Transformer bobbins/coil formers

[Clive Arthur]

For an RM ferrite core (RM8 in my case) you can buy two types of bobbin,
a single section and two sections, though the latter is less often stocked.

Assuming there are two windings and that the wire will fit on either
type, it seems to me that the two section bobbin would have less
inherent inter-winding capacitance and better isolation, though you
can't easily wrap a copper tape screen between windings.

What other differences/pros/cons are there? When would one type be
preferable?

Cheers
--
Clive

 

[Bill Sloman]

On Monday, October 28, 2019 at 9:43:40 PM UTC+11, Clive Arthur wrote:

For an RM ferrite core (RM8 in my case) you can buy two types of bobbin,
a single section and two sections, though the latter is less often stocked.

Assuming there are two windings and that the wire will fit on either
type, it seems to me that the two section bobbin would have less
inherent inter-winding capacitance and better isolation, though you
can't easily wrap a copper tape screen between windings.

What other differences/pros/cons are there? When would one type be
preferable?

Two sections formers have a smaller copper window, so your winding resistance ends up higher.

On the other hand being able to wind two banks does offer you smaller intra-winding-capacitance and a higher self-resonant frequency, which can be important if you need high turns ratios.

I posted a bit about this here back in 2013 and there's discussion about it on my web-site. The example is a different core, and there wasn't a two-section former available, but I did discuss winding in banks anyway.

http://sophia-elektronica.com/PMT-transformer.html

--
Bill Sloman, Sydney

 

[Winfield Hill]

Clive Arthur wrote...

For an RM ferrite core you can buy two types of bobbin,
a single section and two sections, though the latter
is less often stocked.

Exactly, they are useful but hard to get, so whenever
you see a two-section version in stock, you should
purchase some, unless you're already well stocked.


--
Thanks,
- Win

 

On 28 Oct 2019 07:22:49 -0700, Winfield Hill <winfieldhill@yahoo.com>
wrote:

Clive Arthur wrote...

For an RM ferrite core you can buy two types of bobbin,
a single section and two sections, though the latter
is less often stocked.

Exactly, they are useful but hard to get, so whenever
you see a two-section version in stock, you should
purchase some, unless you're already well stocked.

Mouser has some:

https://www.mouser.com/Passive-Components/EMI-Filters-EMI-Suppression/Ferrites/Ferrite-Cores-Accessories/_/N-bw7tb?P=1yxgwjo




--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Winfield Hill]

Clive Arthur wrote...

Thanks, I'd need 12 pin ones to be compatible with
existing PCB and the only one they have is 5 pins.

Picky, picky, picky!


--
Thanks,
- Win

 

[Clive Arthur]

On 28/10/2019 15:30, jlarkin@highlandsniptechnology.com wrote:

On 28 Oct 2019 07:22:49 -0700, Winfield Hill <winfieldhill@yahoo.com
wrote:

Clive Arthur wrote...

For an RM ferrite core you can buy two types of bobbin,
a single section and two sections, though the latter
is less often stocked.

Exactly, they are useful but hard to get, so whenever
you see a two-section version in stock, you should
purchase some, unless you're already well stocked.

Mouser has some:

https://www.mouser.com/Passive-Components/EMI-Filters-EMI-Suppression/Ferrites/Ferrite-Cores-Accessories/_/N-bw7tb?P=1yxgwjo

Thanks, I'd need 12 pin ones to be compatible with existing PCB and the
only one they have is 5 pins.

Cheers
--
Clive

 

[Tim Williams]

"Clive Arthur" <cliveta@nowaytoday.co.uk> wrote in message
news:qp6goh$h21$1@dont-email.me...

For an RM ferrite core (RM8 in my case) you can buy two types of bobbin, a
single section and two sections, though the latter is less often stocked.

Assuming there are two windings and that the wire will fit on either type,
it seems to me that the two section bobbin would have less inherent
inter-winding capacitance and better isolation, though you can't easily
wrap a copper tape screen between windings.

What impedance is the transformer?

Split bobbin design probably has too much leakage inductance for use as a
SMPS transformer.

Heck, they're barely passable at mains frequency -- the leakage inductance
is high enough that the smaller sizes can be rated as "impedance limited".
(The high impedance also limits rectifier harmonics, increasing power
factor -- both come at the expense of output voltage regulation.) They have
a bandwidth of a few kHz tops.

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

 

[Winfield Hill]

Tim Williams wrote...

What impedance is the transformer?

Split bobbin design probably has too much leakage
inductance for use as a SMPS transformer.

Heck, they're barely passable at mains frequency --
the leakage inductance is high enough that the
smaller sizes can be rated as "impedance limited".
... They have a bandwidth of a few kHz tops.

Maybe some of them. The common-mode chokes I've
been testing as 1:1 transformers to give me 5V on
the secondary, with 10kV insulation (vacuum potted
in Sylgard), are working well. Primary inductance
1.5mH (spec says 1mH), and leakage inductance 28uH,
or under 2%. These are Taiyo Yuden, type TLF9UA.


