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15th January 2019, 00:17 #1
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half bridge output inductor
hi
i made this half bridge converter but i change frequency to 70 KHz
for output inductor i dont know which core material should i use
MPP core or Ferrite?
i use a M130127A core(11 turn Litz winding and inductance is about 35uH ) and by 7 ampere output current it become very hot above 150 degree.isn't MPP core suitable for this application?is ferrite core like an ETD34 better choice?

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15th January 2019, 00:44 #2
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Re: half bridge output inductor
Hi,
It was originally designed for 50kHz switching frequency and then you changed it to 70kHz. Why do you have to change it in the first place? You changed only the frequency? What else did you change?
Here is the starting point of your problem. As you increased the frequency by 20kHz/50kHz=0.4=40%, you needed an inductance that is smaller in the same proportion. So using the same old inductor means you didn't underrated inductance. You had 40% extra (or excess inductance) and so what happens to the flux?
Akanimo.
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15th January 2019, 01:45 #3
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Re: half bridge output inductor
i use an etd39 so i change frequency to 70 KHz
but original schematic frequency wasn't 50KHz and it was 35KHz and actually i change frequncy from 35 to 70 KHz
i think this MPP core isnt suitable for this frequency and current.if frequency was about 25KHz or current was under 3 ampere then core temp was under 100 degree
so i think ferrite core has less losses than this MPP at 70KHz and 7 ampere current.is that true?

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15th January 2019, 07:04 #4
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Re: half bridge output inductor
As a general rule when you increase frequency, it pushes the inductor/transformer further into CCM (continuous conduction mode). This means the windings carry greater amplitude of DC.
By experimentation it should be possible to find a frequency that creates lower temperature in your inductor/transformer, for a certain output power. Of course there are all the other factors that limit maximum power (saturation, wire gauge, core, etc).
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15th January 2019, 08:12 #5
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Re: half bridge output inductor
I have looked up certain parameters for MPP against ferrite. MPP has higher Bmax and relative permeability compared to ferrite. This means it is suitable. I do not know what analysis and measurements you have made so far. But I feel the inductor may be saturating at that current. The best way to go about it is to analyze the core size against the current (and voltage) using maybe the kg method. This will aide adequate sizing of the core.
And yes, MPP has relatively higher core losses.
Akanimo.
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15th January 2019, 10:02 #6
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Re: half bridge output inductor
you right MPP has higher Bmax
but now my MPP core inductance is 35 uH and current is 7 ampere, so by using formula core work at B=0.21 Tesla
but look at losses of MPP and Ferrite below,ithink MPP is suitable for frequnct about 125 KHz and above it core has a huge losses.for example at 70KHz in B=2.1 losses is 2W/cm3 which in my core result is 2*6cm3 =12watt which result 400 degree temp rising(by using ferrite core worl at 130mT)
for decrease B i have to change currnet to about 23 ampere or change inductance to about 510uH
now you think for this situation is MPP suitable?Last edited by yoosefheidari; 15th January 2019 at 10:11.

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15th January 2019, 17:37 #7
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Re: half bridge output inductor
I see.
At 70kHz, MPP has about 2W/cm3 whereas for ferrite it's about 0.6W/cm3. You should probably use ferrite then.
But you still need to properly size the core to avoid 2saturation and excess cost due to oversizing.
Akanimo.
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