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LM5180-Q1: Unstable Output Voltage Across Varying Load Conditions

Anirudha Ram
Prodigy 110 points

Part Number: LM5180-Q1
Other Parts Discussed in Thread: LM5180,

Hello!

    I'm working on a design utilizing the LM5180 primary-side regulated (PSR) flyback converter to generate an isolated 5V output, but I am experiencing stability issues across my expected load range. 

    The Issue: The output is failing to maintain regulation as the load varies. It varies from 5.6V to 4.4V with load current ranging from 50mA to 800mA.

Operating Parameters:

  • V_IN Range: 10V to 16V

  • V_OUT Target: 5.0V

  • I_OUT Range: 50mA to 800mA

Transformer Used: 750317605

References:

    1) LM5180-Q1 EVM Single Output User Guide

    2) WeBench Calculation for LM5180

    3)LM5180 Calculator Tool (Excel Sheet). 

The following is my schematic:

image.png

Thanks in advance for your insights.

  • 0
    TI__Intellectual 1800 points

    Hello Anirudha,

    Schematic looks good and the device has to work for your output requirements. I see your design is similar to TI EVM, did you evaluate TI EVM? 

    I see L5 0.1mH is too large at the input filter, could you please short L5 and perform the tests.

    Regarding stability, the compensation loop is internal and only parameters we can work with are RFB and RSET values. Could you please check whether these values are correct?

    Could you please provide with this information,

    • I think Zener is clamping to 5.6V at light load, can you probe output voltage without output Zener at light loads.
    • Could you please share the SW node waveforms and output voltage waveforms at 50mA and 800mA.

    Thanks,

    Hareesh.

  • 0
    Prodigy 110 points

    Hello Hareesh,

        Thank you for the reply!

    As suggested, I have shorted the L5 pads and conducted the tests without the 5.6V Zener.

    Observation 1: The output shoots up to 9V - 9.5V under no load condition.

    The SW node waveforms and output voltage waveforms at 50mA and 800mA are as follows:

    1. 50mA Load at 10V Input:

    2. 800mA Load at 10V Input:

    It seems that there was a grounding issue previously, I am able to get 5.28V to 5.13V at the output for the load varying from 50mA to 800mA.

    Tuning (Reducing) the Rfb makes the output 5V as desired.

    How good is the temperature compensation? Is it better to add a 5V LDO and tune the LM5180 sub-circuit output to 6.5V -7V? 

    Thanks and Regards,

    Anirudha Ram

  • 0 in reply to Anirudha Ram
    TI__Intellectual 1800 points

    Hello Anirudha,

    Good to hear! The waveforms look as expected and at no load it is expected to see higher output voltage, which is the behavior of flyback topology.

    For temperature compensation, we have TC pin integrated with the device which is used to compensate the changes in secondary diode forward voltage drop due to temperature, please refer Section 7.3.3.1 Diode Thermal Compensation from datasheet: LM5180-Q1 65-VIN PSR Flyback DC/DC Converter with 100-V, 1.5-A Integrated MOSFET datasheet (Rev. C)

    Regarding LDO at output, what is your application? Adding LDO will not affect the device performance, if your requirements need LDO, you can add.

    Thanks,

    Hareesh.

  • 0 in reply to Hareesh Babu Takasi
    Prodigy 110 points

    Hello Hareesh,

        The +5V is being used to power up the precision rail to rail amplifiers. I desire efficiency of switching with the silence of linear.

    Thank you for your help!

    Regards,

    Anirudha Ram