Pc psu toroidal inductors?

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filthyone

Member
Joined 2015
#1
It just occurred to me....

How come PC switching psus use toroidal inductors at the output, after rectification?

Aren't toroidals, which are lacking a gap, supposed to saturate at DC and at the (relatively) high currents a modern desktop PC uses?

I think i've seen toroidal chokes even on older motherboard regulators.

Is it maybe because, they are used right after rectification, where there is only rectified switching DC, and not pure DC?

Any thoughts?
Thanks in advance!
 
#2
Depends on the mix used, iron powder toroids can have very high saturation flux, while the ferrite ones tend to higher Al values, and there are special blends for high flux power supply applications..

Given a conventional forward converter you need an inductor there to allow the duty cycle to control the output voltage.
 
#4
Yes, those iron powder cores are internally gapped, that's why. The ATX bigger yellow is about 96nH/turns^2. You (OP) will be surprised to know how well they operate without saturating! However, do note that these cores are not suitable as Class-D output chokes or crossover inductors, as they exhibit a soft saturation characteristic.
 
#5
filthyone said:
Aren't toroidals, which are lacking a gap, supposed to saturate at DC and at the (relatively) high currents a modern desktop PC uses?
Click to expand...
Greetings.

There are ferrite toroids which do not have air gap and they usually have high AL factor ( on the order of thousands of nH/N^2 ), and lower saturation point ( maybe 0,3, 0,35T or something like that ), and there are iron powder toroids that do have a distributed air gap and they usually have low AL factor ( on the order of tens or sometimes hundreds of nH/N^2 ), and higher saturation point ( 0,7T for example ).
Meaning that for the same needed inductance value you would need a much higher number of turns with an iron powder toroid than with a ferrite one.
PC power supply ( ATX ) mostly use T90-26 ( really cheap ones ) and T106-26 core, some higher quality supplies use T106-52 which have almost the same AL ( 95 ) but half the core loss for the same flux density, meaning they can be used for higher power.
T106 describes the core dimension, and the suffix -xx the core mix.
T106-52 has a AL factor of 93nH/N^2, meaning that for let say 100uH of inductance you would need about 33 turns.
A similar size ferrite core may have 2000nH, of even more, meaning that for the same inductance of 100uH you would only need 7 turns or less.
What that does is influence flux density, less turns for the same inductance, means more flux density, on the other hand more turns for the same inductance, means less flux density, meaning the peak flux density never reaches the saturation point with iron powder cores just because it's distributed air gap.

newvirus2008 said:
However, do note that these cores are not suitable as Class-D output chokes or crossover inductors, as they exhibit a soft saturation characteristic.
Click to expand...
-26's problem with class D is it's permeability, class D work better with very low AL because that means much more turns and much lower core loss.
T106-2 for example has an AL of 13,5, compared to 93 of -26.
From there it's simple math.
 
#6
MarianB said:
...-26's problem with class D is it's permeability, class D work better with very low AL..
Click to expand...

Yes, not only is the AL value is high for the yellow (-26) when compared to the red (-2), the permeability (slope of the BH curve) is also not very linear, which is not a desirable property when it comes to audio applications. The BH curves from Micrometals datasheet (below) clearly show why Class-D designers are literally in love with -2 mix, while the -26 is meant for SMPS chokes.

Untitled-1.jpg
 
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