Forums

Transformer bobbins/coil formers

Started by Clive Arthur October 28, 2019
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
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
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
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
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" <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/
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 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
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.)&#2013266080; They have a bandwidth of a few kHz tops. > > Tim >
About 1mH, 10kHz to 100kHz, linear. Cheers -- Clive