Magnetics- Safe operating temperature

[Bjtpower_magic]

Hi,
Someone has designed transformer 10 years ago and left company and i am poor at Magnetics, Can you please help?
Specifications: Flyback Converter
Input Voltage range: 20-28VDC
Output Voltage: 5VDC
Output Current: 8A
Switching Frequency: 130KHZ

What is safe temperature to magnetics to operate? Where we can get this data from Datasheet?
Core Data:
Primary Inductance:20uH (No of Turns:12)
Aux Inductance: 8.9uH (No of Turns:8)
Secondary Inductance:2.2uH (No of Turns:4)

 

[D.A.(Tony)Stewart]

It depends on the material Tc and design thermal shock.

For reliability, I might say 85'C max but for self-destructive limits, I suspect it is twice this or the Curie Temp Tc, whichever is lower. But the real risk is self-heating with thermal shock.

Magnetics - Designing with Magnetic Cores at High Temperatures

Magnetics offers a number of inductor and transformer design tools and literature to assist engineers in optimizing their Magnetics components. Contact our Applications Engineers with your design questions.

www.mag-inc.com

 

[FvM]

Safe operation temperature is usually limited by magnet wire insulation class. Lowest limit temperature is 140 °C, can be also 175 °C. As shown above, power ferrites will work up to about 200 °C, also true for 3F3.

Saturation flux is decreasing with temperature but usually only specified up to 100 °C. If the inductor is designed for high AC flux, you may experience thermal runaway. Also bobbin material or epoxy glue may limit operation temperature.

We have seen bobbins crumbling after years of high temperature operation.

 

[Easy peasy]

Seems unlikely you will get the full 40W out from an EFD20 core without a very high temp rise: -

if you know the current wave shapes at a fixed operating point you can then compute the rms values and knowing the wire resistances at 20 deg C ( x 1.25 for 80 deg C )

you can compute the "DC" losses in the wires

as to the core - I can tell from the specs given that it is a CCM design at full power - the sec side current ripple is 6.65 A peak, 3.35 A trough

and the current ramp down ( assuming 5v5 load - i.e. diode ) takes 1.33uS, leaving ( 130kHz = 7.69uS ) 6.36 uS for the current to ramp up on the driving pri side

for the same change in energy ( increase this time ) on the pri side ( 338uJ ) and an average I in of (( 8 x 5.5 ) x 0.9 eff ) / 20 V = 2.45 A

We can see the current change in the pri side at full power is (( 5.81 A )^2 - (( 0.0 A )^2 ) x 20uH x 0.5 = 338 uJ

Lets just check if we can ramp to 5.81 amps in the time avail at low line: V/L = di/dt = 20V / (20uH x 5.81 A ) => 5.81uS so we are just OK

We can now compute the flux ripple in the core and hence the watt losses in the core as you requested:

dB / dt = V / (N . Ae ) so for 20 V in, dt = 5.81uS, N = 12 Turns and Ae = 31mm^2 ( data sheet ) we get Bpk = 312 mT, and delta B @ 130kHz = 156mT

from the data sheet for 3F3 material the supposed losses @ +/- 156mT & 80 deg C are then about 400mW per cc of core material ( 1.46 cc for EFD20 )

giving: 600mW core losses - which is getting up there for an EFD20

when we add in the wire losses for this design at full power we see that the temp rise could easily get to 80 deg C or more in an enclosed environment - and possibly thermally run away if we go over 110 deg C absolute ( 3F3 properties )

To finally answer your question - it depends on the materials used and if full power is really required - but 100 deg C abs max is a good absolute upper limit.