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Q&A

Transformer - Vsec or Current rating parameter

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I am looking at the datasheet of the 7508170315(link) transformer and something seems strange to me. Itable from datasheet

If I connect this to a 12V supply and expose it to 10us pulse: 12V*10us/475uH = 252mA which is apparently less than the 350mAAC - is it actually? What 350mAAC mean in this case?

However, this is clearly violating the 11Vus spec(12V*10us=120Vus) which will certainly saturate the transformer core.

Which parameter should I look at here?

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2 answers

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If I connect this to a 12V supply and expose it to 10us pulse: 12V*10us/475uH = 252mA

$$$$ This current flows in the primary magnetization inductance (475 μH) after applying 12 volts for 10 μs. This causes core saturation based on the volt-second figure limit of 11 volt-μs.

$$\dfrac{V\cdot dt}{L} = dI$$

$$$$ I believe when the data sheet states the 475 μH figure, it relates to primary inductance despite them calling it $L_S$. If you have any doubts about this, then contact WURTH electronics for clarification. It does look like an error because primary inductance is the driven winding and it is solely responsible for core saturation.
$$$$ The 350 mA AC figure relates to load currents - this is unrelated to the magnetization current. Load current doesn't cause magnetization hence it can't create core saturation. This is because load current primary ampere-turns are cancelled entirely by load current secondary ampere-turns. $$$$ 350 mA RMS is the primary load current limit and, given that the transformer steps down voltage by 2:1, the secondary load current limit is 700 mA RMS.

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Thanks, Andy! How does this current limit of 700mA RMS apply on a secondary when potential capacitor there acts as a "short" during the first pulse? If we assume pulse not exceeding 11Vus. 2kind‭ 4 months ago

@2kind - What capacitor? What pulse? I know nothing about your target circuit and, to answer your question, I didn't need that information. Andy aka‭ 4 months ago

If we assume there is a forward based diode followed by a cap to GND on secondary. 2kind‭ 4 months ago

@2kind - if that is all you have on your secondary then the 700 mA applies when charging the capacitor. But, bear in mind that I don't want you responding with another variant of what load might be connected for me to have to make another comment because where would this process stop? Andy aka‭ 4 months ago

+1
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In addition to what Andy said (+1), the 350 mA AC figure is probably limited by power dissipation, and therefore temperature. It applies during normal forward operation where the secondary current "unloads" the core as fast as the primary current tries to load it.

The 11 V⋅µs figure is about core saturation with the secondary unloaded. This would be a meaningful limit, for example, in a flyback design.

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