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LM5180: Functionality with Q4437-BL transformer

Radek Jahoda
Prodigy 85 points
Part Number: LM5180
Other Parts Discussed in Thread: LM5181,

Hi, I need to design isolated power supply with input voltage range 12-48V and 3 outputs - 3.3V, +15V  and -15V. LM5180/LM5181 seems to be good choice, but there is problem with available transformer for this application. I found only Q4437-BL transformer, which is designed for LM258X with 100 kHz switching frequency, but LM5180 works at 350 kHz. I have designed it with power outputs +8, +18 and -18V with LDO at the outputs. It works, but it seems not to be very stable, especially at light loads (I need only about 3W). The switching frequency changes with voltage and also chaotically so the output is not very nice, I can also hear the transformer noise. Best it works at 15-20V input, where it is acceptable. Do you thing that this transformer is suitable for LM5180? Or do you have some recommendation for different transformer (acceptable is splitting to two supplies with +3.3 and +-15V)? Or would it be better with LM5181?

Thank you very much.

  • 0
    TI__Guru 73072 points

    Hi Radek,

    What are the output currents for your design? Please fill out the LM5180/1 quickstart calculator (available by download from the product folder). Choose the two highest power outputs and insert the values in this file to get an idea of transformer specs -- turns ratio, mag inductance, etc.

    The Q4437-BL is a very large footprint transformer, not something we would normally recommend for the LM5180. However, it seems feasible given the turns ratio and sat current (albeit with high parasitic capacitance). In terms of stability, just ensure that the output cap is sufficient, e.g. 22uF/25V ceramic on each 15V output and a 100uF/6.3V on the 3.3V output.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 85 points

    Hi Tim,

    thank you for your reply. Current requirements are low - maximum is 100mA @ 3.3V and 75mA @ +-15V, that is maximum about 2.5W, but the typical consumption is a little lower - below 2W. Output capacitors are 2x 22uF at 15V and 4x 22uF at 3.3V. I have added another 47uF electrolytic to 3.3V output and it make it a little better.

    According to quick calculator it seems good. It shows with this transformer that maximum output power is about 3W at Vmin 10V which is surprisingly low. Also the first graph (picture below) shows why the frequency is changing with input voltage - at Vmin there is no DCM mode of operation, only the FFM and BCM (figure 20 in datasheet). With increasing voltage the DCM takes place. This corresponds to the actual behavior, at lower input voltage the frequency is quite stable (up to 17V) and with larger voltage discontinuous mode will make the transformer "noisy". I don't know if it is wrong or OK. What do you think?

    I know that Q4437-BL is large but I don't have another one. I'm thinking on using YA8779-BL for single 3.3V output and create separate power supply for +-15V, but I cannot find proper transformer either.

    Thank you

    Best regards,

    Radek



  • 0 in reply to Radek Jahoda
    TI__Guru 73072 points

    Hi Radek,

    Send on the quickstart file and layout for review. Also, what is the winding capacitance of the transformer?

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 85 points

    Hi Tim,

    unfortunately I don't know the winding capacitance, only leakage inductance whioch is 2uH. All info about the transformer is here.

    I'm attaching the quickstart file and layout. There is LDO after each output (U32, U31, U34).

    Thank you

    RadekLM5180 PSR Flyback Quickstart Calculator r2.xlsx