Author Topic: SMPS transformer insulation best practices for higher voltages ?  (Read 190 times)

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Offline Miyuki

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Hi folks,

I want to for your experience about designing transformer insulation for reasonable reliability
I have a flowing transformer primary is push-pull 100-150V with ration 1:4 to secondary where center tapped bipolar output to get positive and negative 350V rail, output center tap is grounded, input is not ground referenced (battery supply)
So secondary can have up to 1200V plus some overshoot from end to end, this is not a low voltage

Question is single vs double coated wire
Layer separation but still keep reasonable coupling
Or is not that big deal with double coated wire ?
 

Online jbb

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Re: SMPS transformer insulation best practices for higher voltages ?
« Reply #1 on: August 21, 2019, 08:24:30 pm »
What size transformer are you looking at (i.e. frequency, power level)?  The core size will make a big difference.

I'm sure others here have more experience, but here are some initial thoughts.

For a small core, Triple Insulated Wire is probably the way to go.  With this, you can wind almost all the way along the bobbin.  You'll still need a coupe of turns of tape around the bobbin to contain the layers mechanically.

Triple Insulated costs more money and also has a lower winding fill factor (because the insulation is thick).  Exotic types like Triple Insulated Litz wire are available, but hard to source and expensive.

For a larger core, you could use single insulated wire, but you will need margin tapes at the edges of the bobbin (maybe 3 or 4mm at each end!) to manage the creepage distances. And insulting sleeves where the wires come out of the core.  Layer to layer insulation can be the yellow mica tape - it's very thin, so getting 3 turns around won't make much difference.

Single insulated is cheaper, easier to source, and can result in a higher winding fill factor.

Edit: Could anyone be hurt (i.e. exposed to high voltages) if the primary to secondary insulation fails? This will have a big impact on insulation design.
 

Offline TimNJ

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Re: SMPS transformer insulation best practices for higher voltages ?
« Reply #2 on: August 21, 2019, 08:57:06 pm »
What's your definition of "single" and "double" coated wire? Do you mean regular enameled wire with different thicknesses of enamel coating? A medical safety standard is a good starting point, though extracting information from them is a pain. Try IEC60601.

The "old-fashioned way" is to use regular enamel wire, and typically 3 wraps of yellow polyester tape (3M #1350) between layers, and typically 4mm+ of margin tape (3M #44) on either side of the winding. When using triple insulated wire (TIW), you can basically do-away with this type of construction. For TIW, you might just use 1-2 wraps of tape between layers, mainly just for mechanical purposes. No need for  margin tape. But, still need to be careful about lead-breakouts, where wire is exposed.
 

Offline blueskull

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Re: SMPS transformer insulation best practices for higher voltages ?
« Reply #3 on: August 22, 2019, 02:09:20 am »
If your transformer is not too bad and you have adequate snubbers, you shouldn't see 4x ringing. 2x ringing is already bad enough.
It's a push pull, not a flyback.
 

Offline Miyuki

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Re: SMPS transformer insulation best practices for higher voltages ?
« Reply #4 on: August 22, 2019, 08:32:21 am »
It is relative big core E55/28/21
Actually chosen wire is SynWire W 210 : class N, dual-coat wire, Grade 2 as it is cheap and available here
 


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