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Thread: Vacuum Tube SMPS

  1. #1

    Vacuum Tube SMPS

    The lengths that I will go through to save a couple of pounds of iron. Got it in my head to build a (roughly) +350V 0.1A, 6.3/12.6V supply for a guitar amp. Want to use a TOP202YAI, . Thought of grabbing a core and input bits from an ATX ps.

    Been sifting through threads here which prompted me to look up datasheets, I have a lot of learning to do. I do have a background in electronics, blew up my share. Mainly into tube amps for now, the paltry 20W amp I want to build seems rather out of place with you guys doing 1kW supplies. Parts are coming on a slow boat from China so not a lot will happen quickly, give me time to figure out what I really am doing. Hopefully you guys can keep me from doing anything really wrong. In return I might give you something to laugh at.

  2. #2
    A rumbling soft motor lynxlynx's Avatar
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    Apr 2015
    Russia, Siberia
    Your power requirements are adequate for flyback supply, but output voltage will not probably fit. If you want to use flyback type, then for your +350V output you will need probably five-ten times higher rating output diode, i.e. 1-2kV. It's common for flyback supplies output of 24V for example to have 100V output diode and more. You can vary this voltage by increasing primary inductance, but this will also put a stress on primary switching element, however, decreasing current.

  3. #3
    Hi, thanks for the response. I am new at this so I expect to take some wrong turns, learning something new sometimes has bumps in the road. I tried to find a circuit to do what I want and came up empty handed, might just used the wrong search terms. I was going to go real cheap and dirty taking two SMPS's, take the transformer out of the one and step up the voltage by connecting the low voltage sides of the transformers together. Then there was the idea of using a 30V PS and hook a 10X voltage multiplier to the secondary. Just need a bunch of fast diodes without a high voltage rating.

    But I had a SMPS built for an alarm system battery charger using the TOP202YAI. It had some fried bits at the alarm portion of the circuit board but the supply works. So I thought to keep it intact as a working example of what the circuit should be doing if I do my own variation of it. Otherwise I am winging it. I know I have no clue if a transformer from a computer supply would work with the chip. Might be the wrong frequency range, wrong inductance if wired up for my application. It is best to say I kind of know how much I don't know. I should scope out my alarm ps board but too busy getting some guitars done (amateur builder). I did look for some diodes for the secondary, found the BYV26G, not sure if it will be fast enough or have a high enough voltage rating. I hope to find a different one as I would have to buy 10 and it starts to get pricey.

    Found a BYT54M 1000V 1.25A, not sure if that is good enough voltage wise or speed.

    Could do some more searching, seems like I should be able to find something. Really the black hole of my knowledge is in the transformer. Not a surprise I guess.

  4. #4
    A rumbling soft motor lynxlynx's Avatar
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    Apr 2015
    Russia, Siberia
    I did some "example" calculations and I can say it will work if you will go with 1400V one. It nicely fits into requirements + some safety gap. I did calculation if I would to build it today with my "example" transformer. Primary switch voltage is about 600V and secondary diode voltage is 920V and peak currents are in safe limits. 100nS recovery is fine if you will not overdrive it with hundreds of kHz, 100kHz of TOP is fine I think. I did used FR207 with it's 500nS with 100kHz and it did worked, but in other switching circuits it will blow.
    However if I wanted to design such a circuit I probably would go with separate voltage for second small push-pull supply that will provide +350V separately. I would power it from either 12 or up to 20V because of gate voltage. Just it's hard to find really high voltage fast type diodes there where I live and long waiting for them to be shipped. It's also more safer. And overpaying for damper diodes when I can buy some MUR460 which are proven fast and stable (however you will need 2 or 4 of them ).

