet?), this is most definitely not an audio application.
The circlomos is interesting, but as you said it is definitely not powerful enough.
As a PPS, the goal is to be able to have one unit which can be set to just output a DC level, do a transient square pulse, sine waves, etc... so that we can look at the transient behavior of the plasma.
EMI is not too concerning, I would classify our lab as electrically noisy. Spectral purity would be nice, thus my interest in a class H.
My understanding is that class D amplifiers cannot reproduce a DC level, is this correct?
Thanks,
Jon
You shouldn't have any special problems with a class D amplifer holding a DC output. Really one of your larger problems is going to be the top end of the spectrum. If you can get away with 10 times less (or at least half) bandwidth you should definitely put away the calculator and start searching the net for shaker table amps, where that kind of power a channel is very common. Who knows, maybe these days they make them that fast too.
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I just say shaker table amp because I think there are some companies with many years experience dedicated to producing very high power amps and while a shaker table probably would have no use for your kind of speed units may be produced that can do it for other applications. It's just that it is probably not most practical to bury yourself with the insanity of designing such a system from scratch until you find out that what you need actually isn't handled by a reputable company already. The last thing you want is stability and reliability problems at those power levels when all you really want to do is push some plasma around?
One problem you'd have with class D at 30kHz is unless your switching frequency was in the low megahertz your output filter will start drawing big current. Megahertz switching means more switching loss and reduced duty cycle dynamic range because Ton/off starts getting to be a big slice of the period. So in a way class H may not be too far off the mark though getting rail switching working well at that speed isn't going to be a piece of cake. Either way you're looking a Real spiffy amplifer.
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Hi Jon,
That sounds like a pretty neat application, but if you want to focus on the plasma research side of things, then I'd strongly suggest buying a complete class D solution from one of the more reputable PA manufacturers. Some of them make amplifiers that will come pretty close to what you need, in a ready to run package, minus the 30kHz bandwidth. Maybe you could relax that to 10kHz?
They're not cheap, but they'll cost a small fraction of what it would be to develop one from scratch.
A class D amp has no problem maintaining DC at the output, but you'll need a full bridge if you want to get 200VAC (RMS) output from a 200VDC bus. You won't be able to get 200VDC output from this setup however.
Cheers,
Owen
That sounds like a pretty neat application, but if you want to focus on the plasma research side of things, then I'd strongly suggest buying a complete class D solution from one of the more reputable PA manufacturers. Some of them make amplifiers that will come pretty close to what you need, in a ready to run package, minus the 30kHz bandwidth. Maybe you could relax that to 10kHz?
They're not cheap, but they'll cost a small fraction of what it would be to develop one from scratch.
A class D amp has no problem maintaining DC at the output, but you'll need a full bridge if you want to get 200VAC (RMS) output from a 200VDC bus. You won't be able to get 200VDC output from this setup however.
Cheers,
Owen
Hi Jon
I believe Eva has experience with high-power design. Perhaps it would be worth sending him/her a private message here: http://www.diyaudio.com/forums/members/eva.html
Maybe also star882. She can be contacted here: http://www.diyaudio.com/forums/members/star882.html
Cheers - Godfrey
I believe Eva has experience with high-power design. Perhaps it would be worth sending him/her a private message here: http://www.diyaudio.com/forums/members/eva.html
Maybe also star882. She can be contacted here: http://www.diyaudio.com/forums/members/star882.html
Cheers - Godfrey
Thanks for the information about shaker table amps, they are nice in that commercially available modules can easily handle my power requirements in a 3U package, but the bandwidth is not optimal. In the end, we may have to compromise on bandwitdh, but I'm still interested in the possibility of constructing an amplifier myself.
Thanks for the information on class D amps, they do seem the best way to proceed. I like the concept of Crown's BCA class D amp in that the effective switching frequency is double the actual switching frequency. Do you know of any reference designs for high speed switching? The highest switching frequency I have seen implemented is 250kHz. Thanks for the help,
Jon
Thanks for the information on class D amps, they do seem the best way to proceed. I like the concept of Crown's BCA class D amp in that the effective switching frequency is double the actual switching frequency. Do you know of any reference designs for high speed switching? The highest switching frequency I have seen implemented is 250kHz. Thanks for the help,
Jon
The best output for such high-power work you are doing will be a power MOSFET drain-output topology. To prevent individual MOSFET output failure, it is necessary to sense the current flowing in each of the output devices and then force all of the output transistors in the same cascode (+ or -) to conduct current equally. This is done with a local nested servo-circuit.
Another benefit from doing this is that the current being conducted by the upper and lower cascodes is available for feedback. The upper and lower cascode current signals are multiplied together and the product is forced to equal a constant. This keeps one output 'warmed up' while the other cascode is delivering 40A. It may be shown mathematically that forcing the product of the cascode currents to equal a constant results in the lowest-possible third harmonic distortion.
I have built such an amplifier to deliver 500 Watts at +/-200V at a full-power bandwidth of 1MHz and a small-signal bandwidth of 5MHz (to drive the horizontal deflection coil of a projection CRT). Because each MOSFET is under tight current-sharing control, the output may be simply expanded up to any power level, such as 10,000W, with little impact on specs.
The best linear DC - 100KHz power amplifiers ever designed were used to drive the horizontal deflection coils of large medical CRTs and military projection CRTs. These amplifiers were required to slew a pixel to a location and settle within several usec - requiring extreme bandwidth, low noise, linearity and damping.
You may be able to find some of these in the US patent database or search Google patents. Another way is to research the companies who made these amps and try to get a manual.
Hi Jonjara,
Class-H if done with 4 steps in monorail grounded bridge design can match your requirements while keeping dissipation low. If you are intrested we can discuss about it.
I am currently working on 10kW 8-phase Interleaved Class-D resulting output ripple frequency is 800khz to target low impedance drive capability down to 1 ohms and yes this is for Audio [DC-40khz], but same could be used for shaker tables also.
regards,
Kanwar
Class-H if done with 4 steps in monorail grounded bridge design can match your requirements while keeping dissipation low. If you are intrested we can discuss about it.
I am currently working on 10kW 8-phase Interleaved Class-D resulting output ripple frequency is 800khz to target low impedance drive capability down to 1 ohms and yes this is for Audio [DC-40khz], but same could be used for shaker tables also.
regards,
Kanwar
hi workhorse
greetings can you tell me if a 3 step h class power supply can
be added to ab class amplifier after building apexaudios h 900 i want to apply
3 step supply.Please can you tell me which commercial amplifier circuits to look
into searched a lot but could not come across any schematics hoping you can
help me out
thanking you
andrew lebon
greetings can you tell me if a 3 step h class power supply can
be added to ab class amplifier after building apexaudios h 900 i want to apply
3 step supply.Please can you tell me which commercial amplifier circuits to look
into searched a lot but could not come across any schematics hoping you can
help me out
thanking you
andrew lebon
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