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SMPS for vacuum tube power amplifiers.(status: back at it)

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SK_Caterpilar_SK:

--- Quote from: T3sl4co1l on May 13, 2019, 01:24:39 pm ---Flyback for both is fine.  Or forward for heaters, but not for HV because flyback is better there.......
Cheers!
Tim

--- End quote ---

Man thats one hell of a big fat reply xD. Absolutely amazing Tim. And I love the radio stuff maybe end up some day building that pulse counting tube radio I showed because of its relative simplicity and yeah lol.

Here is the thing. I dont know how to design smps transformers properly. I see your powersupplies are quite powerfull and you probably did the math on your transformers yourself. So the deal is, I want to do my own math too lol.

At this point I decided to have two separate switching  supplies. One for the heaters and negative bias voltage, and the other one will be the big fat 100W 500V smps. This way I can really get tube lifetime out. One of the most important things with vacuum tubes is to keep the heater voltage at an exact level with the least heater voltage deviation. As low as possible. Like hopefully within 1-2% is achieveable at all. And this way the HV can be also regulated tightly which works out just about perfect this way.

And btw you kind of forgot about my problem of not having the literature for the job so if you could please point me to any literature. Or help me designign my transformer, I would be more than thankfull.

T3sl4co1l:
I'm a terrible reference for literature unfortunately; most of what I know has been synthesized from piecemeal articles, and experiments, supported by a strong foundation of theory.  Hopefully others can volunteer something...

Offhand, these may be useful:
Magnetics Design Handbook, Unitrode/TI https://www.ti.com/seclit/ml/slup132/slup132.pdf
Transmission line transformers -- http://een.iust.ac.ir/profs/Tayarani/files/transmission%20line%20transformer.pdf
Handbook, Amidon http://www.introni.it/pdf/Amidon%20-%20Transmission%20Line%20Transformers%20Handbook.pdf
An original, in a sense; G. Guanella, New Method of Impedance Matching in Radio-Frequency Circuits https://hamwaves.com/chokes/doc/guanella.1944.pdf

I emphasize transmission lines, because they are the most general, and in my opinion not onerous to understand (but, prove me wrong; these things come naturally to me so I underestimate how much complexity goes into them).

The basics are this: a transmission line is formed by two pieces of wire being nearby.

That gives a characteristic impedance (which depends on the relative cross section only), and a characteristic length.  We instantly know all the other high-frequency properties of the structure (delay, cutoff frequency, leakage inductance, parasitic capacitance), and need only ask the impedance of the core to figure out the low frequency properties (magnetizing inductance, coupling factor).

In a transformer, we have a primary and secondary wound together, for example as a pair of single layer (solenoid) windings.  If these are wound in the same direction (say, clockwise, left to right, start to finish), then they act like bifilar wire wound edgewise, and the impedance is that of the bifilar pair (well... nearly).  The length is simply the wire length.

Or we can just wind bifilar wire flat as usual, and then we have two helical windings within each other, 180 degrees apart.  The impedance is again the bifilar pair (...nearly).

Or we could wind two layers, one left-to-right and the next right-to-left.  Now the helices are opposite handed, and the wires don't perfectly line up -- they're crossing over and under each other, every turn.  Well, in this case, we still have a given wire in proximity to others, it's just less uniform.  (As long as we aren't considering waveforms with edges as fast as the electrical length of a single turn, we don't care.)

What violates this sort of approach, is having multilayer windings.  Say we put down two layers of primary, then two layers of secondary.  The first primary layer "sees" its neighbor, not the secondary, so it doesn't couple directly to the secondary.  And likewise for the far secondary layer.  The inner layers see primary and secondary respectively, but also see the outer layers.  We see two things in this situation: the image currents from the outer layers flow on the inner layers, dramatically increasing their losses (proximity effect); and, the impedance between outer layers is a good, what, triple or so the impedance you might've been expecting given the layer-to-layer distance.  But the wire length is double, so the inductance is sextuple and the self-capacitance (capacitance between ends of a given winding) is equal to the isolation capacitance (between windings).

In short, multilayer windings have considerably lower bandwidth, and fairly higher impedance, than a proper single-layer winding in transmission line style.

What if you want a high impedance transformer?  Well, you can open up the distance between wires (higher winding pitch, thicker inter-layer insulation); but this does waste a lot of space.  You can at least use a longer bobbin (to get a wider single layer winding), or maybe a toroid if you don't mind winding one.  You also tend to need a lot of turns, though, especially at very high voltages.  So the reduction in bandwidth is inevitable, and at some point you'll need to use another approach, like a resonant supply instead of a switcher.