    Flyback transformer is indeed complex thing. You need to be cautious when you select primary voltage because you need to send all of parasitic kickbacks into it's RCD snubber across primary so it will not kill your switcher. You need to pick up current protection resistor if you build it discrete type/UC384x, which is also sometimes maybe a hard task (but you're using TOP, so you are safe). All switching and rectifying elements must have avg. 2-3 times more ratings than your requirements (voltage, current) because of how flyback works - delivering energy by short but hard spikey packets. However you can vary impulse duration.
    It's not like you can salvage random transformer from ATX. I think you will need to rewind it. If you thinking about taking main transformer then do not ever do that without creating a gap with paper between halves - it's usually gapless and you will need gap in your transformer core. In fact, flyback transformer is just an multiple winding inductor and it needs to be protected from saturation.

    If you do not know how flyback transformer work, you can try to simulate it. Download LTSpice and I will send you an example (or even example for your +350V needs). For transformer calculations, seek for Starichok51 Flyback program. I have another calculator, but it's text mode based and still experimenting with it. I hope TOP has programs for it and you will probably skip all this nightmare of picking up random parts

    For this task you probably will need LC meter otherwise it's going just to be guessing and burning parts. Oscilloscope also is a very good option, but only with 100x probes!!!
    Last edited by lynxlynx; 04-01-2016 at 03:29 AM.

  5. #5
    You have been working overtime. I think there is a program from the manufacturer to select the transformer. Did a little reading the last day or so, you SMPS people are insane. I can find my way around the average schematic but this stuff is almost in the realm of black magic. Guess the programs help. I do have LTSpice on one of my computers, never really found the time to really learn to use it. Might as well jump in with both feet.

    I assumed I would have to wind my own transformer, was hoping to do a lower voltage test circuit and then up the voltage a few times till I get close to my target. I could be wrong but I get the feeling adding some windings will change the parameters of the transformer and might throw off the circuit. I have been trying to figure out the size of transformer needed and looking on EBay to see if there is one to be had at a reasonable price. Saw this, don't know enough if it is something I want or not. While I am all for being thrifty and recycling stuff I won't know what I will be getting out of my junk pile. If it saves me grief I could pick something like that up.

    I have a DMM that will measure capacitance and inductance. Don't have the 100X probes but I can hit up my brother when I get that far, one of the products the company he works for is a fencer to keep animals contained. A few loops of small gauge wire should be no problem also.

  6. #6
    "I assumed I would have to wind my own transformer, was hoping to do a lower voltage test circuit and then up the voltage a few times till I get close to my target."

    This is a very good idea. Start with a 12 to 24 vac transformer to power your flyback and make lower output voltages. Now you can poke your fingers all over the circuit and use regular scope probes.

    The transformer you picked out on Ebay is probably good for 30 to 50 watts and has a fair price. It has a lack of specs but the one you need the most is there. AL=285025%.

  7. #7
    Well that is great, one less thing to scratch my head about, I'll pick up the cores. Good to know I can muck around at lower voltages. Not too squeamish about higher voltages, play with my tube amps and need to troubleshoot 600V stuff at work. Have a healthy respect for HV, haven't got poked by it yet, 120V a few times.

  8. #8
    Been looking at diodes I can get and thought about a couple of UF4007's in series. I already have some, any problem using them?

  9. #9
    A rumbling soft motor lynxlynx's Avatar
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    Apr 2015
    Russia, Siberia
    Maybe, because diodes are not resistors or other such passive components. They can slightly differ in specs, resulting in one diode taking all voltage on itself and burning because of result overvoltage (same applies to current rating). If you concerned, you can put 100k-1M resistors across each diode. And UF4007 was not good for me, they nicely overheat and puke already at 400mA 50kHz for me (because of their poor forward voltage and very small size for this dissipation). So watch not to overload it.
    Last edited by lynxlynx; 04-07-2016 at 05:56 AM.

  10. #10
    I'll keep an eye out for another diode, the 1500V one is only sold in a group of ten and if I were to make a number of PS's I would go for it but I am doing this more of a proof of concept exercise. Or in another words, I get an idea and then want to see if it is doable. I know about balancing resistors across the diodes, hope to keep the average current below 100mA.

    HER308 is reasonably priced but need to do two in series. 1000V 3A 75nS


    1200V 15A 75nS, does not look too bad. So some options. Will have to think more on this this weekend.

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