--
Thanks,
- Win

 

[Winfield Hill]

Winfield Hill wrote...

Tim Williams wrote...

What impedance is the transformer?

Split bobbin design probably has too much leakage
inductance for use as a SMPS transformer.

Heck, they're barely passable at mains frequency --
the leakage inductance is high enough that the
smaller sizes can be rated as "impedance limited".
... They have a bandwidth of a few kHz tops.

Maybe some of them. The common-mode chokes I've
been testing as 1:1 transformers to give me 5V on
the secondary, with 10kV insulation (vacuum potted
in Sylgard), are working well. Primary inductance
1.5mH (spec says 1mH), and leakage inductance 28uH,
or under 2%. These are Taiyo Yuden, type TLF9UA.

Oh, and optimum operating frequency, 70kHz.


--
Thanks,
- Win

 

[Tim Williams]

"Winfield Hill" <winfieldhill@yahoo.com> wrote in message
news:qp7c680nla@drn.newsguy.com...

Maybe some of them. The common-mode chokes I've
been testing as 1:1 transformers to give me 5V on
the secondary, with 10kV insulation (vacuum potted
in Sylgard), are working well. Primary inductance
1.5mH (spec says 1mH), and leakage inductance 28uH,
or under 2%. These are Taiyo Yuden, type TLF9UA.

Oh, and optimum operating frequency, 70kHz.

Yeah, those work alright as transformers for a couple of watts; it's hard to
get much more, even with a resonant controller. The bandwidth is pretty
narrow. (What's so special about 70kHz? Exactly, the transformer is. ;-) )

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

 

[Winfield Hill]

Clive Arthur wrote...

About 1mH, 10kHz to 100kHz, linear.

It's a strong function of low source impedance
and low load capacitance and current (high-Z).


--
Thanks,
- Win

 

[Clive Arthur]

On 28/10/2019 17:13, Tim Williams wrote:

"Clive Arthur" <cliveta@nowaytoday.co.uk> wrote in message
news:qp6goh$h21$1@dont-email.me...
For an RM ferrite core (RM8 in my case) you can buy two types of
bobbin, a single section and two sections, though the latter is less
often stocked.

Assuming there are two windings and that the wire will fit on either
type, it seems to me that the two section bobbin would have less
inherent inter-winding capacitance and better isolation, though you
can't easily wrap a copper tape screen between windings.

What impedance is the transformer?

Split bobbin design probably has too much leakage inductance for use as
a SMPS transformer.

Heck, they're barely passable at mains frequency -- the leakage
inductance is high enough that the smaller sizes can be rated as
"impedance limited". (The high impedance also limits rectifier
harmonics, increasing power factor -- both come at the expense of output
voltage regulation.)  They have a bandwidth of a few kHz tops.

Tim

About 1mH, 10kHz to 100kHz, linear.

Cheers
--
Clive

 

[Tim Williams]

"Clive Arthur" <cliveta@nowaytoday.co.uk> wrote in message
news:qp7dlk$5cb$1@dont-email.me...
> About 1mH, 10kHz to 100kHz, linear.

1mH where? What's the source and load impedances?

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

 

[legg]

On Mon, 28 Oct 2019 10:42:59 +0000, Clive Arthur
<cliveta@nowaytoday.co.uk> wrote:

For an RM ferrite core (RM8 in my case) you can buy two types of bobbin,
a single section and two sections, though the latter is less often stocked.

Assuming there are two windings and that the wire will fit on either
type, it seems to me that the two section bobbin would have less
inherent inter-winding capacitance and better isolation, though you
can't easily wrap a copper tape screen between windings.

What other differences/pros/cons are there? When would one type be
preferable?

Cheers

You can always remove a barrier, in prototyping. Three sections is not
uncommon. Bobbins dedicated to HV inverters may not be available
without them.

It's only in large volumes, that you have to worry about exact
dimensions, part numbers etc.

.. . . . but it's pointless prototyping using shapes that aren't
actually available, if you're not prepared to go through hoops to make
them available.

RL

 

[Clive Arthur]

On 28/10/2019 12:51, Bill Sloman wrote:

On Monday, October 28, 2019 at 9:43:40 PM UTC+11, Clive Arthur wrote:
For an RM ferrite core (RM8 in my case) you can buy two types of bobbin,
a single section and two sections, though the latter is less often stocked.

Assuming there are two windings and that the wire will fit on either
type, it seems to me that the two section bobbin would have less
inherent inter-winding capacitance and better isolation, though you
can't easily wrap a copper tape screen between windings.

What other differences/pros/cons are there? When would one type be
preferable?

Two sections formers have a smaller copper window, so your winding resistance ends up higher.

On the other hand being able to wind two banks does offer you smaller intra-winding-capacitance and a higher self-resonant frequency, which can be important if you need high turns ratios.

I posted a bit about this here back in 2013 and there's discussion about it on my web-site. The example is a different core, and there wasn't a two-section former available, but I did discuss winding in banks anyway.

http://sophia-elektronica.com/PMT-transformer.html

Thanks Bill.
--
Clive