LM5155: designing a 24v to 5.45 dc to dc sepic converter using LM5155

Prajwal Nataraj

Part Number: LM5155

Hello TI Engineers,

I am currently designing a 24 to 5.45V DC to DC converter using sepic topology i have used the quickstart calculator to design the circuit but the gate voltage seems to go down to zero after 5 microseconds. I have attached the converter design below.sepic_converter.TSC

Thanks,

6 months ago

+1 Niklas Schwarz 6 months ago

TI__Mastermind 19770 points

Hi Prajwal,

Thanks for using the e2e forum.

When I open the schematic, it looks like the power stage is connected in the wrong way.
Your primary side inductor (L2) is connected from VIN to the C1 cap. This means C1 just gets charged with the VIN voltage and then no more current is flowing.
Please check once more the schematic of a SEPIC controller and adjust the inductor placements.

Best regards,
Niklas

0 Prajwal Nataraj 6 months ago in reply to Niklas Schwarz

Prodigy 30 points

Hi Niklas,

Thank you for the response, I have made the changes that you have pointed out but the gate voltage still does not switch is there a problem with Undervoltage or CS or SS pin .I have attached the schematic below.

drive.google.com/.../1Zztm2qKtwapgv8FNvVirhjTa7n36eV1n

+1 Niklas Schwarz 6 months ago in reply to Prajwal Nataraj

TI__Mastermind 19770 points

Hi Prajwal,

Sorry for the inconvenience, but I cannot access google drive folders due to company policy restrictions.
Can you attach the files directly just like the previous schematic files?

Thanks and best regards,
Niklas

0 Prajwal Nataraj 6 months ago in reply to Niklas Schwarz

Prodigy 30 points

Hi Niklas ,

thank you for the response after some adjustments in the circuit the gate has started switching but the output voltage is not stable 5.45 but an unsteady slope.I have attached the schematic below.

Thanks and best regards,
Prajwalsepic_design_new.TSC

+1 Niklas Schwarz 6 months ago in reply to Prajwal Nataraj

TI__Mastermind 19770 points

Hi Prajwal,

Thanks for the update.
It is good to see that the device is at least switching for now.
To fix the hiccup/oscillation behavior, I recommend to check the following things:
- compensation network: is the system stable?
- sense resistance: is the sense resistor fit for the load conditions, or is there unintended triggering of overcurrent protection?
- inductor: is the inductance fit for the application, or is it too high/low?

You can calculate all these values with the help of our quickstart calculation tool.
https://www.ti.com/tool/download/SNVR481
Please enter all your design parameters into the calculator and make component optimizations where necessary.

Best regards,
Niklas

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LM5155: designing a 24v to 5.45 dc to dc sepic converter using LM5155