What if you want a low impedance transformer?  Use wider wires -- multifilar, lots of wires in parallel abreast -- or foil.  Planar transformers are very attractive here, where you can stack alternating layers in parallel (primary and secondary), more than halving the impedance compared to a single layer pair.

You can use some cheats, like connecting windings in series at DC, but stacking them (one diode per layer) so they act in parallel at AC (the capacitance between windings cancels out).  CRT flyback transformers are made this way.

I did a sort of this in this module,
https://www.seventransistorlabs.com/Images/DCDC_800V.jpg
which is a 12V input, 800V output (adjustable 100-800) module, and uh, 50W or so rated, I forget exactly.  The transformer has the windup,
https://www.seventransistorlabs.com/Images/DCDC_800V_FoilWindup.jpg
the primary is low impedance (12V DC input, some amps), so foil is appropriate.  The secondary is high impedance (400V and fractional amps).  The two sections are wired in series, and the rectifier is wired as a bipolar output (+/- 400V).  The "-400V" node is merely grounded, giving 800V total output.

Here, it's not so much that the secondary layers act together -- they can't, there's a foil turn between them -- but that they are in the same environment, it's symmetrical, so whatever happens to one, happens inversely on the other.  With the diodes similarly symmetrical, the output voltage is symmetrical, and the common mode noise is symmetrical (well, around the middle of the primary it is).

This is the key feature that my 100W offline supply was missing -- I didn't consider the relative voltages at the start and end of the windings, and ended up getting one backwards, so the full switching waveform gets induced across the isolation capacitance.  Huge fuckin' common mode noise.  This module, well, it doesn't actually matter any because it's common ground, but -- if it were isolated, it would be a far sight better, despite its much higher output voltage.

The windings are also pretty short, like 1-2m of wire, so the bandwidth remains high despite the impedance mismatch (the impedance of a round wire winding sandwiched between two plates, and a couple layers of tape, is around 50 ohms, whereas a kohm or so would be more appropriate).  In fact I see very little if any overshoot and ringing on the waveforms, I was quite impressed.  (Part of the short windings is the oversized core: it's good for about 100W in flyback, but only being used for 30, maybe 50W.)

Tim

SK_Caterpilar_SK:
So it has been a while since I posted something and now its time for a question.

I have not had any time to make the powersupply at all. I still have the second version on paper not an actual working device but I have got some time toi sniff arround my prototype. the previous problem was that the entire PSU seems quite inefficient. The mosfet gets blistering hot and so does then switching transformer.

I have looked at the gate voltage and it seems fine to me, but the  transformer has an extreme amounth of oscillation. More specifically I was measuring ground to drain of the mosfet. That said on the high side of the G voltage it pulls short 2 ground on the drain (just saying so you dont have to figure out on your own.). Could this be causing the heating of the transformer and the mosfet? Interesting is that any capacitor in the input (bypass capacitor- electrolitic) right close to the switching transformer it gets very, very hot.

I dont really know why this is happening, so far my knowledge about SMPS are rather basic. I did design another powersuply completely, but since I did not know about this issue Im pretty sure it will happend again.

I have hope in your reply Tim :D. Would just a snubber network solve this or it is something that needs a more proper solution. I have a feeling it is going to require a proper solution.

On the oscilloscope where there is only one waveform I have measured between D and S  (G30N60-its not a mosfet-IGBT but a really expensive one that works really well, does not heat up as much as a IRFZ44. This heating issue stays a problem even for really low on resistance mosfets like for 100A above mosfets). The frequency of oscillation is 840kHz.

So how can I make it more relyable and efficient? Assuming the controller will be a TL494 in the future powersupply. Also I would not mind using a different chip if that makes it better.

SK_Caterpilar_SK:
Also in reply to my previous reply, the first waveform where the oscialltion is way more pronounced im switching the transformer at 75kHz (unintentionally, I have set it to 50kHz just as specified by the manufacturer but seems under load it changed the whole thing, it is the fault of my crappy controller, the traces all act like antennas.

The second measurement was taken at 30khz switching frequency.

Also in the second measurement the 1st probe is in 10x , I forgot to set the scale properly on the scope.

T3sl4co1l:
Looks like a shitton of leakage in the transformer.

Tim

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