3/8 ring core in 3F4

I think that Fair Rite type 77 is superior to 3F4. Take a look at the

5977000201 toroid. http://64.162.105.66/lspservices/xsinvfr.asp?PageNo=2&Mv=Page+Down specs on page 137 of:

I was just about to argue with you and I realized that I had misread the frequency scale on the data sheet. Sorry.

Hello Ken,

Check Kaschke K2003 material:

They have a rep in California (assuming you are in CA).

Or Magnetics "A".

Tried other materials such as #77 or J?

Regards, Joerg

In article , Joerg wrote: [....]

Thanks for that lead.

[...]

Type 77 core material gets too lossy above about 100KHz. It isn't as good as the 3F3 material I can easily get. I can easily give up a bit on the Al number for the core, since my copper losses are quite low.

I'm switching at about 2MHz in this design.

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kensmith@rahul.net   forging knowledge

In article , John Popelish wrote: [...]

No, the numbers seem to say that type 77 is a bit worse at high frequencies than the 3F3 material. Type 77 is not recomended above 100KHz.

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kensmith@rahul.net   forging knowledge

In article , John Popelish wrote: [...]

Yes, it happens.

Thanks for confirming my, as it turns out, correct reading of charts.

If we could just get all the makers to use the same format, life would be much easier.

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kensmith@rahul.net   forging knowledge

much easier.

Yes, Yes, I agree with that, and even different formats in a single manufacturers catalog. Mike

Hello Ken,

Wow. I am not sure 3F4 is right for that. Best would be to talk to an engineer at one of the ferrite manufacturers about which material may be best. I usually went to #43 when going into the MHz range but there is some stuff in between as well, most likely more expensive though.

Regards, Joerg

In article , Joerg wrote: [...]

Thanks for pointing me back to the #43 material. It looks like I can get it in toroids and the numbers on it look good.

The maker's suggested use for it is for EMI at frequencies over IIRC,

30MHz.

Bah, cost is no object :)

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kensmith@rahul.net   forging knowledge

There aren't too many power inductor/transformer applications that will unequivocally suit a toroidal ferrite core - a small gap is often beneficial.

Although cost may be no object, availability generally dictates the use of parts commonly used elswhere.

`3/8 OD toroids are metric T8 orT10 (TDK), R9.5 or R10 (EPCOS), TN9 or TN9.5 (Ferroxcube).

Low profile bobbin and core assemblies (eg ER14.5) are available from EPCOS, TDK and others in low loss material, gapped or not.

How have you established that core losses are dominant (zero load rises)? Can you reverse this imbalance, to prove it?

Tolerance of core loss in production of larger parts generally prevents you from allowing it any sign of dominance in a prototype - it can easily double. In a smaller part, the responsibility for heat removal is likely shared by the total assembly - it's harder to pinpoint. This can even be useful.

RL

In article , legg wrote: [...]

Yes, in general, the core used in a DC-DC converter is normally gapped in some way. Toroids can have a "distributed gap" where the magnetic material is mixed with some non-magnetic material to provide the gapping. In my case, the availability of the part off the shelf is driving the selection of the core. If the core was optimized, it could be quite a bit smaller.

[...]

The design is basically a SEPIC converter and an isolated CUK converter sharing a core to make two outputs. As a result, I can't use the no-load losses method to do an in application measurement.

I this situation the measurement is made by measuring the temperature rise of the copper and the core in the circuit and then reproducing the rise out of the circuit. Because the inductor is in a confined space, it doesn't take much power to cause the rise I'm seeing. My best estimate is that just about 0.3W is being lost in the inductor when I'm making 3W of output power. The semiconductors losses are about another 0.3W.

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kensmith@rahul.net   forging knowledge

Ok Ken, you got my attention. Let's see this SEPIC + CUK sharing one core. Sounds interesting. Can you post a schematic or sketch? Harry

Hello Ken,

And it's dirt cheap. The variety is larger than that of the beer aisle at Safeway.

I have used #43 material for pulse transformers in the several 10nsec range. Also for EMI. The FT-37-43 core is 0.375" OD and 0.125" high. If that saturates in your application you could stack two for experiments.

Our tax Dollars at work? Lucky you. My toroid inductors often have to come in under 50c, custom wound and all. But that is in huge quantities.

Regards, Joerg

Hello Ken,

Converters like the SEPIC generally do not need gapped cores. Also, as Harry pointed out, in a SEPIC you can "core share" (ain't that a nice word?) and wind both inductors on the same core.

0.3W core loss in a 3W converter is huge. Typically core loss would contribute a percent or so, if that. The thing to watch out for is saturation at the end of the charge cycle. If it isn't a current mode concept you are using install a very small resistor in the source leg of the FET. 100milliOhms or so, something a current mode concept already has. Look at it with a scope. You should ideally see a nice linear ramp. If you see a marked rise at the end, like a shark fin, then saturation is setting in. This is also a reason why continuous mode may not be such a good thing when the core is on the skimpy side.

Regards, Joerg

In article , Joerg wrote: [...]

I went to the store that doesn't have beer and chips mixed in with the sockets and chips[1], and, *bummer*, They are out of them. I will have to order some. I meant to do the order today but I got busy putting the prototype in a box and then trapped in a meeting.

[1] ie: it isn't Fry's

Right now, this is privately funded. The product will sell for about the same as a new car so a few bucks on a core won't matter.

I have a $300 OCXO, a $100 CPLD and a couple of $50 micros so most other things get lost into the noise.

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kensmith@rahul.net   forging knowledge

In article , Harry Dellamano wrote: [...]

Here's the basic idea:

C1 D1 --[L1]--[L2A]-+-----!!----+----->!--+----- Vout1 ! ! ! !- [L2B] --- C2 ! ! --- !- GND ! ! GND GND

C3 ---!!--+----[L2D]-[L3]-+-- Vout2 ! ! ! ! --- D2 --- [L2C] ^ --- C4 ! ! ! -------+---------------+--

L2 has lots of winding on it and is custom. L1 and L3 are small off the shelf parts.

The input and the Vout2 have low ripple. Vout1 is the one used for the real feedback. Vout2 is only semi regulated.

I'm thinking of dropping the Vout2 back to just being isolated flyback.

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kensmith@rahul.net   forging knowledge

The numbers say that I'm only at about 1/4 saturation. I'm at home and the real numbers are at work but that much I rememeber. This is why I decided it was just the core material that was at fault.

It is a current mose design.

This is sort of what the current does.

................*........ ..............*..*....... ............*....*....... ..........*......*....... ........*........*....... ....*..*.........*....... ....***..........*....... ....**...........*....... ....*............*.......

****.............******** .................*.......

The slight decreasing slope may be from the way I did the measurement or the coupling capacitor being a bit too small.

I see some ringing on the edges of the switching.

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kensmith@rahul.net   forging knowledge

Hello Ken,

Some of the larger Radio Shacks used to have 43 cores. But not these flat versions.

Amidon is pretty good if you need a few cores quickly.

I would be surprised if they had ferrite toroids.

Wow. Guess a 50c ferrite pales in comparison.

Regards, Joerg