Why is current sense transformer wound on a split-section-bobbin?

Why is current sense transformer wound on split section bobbin?

We have had a 300w, 200khz buck converter designed for us for our 170degC ambient environment. (vin = 120-400vdc, vout=100vdc) The current signal to the current mode controller is via a current sense transformer and not a resistor.

Why have they wound the current sense transformer on a split section bobbin, with the primary and secondary in separate sections? Surely this just unecessarily creates too much leakage inductance?

The split section bobbin is ferroxcube part number..
CSH-EP13-2S-10P
https://www.farnell.com/datasheets/1571206.pdf

The ferrite core is ferroxcube EP13, no gap.
Np=1; Ns=50

Why not use a single section bobbin, wind a full layer of secondary, then tape over it, then wind the single turn primary over the top of this?

Re: Why is current sense transformer wound on split section bobbin?

Easier to wind? or more likely, constant number of secondary turns, alter the secondary to sense different currents. How would you overwind a 200A primary?
Frank

Re: Why is current sense transformer wound on split section bobbin?

primary current is only around 3.5A peak at worst case (vin=400v)

Re: Why is current sense transformer wound on split section bobbin?

treez said:

We have had a 300w, 200khz buck converter designed for us for our 170degC ambient environment. (vin = 120-400vdc, vout=100vdc) The current signal to the current mode controller is via a current sense transformer and not a resistor.

Why have they wound the current sense transformer on a split section bobbin, with the primary and secondary in separate sections? Surely this just unecessarily creates too much leakage inductance?

The split section bobbin is ferroxcube part number..
CSH-EP13-2S-10P
https://www.farnell.com/datasheets/1571206.pdf

The ferrite core is ferroxcube EP13, no gap.
Np=1; Ns=50

Why not use a single section bobbin, wind a full layer of secondary, then tape over it, then wind the single turn primary over the top of this?

Click to expand...

EC standard specify split bobbin and 4000V isolation for the low voltage directive.

Re: Why is current sense transformer wound on split section bobbin?

Thanks, but this is for MWD application, and no external regulations apply.
Sufficient isolation can be surely obtained with a layer of tape over the secondary, with the primary wound over the top of this.

Re: Why is current sense transformer wound on split section bobbin?

Might also be an element of convenience, either for the
manufacturer (assemble various ratios from stocked
bobbin-halves) or the end user (if primary is to pass a
fat existing cable through the core, a split bobbin can
be clamshelled around the conductor without rework
or interruption).

Too, split bobbin minimizes primary:secondary capacitance
which in common-mode-noisy systems is almost certainly
a bonus.

But the regulatory stuff may well be the driver of it all,
costing little more to other applications but eliminating
a "hell, no" for anyone with those constraints.

Re: Why is current sense transformer wound on split section bobbin?

Usual reason is that if multi turn secondary on split bobbin occupies half the window area, this often leaves a square hole for the primary.
A square hole can be very convenient for a very bulky single turn primary.

Another more off beat reason might be that this particular core and bobbin was already in use for something else, and the designer decided to use more of the same for this application.

Purchasing officers, the warehouse guys, and production managers all love you if you can cut down on the number of different parts to stock for manufacture.

Re: Why is current sense transformer wound on split section bobbin?

Warpspeed said:

Usual reason is that if multi turn secondary on split bobbin occupies half the window area, this often leaves a square hole for the primary.
A square hole can be very convenient for a very bulky single turn primary.

Another more off beat reason might be that this particular core and bobbin was already in use for something else, and the designer decided to use more of the same for this application.

Purchasing officers, the warehouse guys, and production managers all love you if you can cut down on the number of different parts to stock for manufacture.

Click to expand...

So, in the land down under the boss puts on your desk a handful of components that he got from the warehouse and tells you to design a circuit, he doesn't care what the circuit does as long as you use those components.
It must be fun when many of you think like Paul Hogan.

Re: Why is current sense transformer wound on split section bobbin?

I read Tony's post so that there may be rather trivial reasons why the transformer was designed as is. Asking why is it so may be just a waste of time.

On the other hand, if you know your specifications for the current transformer, you don't need to ask at all.

Re: Why is current sense transformer wound on split section bobbin?

Vbase said:

So, in the land down under the boss puts on your desk a handful of components that he got from the warehouse and tells you to design a circuit, he doesn't care what the circuit does as long as you use those components.
It must be fun when many of you think like Paul Hogan.

Click to expand...

That's just silly. On technical matters once you earn a good reputation as a successful designer, management will both listen and take advice.

Its more than just designing something that works. It must also present minimal problems of parts procurement , manufacture, and future repair.

If you want to design some small non critical transformer or inductor, and some suitable ferrite parts and bobbins are already in stock (sometimes in the tens of thousands) its easier for everyone involved, including myself to just grab a few, and base my new design around that.

It may be one size larger than absolutely necessary, but bulk buying can often make it cheaper than the next size down.

It may all be high comedy to you, but this is all a very serious business, and to be successful at it you need to take a very broad view of the whole process and create as few problems as possible for others working with and around you.

Re: Why is current sense transformer wound on split section bobbin?

Brings to mind a coil that I was involved with. It was wound with 39 SWG on a Mullard pot core which had its air gap especially widened for us. It was use as the BFO oscillator coil on a communications receiver of which thousands were made.
The 39 SWG wire eventually became unobtainable, so I was asked to redesign it with a better gauge wire (38 SWG?). this is when I found out about the very expensive core. I rewound it on a standard air gapped pot core with standard wire. The BFO was then temperature coefficient tested and nothing needed to be changed.
One of life's little mysteries, still my mod, saved the firm thousands of pounds over the remaining life of the receiver production.
Frank