LM25180: Alternative transformer for flyback topology

Stefano Infante

Expert 2320 points

Part Number: LM25180

Hi all.

I have to design an isolated power stage with these requirements:

- Vin: nominal 12V, min/max: 10/15V

- Vout: 24V nominal (not a strict requirement: any voltage between 20 and 28 V is ok for me)

- Iout: 40-50 mA typical, 110 mA maximum

- 1 kV functional isolation (2.5 kV would be better)

- the temperature could go up to 60 °C

- maximum possible efficiency

I used Webench in order to find a possible device and I found a design based on LM25180, with a 91% efficiency rating (very good for me). But I have to use an off-the-shelf transformer, and the ones suggested are far too big. I found some devices: Wurth Elektronik 7448845470, Bourns SRF1260-220M, etc (many have different devices in the same series, so there are many possible choices). Are they ok? What should I look for in order to find a suitable alternative transformer?

Thanks in advance.

Best regards,

Stefano Infante

over 5 years ago

0 Arief Hernadi over 5 years ago

TI__Mastermind 23845 points

Hi Stefano,

I think the datasheet guideline on the selecting the transformer would be a good start. In terms of current requirements as well as the magnetizing inductance

https://www.ti.com/lit/ds/symlink/lm25180-q1.pdf#page=18

Alternatively you can use the calculator below to see what magnetizing inductance is required for the transformer

http://www.ti.com/tool/LM25180DESIGN-CALC

I tried to enter your parameters and found the result attached.

LM25180 PSR Flyback Converter Quickstart Tool -- 24V_110mA.xlsx

Thanks and i hope that answers your questions

-Arief

0 Stefano Infante over 5 years ago in reply to Arief Hernadi

Expert 2320 points

Hi Arief,

with your Excel sheet I got a #VALUE error for the magnetizing inductance, but by introducing all values in a new one it seems that the error is due to the selection of a 1:2 transformer. Why do you think I should use such a ratio?

By the way, the resulting magnetizing inductance is 21 uH (or 42 uH when I select a 1:1 transformer); if I only look at the inductance value, the devices I suggested in my first post should be ok. Is it correct?

Thanks

Stefano

0 Arief Hernadi over 5 years ago in reply to Stefano Infante

TI__Mastermind 23845 points

Hi Stefano,

When i select 1:2 transformer on the excel it seems to be ok.

If choosing 1:1 transformer, i would recommend this one SRF1260-470M

The reason being is that if you look on the excel 2nd tab. The transformer should have saturation current greater than 1.65A. Therefore you want to look at something around 2A or so saturation. The Wurth transformer 7448845470 has a much lower saturation current.

Thanks

-Arief

0 Stefano Infante over 5 years ago in reply to Arief Hernadi

Expert 2320 points

Hi Arief,

thanks for your reply. I don't know why my Excel sheet is not ok with a 1:2 transformer, but I can surely go for a 1:1 one.

I did not check the Bourns part in details, but it cannot be used for isolating, as it has the two inductances crossed.

Now I found P/N 7448990470 from Wurth. It has a 6 A saturation rating. What do you think about it?

Thanks again

Stefano

0 Arief Hernadi over 5 years ago in reply to Stefano Infante

TI__Mastermind 23845 points

Hi Stefano,

Yes the new transformer looks reasonable.

I have attached the spreadsheet with the 47uH transformer

LM25180 PSR Flyback Converter Quickstart Tool -- 24V_110mA_TransformerUpdated.xlsx

Thanks

-Arief

0 Stefano Infante over 5 years ago in reply to Arief Hernadi

Expert 2320 points

Thank you Arief!

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LM25180: Alternative transformer for flyback topology