SMPS inductor clone

** You sure it has no gap ???

L9 is a PFC choke and they use powdered iron cores or gapped ferrite.

Normally, the middle leg is ground short to create a gap of up to 1mm to allow DC current to pass without saturating the ferrite. Un-gapped ferrite cores are good for push pull, square wave transformers only.

You have the original core, so can measure its inductance and have a damn close look for the missing GAP.

Meanwhile, this link should supply some useful clues.

..... Phil

** You sure it has no gap ???

L9 is a PFC choke and they use powdered iron cores or gapped ferrite.

Normally, the middle leg is ground short to create a gap of up to 1mm to allow DC current to pass without saturating the ferrite. Un-gapped ferrite cores are good for push pull, square wave transformers only.

You have the original core, so can measure its inductance and have a damn close look for the missing GAP.

Meanwhile, this link should supply some useful clues.

..... Phil

Thanks Phil, I'm pretty sure it has no gap. It's almost impossible to take these things apart without breaking the very fragile core. I've kept all the bits so will go back and glue them together to recheck this. I've taken apart other SMPS transformers where the gaps are quite obvious, even with a core in several pieces, and measurable with a micrometer without having to reassemble.

One other thing, Digipro use enamelled stranded wire on this inductor. I gather this has better HF performance than the much more common solid copper enamelled wire. A quick look for stranded enamelled wire of this guage on the Interweb yesterday showed it is not widely available in small amounts.

Would you need to adjust the number of turns if you used solid copper instead?

BTW, this seems to be a common failure mode of these Digipro amps - this is the second I have seen. The windings short together, and there is no insulation or tape applied anywhere on the windings except the final outer wrap.

Cheers,

Gareth.

Did you search under the name "Litz wire"? (That's just something dredged up from my electronics of decades ago.) I just got some hits, but did not look for suppliers...

Mike.

** There is one - it might be only a few thou of an inch, but it must be there or the whole thing would blow up. ** This is NOT a SMPS transformer, it's a ferrite CHOKE that must pass a couple of amps of DC current. It simply HAS to have an air gap !!! ** FFS, make your own stranded wire.

Dare to be great !!!!!!

** Use higher temp wire, 220C stuff is readily available, the usual magnet wire is only rated at 120C at best.

Do not muck about: an active, high frequency, PFC choke is critical component.

.... Phil

** There is one - it might be only a few thou of an inch, but it must be there or the whole thing would blow up. ** This is NOT a SMPS transformer, it's a ferrite CHOKE that must pass a couple of amps of DC current. It simply HAS to have an air gap !!! ** FFS, make your own stranded wire.

Dare to be great !!!!!!

** Use higher temp wire, 220C stuff is readily available, the usual magnet wire is only rated at 120C at best.

Do not muck about: an active, high frequency, PFC choke is critical component.

.... Phil

Well there's my biggest problem, then. Unless I know what the original core material is, I can't make one the same, the original core is in pieces. It may not be any of the TDK formulations I can buy at Farnell.

And I guess I would need to use the same type of wire, not solid enamelled. (or possibly rescue/repair and re-use the old winding)

Gareth.

Did you search under the name "Litz wire"? (That's just something dredged up from my electronics of decades ago.) I just got some hits, but did not look for suppliers...

Mike.

Thanks for that info, Mike.

Gareth.

** There is one - it might be only a few thou of an inch, but it must be there or the whole thing would blow up. ** This is NOT a SMPS transformer, it's a ferrite CHOKE that must pass a couple of amps of DC current. It simply HAS to have an air gap !!! ** FFS, make your own stranded wire.

Dare to be great !!!!!!

** Use higher temp wire, 220C stuff is readily available, the usual magnet wire is only rated at 120C at best.

Do not muck about: an active, high frequency, PFC choke is critical component.

.... Phil

It is becoming increasingly obvious to me that L9 in this design is ANYTHING BUT an inductor of the appropriate inductance and the required power handling for the job required.

It is clearly the Achilles heel of this particular design.

As such, I have given up any such silly idea as to how I might improve on it, as I am not qualified to do so.

Thanks,

Gareth.

The trace of the inductor in the schematic means with a core.

No Litz wire, it is reserved for high frequency low intensity windinds.

SMPS works at "low frequency" (some 10 kHz).

If the core is ok, just unwind and rewind with a new wire.