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LM5156: Boost converter 12V to 300V with LM5155 / LM5156

Part Number: LM5156
Other Parts Discussed in Thread: LM5155, , LM555


I'm trying to design a boost converter to have 300VDC in the output from 12VDC (regulated) in the input, so I was thinking on using the switching controller LM5155/LM5156. For this purpose, I used the Excel file for the calculations and to reach this voltage, the duty cycle must be 96% which is out of the range for the calculations in the Excel but apparently is the upper limit for the LM5155/LM5156 from their datasheets.


 My question is, could be working fine the LM5155/LM5156 at a 96% duty cycle? I did the calculations for a duty cycle of 95% and the voltage in the output would be 238V instead of 300V, which could be acceptable but not ideal.

Thank you and regards.

  • Hello Angel,

    A robust design needs to have a duty cycle limit lower than the low limit of the maximum duty cycle.

    A duty cycle of 96% is extremely high for any boost converter, so I recommend using a flyback converter instead of a boost converter or using 2 stages of boost converter as the power is relatively high for a flyback converter as well.

    Best regards,

  • Hello Brigitte, thanks for your answer.

    I have already implemented a solution based on a flyback converter, but I need to reduce the size off the board so I want to remove the transformer, this is why I'm looking for a boost converter like the LM5155/LM5156.

    Additionally, I tried with a boost converter based on a LM555 in astable mode but I was simulating the circuit and I'm not able to design a correct feedback for reducing the duty cycle or the frequency to get an stable output voltage.

    Thanks again for your answer.

    Kind regards.

  • Hi Angel,

    Thanks for the feedback.
    If a flyback topology is not possible due to transformer size, two boost stages with a LM5155 and another controller might achieve smaller solution size.
    There are also alternative topologies, which achieve higher boosting ratios, like a boost with tapped inductor, or a cascading diode output.
    A reference design for this is shown below:

    Best regards,