RECOMENDATION for 40khz push-pull smps

[alfcoder]

hello!

i'm working on a 40khz push-pull converter, the primer is 12v and i have
3 outputs with minimum 5V at 5A each, the non loaded voltage can be
few volts higher, basically is is a non-regulated psu

i'm studied the great book: switching power supply design by abraham pressman, kaith billings, taylor morey

and i'm become very familiar with the concept and as well the equations
but after i found in my junk a toroid ferrite with a proper size (40mm diameter, 27mm inner diameter, 15mm height) and tested it, i found that
the core getting real hot, it is clearly not from the copper because i only
wound one portion of the toroid and my input current with only the primary
was 650ma, also the copper was cool, i was unable to measure the exact
temp but after a 10-20 minutes the core was so hot that i was unable to hold in my hand (i'm not an psu expert so maybe this temp is ok)

some information about the configuration and calculations, maybe i commited some error here:

my power rating is 75w output => the toroid need to be at least 40mm wide

the measured core al=755nH/N^2

magnetic path is 106mm

cross section area is 97mm^2

i let the flux swing to be +/- 1600 gauss so the calculated turn of the
primary is 2x4 turn

the primary wire diameter need to be at least 1.45mm diameter or i can use
5x 0.6mm diameter wires

the fixed pwm is 80%

so if my calculations are correct, then i can only think that i have some
real bad ferrite material, in this case i would like to know some suitable
ferrite types, if it is possible from the farnell distributor, i've seen they
have a very wide selection, but unfortunatly for me is very hard to find
that material, i mean to know for sure if one will be good in my application
or not and honestly i really don't want to buy 10 or more and try those one
by one

so please, experts, give me some recommended core

many many thanks

david

[sawreyrw]

Maybe the core is powdered iron.

[busan]

You need spread the windings on all toroid.

[savu]

to Alfcoder:

Try 3c90 or 3f3 they are good materials for smps. you can find them at farnell

regards,

[odnaizutra]

maybe you can try n27 .

[alfcoder]

thank you all, this is a very nice responsive forum

i ordered two toroid with the 3c90 material one is 42mm x 26mm x 13mm
and the other is 40mm x 24mm x 16mm, so in few days we will see

the claimed Al value is much higher then the current one (2690nH and 3500nH)

and my measured was only 755nH, so maybe this is the big difference

as with this full spread winding i'm still not very familiar with, simply because
it looks a bit strange to wind only 8 turn fully on a 4cm toroid and also as
i seen it not take longer path for the magnetic flux to travel then in the
E -core outside legs, but i will try this spreaded winding for sure

so thanks again

david

[savu]

Originally Posted by alfcoder

thank you all, this is a very nice responsive forum

i ordered two toroid with the 3c90 material one is 42mm x 26mm x 13mm
and the other is 40mm x 24mm x 16mm, so in few days we will see

the claimed Al value is much higher then the current one (2690nH and 3500nH)

and my measured was only 755nH, so maybe this is the big difference

as with this full spread winding i'm still not very familiar with, simply because
it looks a bit strange to wind only 8 turn fully on a 4cm toroid and also as
i seen it not take longer path for the magnetic flux to travel then in the
E -core outside legs, but i will try this spreaded winding for sure

so thanks again

david

spread winding ensures low emi and low skin effect.

regards,

[Electrone]

I think you might be experiencing a DC bias on the core, walking it into saturation. One way to tell is if one MOSFET drain is experiencing a much greater turn-off spike than the other. In push pull designs, it is good to use current mode control because the transformer can't easily be capacitor coupled to its MOSFET drive.

[alfcoder]

Originally Posted by Electrone

I think you might be experiencing a DC bias on the core, walking it into saturation. One way to tell is if one MOSFET drain is experiencing a much greater turn-off spike than the other. In push pull designs, it is good to use current mode control because the transformer can't easily be capacitor coupled to its MOSFET drive.

yes i have very big drain spikes but they are equal in shape, once i have a
saturation when i was not driven the mosfets with the same square-wave
so i heard a ticking sound from the transformer and many times my 10A
limited psu switched off, but as soon as i fixed the square wave i not
noticed any strange things, so from my limited expertise on smps,
i can say i don't have any saturation, also the non loaded input current
was only 650mA, not a very wild thing

i think this current mode thingy is just complicating the design, also it need
some knowledge to implement it, so i rather use some half-bridge config if
i can not get rid off the spikes

i also ordered some 100v mosfets because the other was only 60v and it is
very on the edge because of these spikes, so now i will have 40v more
space, but i sure will do something with the spikes, but it is very hard
because, it doesn't matter how big capacitor i'm using with whatever
resistor for the snubber, when i'm applying the load the big spike is come
along

so we will see

[alfcoder]

Originally Posted by savu

spread winding ensures low emi and low skin effect.

regards,

what we should have is a coated copper ribbon for the winding, in an ideal
world

isn't this skin effect is only affected by the frequency? maybe you meant
some proximity effect when on conductor is affecting the neighbor conductor
if they are in close proximity