# Toroidal inductors - Heat Problem

I'm working on an isolated 2-sw forward converter.

The output section has a toroidal inductor that I've a problem with. Here's how the output section looks like:

simulate this circuit – Schematic created using CircuitLab

The switching frequency is 100kHz and the required value of the output inductor is calculated as about 100uH.

I put a toroidal 100uH inductor (I think it has iron powder core) taken from a broken power supply and left the converter operating under 6A load for half an hour. Then I measured the output inductor's temperature as 50°C which is quite acceptable. So I calculated its $A_L$ (from $L = A_L \cdot N^2$ of course) as about 95nH.

Then I bought an iron powder core (Part Nr: T130-26K - http://www.pe-coils.com/IPC.htm) having the same dimensions as the other one and calculated its $A_L$. Surprisingly, it has the same $A_L$ value! I thought that I "accidentally" found the same core, so I wound a toroidal inductor with the same number of turns and using the same dia wire. As expected, I got a 100uH toroidal inductor. No problem until here.

The problem is this: If I use my hand-wound inductor, and under full load, it gets as hot as 95°C! How can it be? They both have the same wire thickness, the same inductance and cores with the same $A_L$ values. But where does that temperature performance comes from?

More funniness: if I use my hand-wound inductor, I get %87 efficiency; but if I use the other one, I get %92!

I measured Q values of both with a calibrated LCR meter: My hand-wound has 2.4 but the other one has 1.8. So, what can be the thing behind this issue? What should I do?

[SOLVED] EDIT:

I've experienced that it really comes from core losses. The one with model number of T130-26K is a quite lossy one. So I purchased some toroidal inductors from Mouser and tested each under the same conditions. Finally, one beat them all in terms of thermal and electrical performance. For the ones who are interested, it is 2300LL-101-V-RC from Bourns.

PS: Is there a way to close or flag this question as "answered" or something like that?

• Well, the reduced efficiency shouldn't come as much surprise as you are wasting some heat. $A_L$ is not everything to characterize a core, different magnetic properties make things go into saturation regions in different ways. The cores datasheet should have some illustratnig graphs. Nov 10, 2016 at 13:28
• @PlasmaHH Thanks. Switching frequency is 100kHz, btw. So, for example, can I say "My core cannot be used at 100kHz" ? Nov 10, 2016 at 13:32
• strength of the magnetic field i.e. current plays a big role too. Since you seem to have the setup, to learn something about your cores, you could graph current vs. efficiency for both cores, as well as (for a fixed current) frequency vs. efficiency. Nov 10, 2016 at 13:37
• What core did you buy and what's your switching frequency? Nov 10, 2016 at 13:42
• Ditch it then and find a replacement - most likely it cannot handle the frequencies - core losses are the most likely cause. Nov 10, 2016 at 14:55