How to calculate ferrite transformer core size for a push-pull SMPS?

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Redwire

New member
I found some math but confusing results :confused:

Looking at IRF car amp reference design pdf uses Magnetics ZP42915 toroid core to get 500W (Ae=74.9mm^2) pri. 12V/2x4turns, 50kHz.

Magnetics Inc. Power Design with Ferrite Cores Table 4 (50kHz, 1300 gauss, push-pull):
ETD44 good for 450W (Ae=173mm^2)
ETD39 good for 240W (Ae=125mm^2)
ETD34 good for 110W (Ae=97.1mm^2)
ZP42915 good for 215W (Ae=74.9mm^2)

These numbers seem really conservative? Why do they use Area Product Wa*Ac which has core window area?
Mag. Inc. gives ZP42915 215W, even upping to 2000 gauss gives 335W, still way off from IRF's 500W ;ww:
So I'm not sure what math to use.
 
R

Redwire

New member
I am using the equation in Section 4.4/Pg. 4 of mag-inc Ferrite PowerDesign.pdf
It starts with an ideal transformer using Faraday's law E=4*B*Ac*N*fx10^8 (square wave) and then adds some losses and the winding factor to give a bigger equation: "Area Product"

"Core manufacturers have universally adopted a mathematical method of easing the core selection process. The method results in the product of the required wire cross-sectional area needed to couple the electrical power into the core times the required core cross-sectional area needed to support the generated magnetic energy."

Looking into it more, I find Mag Inc. use only 80% efficiency, 1300 gauss (50kHz) and pri. wire current density of 2.5 Amps/mm^2. This is why it calculates such a big core!

Making this more realistic:

1. You can push more current in the pri. windings "...with a small number of turns, a current density of 6-10A/mm^2 is also acceptable." Fairchild AN4140
2. The IR reference design measured 90% efficiency, so 80% is low.
3. The flux density... looks like 2000 gauss is okay for 3C8, F44, N87 materials.
4. But the Mag Inc. formula doesn't include ANY room for secondary windings, so guess what... my bobbin is full when I did all this x-(
 
M

marmis

New member
Redwire, I was looking at the same IR ref design and numbers just don't quite make sense.
We need 700W SMPS. I tried to follow IR example in the PDF, but can't figure out how they chose this ZP42915 core.
Numbers just don't make sense.
How did you make out?

Cheers
 
Dimonis

Dimonis

New member
marmis said:
We need 700W SMPS. I tried to follow IR example in the PDF, but can't figure out how they chose this ZP42915 core.
Click to expand...
Do you know what music power factor means?((@
A constant power supply of 200W can handle up to 500-700W music power.:D
 
R

Redwire

New member
the IRAUDPS1 reference design

Be careful with the IRAUDPS1 reference design; a big difference between a technical paper and something for mass production.
Some of the numbers are off. Example: It's a 490W output design (+/-35VDC at 7A), but the input power spec at full load is 511W (35.5A at 14.4VDC). That's a 96% efficiency, yet the rating is 87-92% which is 532W-563W input. Later, the input current is given as 36.6A (93% eff) at full load... 37W of losses. So I take the paper cautiously. Where did the 37W of heat go.... IMHO without a cooling fan it wouldn't last long yet the paper says it takes 30 minutes until overtemp trips at full load.

Looking at transformer sizing, at a top level, Mag Inc. Power Handling Chart rates the ZP42915TC at 240W at 50kHz, with a conservative flux density of 1,300G (another doc said 215W)

IRAUDPS1 uses 4T primary (no mention of where that number came from) which gives about 2,400G which seems quite high, the core seems to be pushed hard (14.4VDC, 50kHz, Ae=73.9mm^2; it would be great if someone could check my math)

The primary windings (4 strands #18AWG=12AWG) at rated 35.5A input current gives 183 circ. mil/A or 10.8 A/mm^2 current density. Again, pushing the copper quite hard, if my calcs. are correct.

For a consumer product, full power for short periods (a song or two) I guess it's okay.

See also [url]http://www.mag-inc.com/products/ferrite-cores[/URL]
 
W

wally7856

New member
“Looking at transformer sizing, at a top level, Mag Inc. Power Handling Chart rates the ZP42915TC at 240W at 50kHz, with a conservative flux density of 1,300G (another doc said 215W)”

My guess is that this core can run 215 to 240W continuously. So you can get 500W of audio power out of it.

“IRAUDPS1 uses 4T primary (no mention of where that number came from) which gives about 2,400G which seems quite high, the core seems to be pushed hard (14.4VDC, 50kHz, Ae=73.9mm^2; it would be great if someone could check my math)”

I get 2400G also. 6 turns make more sense to me, but with such a small core they probably could not fit all the winding’s.

“The primary windings (4 strands #18AWG=12AWG) at rated 35.5A input current gives 183 circ. mil/A or 10.8 A/mm^2 current density. Again, pushing the copper quite hard, if my calcs. are correct.”

18awg = 1,624Cmil x 4 = 6,496 Cmil
6,496Cmil / 35.5A = 182.98 Cmil/A
 
szhighstar

szhighstar

New member
Magnetics Inc. Power Design with Ferrite Cores Table 4 (50kHz, 1300 gauss, push-pull):
ETD44 good for 450W (Ae=173mm^2)
ETD39 good for 240W (Ae=125mm^2)
ETD34 good for 110W (Ae=97.1mm^2)
ZP42915 good for 215W (Ae=74.9mm^2)

These numbers are not conservative, it is normal selection guidance for PC40 core or equivalent materials.
We can select core according to power and work frequency.
 
M

michelle

New member
power output table

Hello
greetings whats best material for GATE DRIVER TRANSFORMER for smps working at 50KHZ trfo turns 1:1:1 using TL494 with totem
warm regards
michelle:"::
 

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michelle

New member
Hi Dimonis
greetings thanks for your reply and help now i can order these cores the problem here is the shops that stock cores here have mostly chinese made ones and very
little information i am trying to make a simple gate transformer for my smps thank you Dimonis
warm regards
michelle
 
M

michelle

New member
Hi Szhighstar
greetings thanks for helping as you are qualified engineer in this field need some information on this core i will post picture of this core but it has no markings on it
its just for smps design instead of etd cores as its easy to wind hoping you can help me out
warm regards
michelle
 
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