| Electronics > Projects, Designs, and Technical Stuff |
| SMPS for vacuum tube power amplifiers.(status: back at it) |
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| SK_Caterpilar_SK:
--- Quote from: TimNJ on April 18, 2019, 10:55:54 pm ---Popular ways to get a low-voltage Vcc rail for powering your primary side ICs are... 1.) Add another winding to your transformer. Usually called 'aux winding' or similar. You'll still need some sort of "bootstrap" supply to get the thing up at startup. 2.) Use a small offline switcher. Power Integrations makes controllers with integrated MOSFETs. Those are nice. Regarding the output error amplifier. There are rigorous ways to determine those values, and not-so-rigorous ways to determine them. Ideally, you'd like to know the gain/phase response of your entire system. Then, you apply compensation to your feedback network to ensure that the system is unconditionally stable, under all conditions. You can look up feedback network compensation with TL431. You can basically change: 1.) DC Gain 2.) Mid-band gain 3.) Frequency of pole 4.) Frequency of zero --- End quote --- Thanks for the reply. I forgot to mention that I dont have acess to custom wound switching transformers. I also have 0 idea how to calculate them, what core materials to use and everything. It is much easier to just get a part thats already available. I plan to power the controller with a capacitive divider, for now. Later I might go about creating a specific transformer for this purpose but I need bigger minds to help me do that. The transformer part is "Coilcraft HA4060". I simply have no patience of custom winding the thing. This way I get known numbers of the switching transformers like working frequency and peak currents and all the other stuff, that I would have had to measure in the first place. Maybe in the future I might end up teaming up with some switching transformer manufacturer, but that wont be any time soon since I really just want one or two pieces, no company in their right mind would support that. And I have no clue how am I going to solve the issue with the feedback. I dont know what values to choose and or how to do measurements on the original circuit so I can determine the values based on measurements.. |
| T3sl4co1l:
Yeah, lifting ground won't work because you need a change in LED current to transmit a signal, and that current has to flow out the ground, so the voltage sensed by the error amp changes with load... What you can do, is make a little local supply, say 10V or so, with a zener shunt regulator, or an LR8 or something like that. This supplies the error amp, which should be TLV431 (note the voltage limit!) for lower current consumption. The "cathode" current can be cascoded into a pass transistor that drives the opto, or the whole thing supplied from the low voltage supply (either way, you need to handle the dissipation caused by the couple mA maximum consumption). BTW, the opto symbol is upside down? ??? Tim |
| SK_Caterpilar_SK:
--- Quote from: T3sl4co1l on April 19, 2019, 12:33:40 pm ---Yeah, lifting ground won't work because you need a change in LED current to transmit a signal, and that current has to flow out the ground, so the voltage sensed by the error amp changes with load... What you can do, is make a little local supply, say 10V or so, with a zener shunt regulator, or an LR8 or something like that. This supplies the error amp, which should be TLV431 (note the voltage limit!) for lower current consumption. The "cathode" current can be cascoded into a pass transistor that drives the opto, or the whole thing supplied from the low voltage supply (either way, you need to handle the dissipation caused by the couple mA maximum consumption). BTW, the opto symbol is upside down? ??? Tim --- End quote --- Thank you Tim, in the ideal world I would have had a tap at the tranformer for the 431 and the opto which would have been propbably the best but overkill solution. If I would have had a custom tranformer that would have another secondary for the tube filaments (6,3V) then I would have used that. But I think this might be the best solution for my current setup. Simply just a shunt regulator.. now just to measure the current and choose the proper value for R6. |
| T3sl4co1l:
Yeah, heater winding would do perfectly... ah well. :) That's why I added positions for an aux winding on this general-purpose power supply board: https://www.seventransistorlabs.com/Images/100WPowerSupplies.jpg they've mostly gone unused, but are there for when it's needed. :) (I did make one, a 150-300V 100W model, which uses this aux secondary winding.) Also what's nice about custom transformers, and having a bunch of bobbins in stock. Tim |
| SK_Caterpilar_SK:
--- Quote from: T3sl4co1l on April 19, 2019, 09:55:24 pm ---Yeah, heater winding would do perfectly... ah well. :) That's why I added positions for an aux winding on this general-purpose power supply board: https://www.seventransistorlabs.com/Images/100WPowerSupplies.jpg they've mostly gone unused, but are there for when it's needed. :) (I did make one, a 150-300V 100W model, which uses this aux secondary winding.) Also what's nice about custom transformers, and having a bunch of bobbins in stock. Tim --- End quote --- That looks hella professional. The big deal with the heater winding if there would be any, is that for an average amplifier like mine for example mine, two power tubes and two pream tubes, It requires at least 4A of output current with absolutely no headroom, 3A for the power tubes and 900mA for both preamp tubes. So id say a 30W winding would be ideal for an average amp (6,3V 5A). Maybe sometime in the future if I get a chance to work on the transformer with a professional SMPS transformer manufacturer. But the big deal is I got no idea how to calculate the transformers at all. I may ask Coilcraft if they would be willing to help out but I doubt any of that will happen. So I guess its only left to do measurements and go ahead build the thing. for the FB. I just hope it will be sub 5mA cause then I can simplify things and not waste incredible amounths of power lol. |
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