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Design trade-0ffs for HV AC Transformers

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LAN_403

Green Hornet

Thu Aug 27 2015, 03:20PM

Registered Member #55900 Joined: Wed Jul 22 2015, 06:14PM
Location:
Posts: 8

Just discovered this site and am looking forwards to getting a couple of Fiddys transformers but as a retired EE who has built a dozen HV transfortmers, all driven by ZVS circuits, (which I buy from China). I can offer some thoughts on desgin trade-offs of AC HV transformers.

In transformer design it is, "ALL ABOUT THE CORE". The limitation in the pwer handling capacity of a core, in any material, is a function of: core cross sectional area; the material charateristics notably its B sat, and frequency. (the fundamental transformer formula derived from Maxwell). Note that your max primary voltage inceases with frequency but core losses and inductance will drop which offset. That is why there are "trade-offs". Adding in the lowest number of turns in your design, will then get you the max primary RMS before core sat. The small cross section of all flybacks severely limits it power handling capability. Adding an air gap, helps but then you lower the inductance, and unless you add more turns, your ZVS tank circuit will then run above 50KHz, which is where core losses kick in. But then adding more turns reduces your max RMS volgtage it can handle and you are in a viscious circle. I even used two flyback cores in parrallel and wound over them which is an OK solution if you have a bunch of flyback cores. In some disigns, I have increased my ZVS tank capaitance to 1uf from the ,68 in the standard circuit. It took me months but I found a great supplier of larger size ferriteU cores, some with very low core loss. THey are called TSC Ferrite International out of the US Midwsest and they accept small orders and you can order on-line. I have used up to a 4" square U core with a 1" square cross section that has handled >500W at 50 KHz and stays below 50 deg core temp. The tech data on their cores are excedllant. The only issue is that this stuff isn't cheap. I paid $60 for the above "Giant" core although I have sucedsfuuly used some of their smaller U cores for $30 in many designs. FYI, there are some informative videos on YouTube regarding inductor and transformer design.

In my designs, I run out of patience, and sometimes core space, winding more than 1000 Turns, so to get my secondary voltage up, I run with only 4-6T CT, wich means I need to use a high permiable core material or my ZVS tank runs above 100KHz which creates core losses and heat, but higher permiability ferrites cost more. Building a million TV sets means you use the smallest cheapest and flybacks transfer very very little DC current...less than a milliamp. If anyone is crazy like me and starts down this path, I strongly reccommend the purchase of an LC meter which you can get as a small open frame PC boardwith wall wart 5V power, for $25 on ebay. Works great.

Two more issues on "winding your own". Use Kapton tape and lots of it. Amazon sells it. Pay attention to your wire. Not all mag wire is the same. You want to use the double coated, high temp stuff which I found from "Bulkwire.com". Keep your turns away from the edges. My first winding layer is starting up from ground, but each additgional layer should be offset "inside" lower level by 1/8 min and then add extra Kapton at th edge of each layer as well as between layers. This makes your winding like a pyramid. Even then, I have had designs arc from the edges and sometimes using stuff called "Corona Dope" can save it. I have learned the hard way with each winding job, using a corless drill and taking care with each turn, it takes me 5-8 hours to wind a secondary. I use 28 awg.
Last but not least, and this is true for any HV design....GROUND ONE SIDE OF YOUR SECONDARY. And I mean a real earth ground so check that 3 prong socket to make sure. This can save your life as well as making your winding less prone to arcing over.

Goodchild

Thu Aug 27 2015, 04:43PM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

Green Hornet,

First let me say welcome to the forum is no one has said so yet. It's great to see someone with experience join the forum.

I have personally been fascinated with the various ZVS topologies out there. I personally have been working on a phase shifted full-bridge topology that uses ZVS form some time now. I won't be using a core, so that makes it somewhat of an easier beast to deal with.

I also wanted to add to your great list of tips for flyback and general multi-layer HV transformer design. Captive losses on the secondary winding, this in my experience has been a major hurtle in total system efficiency. Simply due to the many number of windings and layers there can be significant parasitic capacitance in the secondary winding that will create losses in accordance with CV^2 on the secondary side.

Simply establishing right or left handed winding only to minimizes the voltage gradient between layers can help tremendously. Also keeping the tape layer between layers thick can help but can be a trade off with winding window.

Wolfram

Fri Aug 28 2015, 12:33PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Nice summary.

At frequencies above a few kilohertz, core loss will be a problem long before saturation, unless you're just running it for short enough periods that the core doesn't get time to heat up. For typical power ferrite grades, the TSC datasheets don't even show the core losses at the saturation flux density, the graphs cut off at a much lower flux density. Down around 25 - 50 kHz, you can usually run at 200 mT with reasonable losses, but they increase fast when going above that.

Introducing an air gap doesn't really help, it just lowers the primary amp turns the core can support. In an inductor, this makes sense, but in a transformer, the secondary amp turns cancel the primary amp turns, and in practice you end up having to supply more magnetizing current due to the lower primary inductance.

I also wanted to add to your great list of tips for flyback and general multi-layer HV transformer design. Captive losses on the secondary winding, this in my experience has been a major hurtle in total system efficiency. Simply due to the many number of windings and layers there can be significant parasitic capacitance in the secondary winding that will create losses in accordance with CV^2 on the secondary side.

Yeah, this is usually the main limitation in HV transformers, and it can lead to a lot of confusion if you don't expect it. The secondary capacitance appears on the primary side scaled by N^2 , and since N (the secondary to primary turns ratio) is often large for these transformers, the reflected capacitance is ususally very large as well. I've seen several hundred nanofarads in practice. The best way to work around it is to use it to your advantage. This capacitance will resonate with the leakage inductance of your transformer, this is why HV transformers often "resonate" around a certain frequency. If you drive it around this frequency, the driver losses from driving this capacitance are eliminated.

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