EMRFD Message Archive 11735
Message Date From Subject 11735 2015-10-05 14:27:28 Alan Ibbetson Powdered iron toroid loss question I am making a 100W amplifier but I need some help with core selection
for the low pass filters.
As a starting point I thought I would copy the filters in the Elecraft
K2/100 PA. All their cores are T50 types.
They look really small for 100 watts so I plugged the values into
LTspice to confirm the component values are OK (they are) and then
looked at simulating 100V peak RF into the filters. The top band filter
has the first inductor as 3.57uH made with 27 turns on a T50-2 (p28 of
the KPA100 build manual). LTspice says there is 80V RMS across this coil.
Micrometals says the T50 cores have an effective area of 0.112 cm^2.
Cranking the numbers I make the AC flux over 300G (30mT) at 1.85MHz.
This is way off the scales of Fig E p4 of
and fig G implies you will get a burned finger if you touch the core,
because it is dissipating somewhere in the region of 1.5W according to
the old Mini Ring Core calculator written by a now-deceased German amateur.
I don't for a minute believe Elecraft's cores catch fire so I would be
very grateful if someone could point out where I made the idiot mistake.
73, Alan G3XAQ
11736 2015-10-05 16:14:35 kerrypwr Re: Powdered iron toroid loss question I couldn't find any obvious mistake in your maths.
The app note does say that "With constant power dissipation it typically takes a core 2 hours to reach its final temperature. Applications involving low-duty or intermittent operation can time average the losses".
From Figure G, that would mean that temperature rise would be about 25C for 50% duty cycle; the paragraph at the bottom of page 7 of the note appears to indicate that 25C rise is acceptable.
11737 2015-10-05 16:26:20 Clint Re: Powdered iron toroid loss question You might also want to review the K5OOR 100 watt low pass filter. I use this filter with my EB63 amp and it works very good. I was very impressed with the compact layout.73ClintW7KEC 11738 2015-10-06 00:54:23 g3xaq Re: Powdered iron toroid loss question Kerry, that fig G is per cc. The T50 core is 0.358 cc so again the temperature rise appears to be very high even at 50% duty cycle. Although a big core may take 2 hours for the temperature to stabilise I find it hard to believe it will take more than a minute at most for a little T50 core to warm through.
Clint's experience shows T50 cores do handle 100W transmitters in the real world without the predicted fire.
I still think I must have something wrong with my analysis but I can't see the mistake. When the 500v SM capacitors arrive in the post I'll try small cores for myself, but don't you just hate it when theory and practice fail to align?
73, Alan G3XAQ
11739 2015-10-06 07:34:16 nm0s_qrp Re: Powdered iron toroid loss question Yes, my core loss calculator confirms this, you will be burning over a watt in the core, and it will reach about 150C for long transmissions. I'd recommend just stacking two T50-2 cores, with 20 turns of wire around them both. This will keep things in a safe range for power dissipation and temperature rise.73 Dave NM0S 11740 2015-10-06 09:03:46 g3xaq Re: Powdered iron toroid loss question Yes Dave, I think I can see that two cores will double the Al value so you need sqrt(2) the number of turns but the magnetic path area is doubled so there is a net reduction in flux of 1/sqrt(2), to around 20mT. I still suspect that with double the core volume the total loss is about the same as with one core but at least there is more surface area to dissipate it.
I remain very surprised that Elecraft cuts corners so heavily but I suppose where there is a dollar to be made then needs must. I shall use T80 cores in my 100W LPF.
11742 2015-10-06 09:16:32 afu Re: Powdered iron toroid loss question Hi,
I think you all have forgotten that, when a core is driven near the saturation, then the inductance is going down because of the change of the change of the permeability !
Driving a core into saturation, does not only change the effective permeability, but by this, there is a chance of generating dynamic distortions. For filter application, any saturation is a no go !!!
The second effect is, that the warming of the core also changes the magnetic parameters. So the "cheap and easy way" of using too small toroide cores is not the right way !
Herbert / CT2ID / DD0PC
Am 06.10.2015 15:34, schrieb firstname.lastname@example.org [emrfd]:
Yes, my core loss calculator confirms this, you will be burning over a watt in the core, and it will reach about 150C for long transmissions. I'd recommend just stacking two T50-2 cores, with 20 turns of wire around them both. This will keep things in a safe range for power dissipation and temperature rise.
73 Dave NM0S
11743 2015-10-06 10:37:21 g3xaq Re: Powdered iron toroid loss question Herbert, the cores are dissipation (heat) limited. They are way way short of saturation, by a factor of 20 or more. They are made from powdered iron, which has a "built in" air gap, not ferrite.