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Buck Regulator with a Isolated DC/DC Converter

Tapajit Mondal
Prodigy 160 points
Other Parts Discussed in Thread: ISO7241C, BQ24105-Q1, TPS57040-Q1, SN6501-Q1

My application requires me to put a CAN transceiver in the circuit. The MCU communicates with the CAN transceiver through a Digital Isolation Barrier IC - ISO7241C. My input supply is from a Battery operating at 24V.

I have used a Buck Converter, TPS57040-Q1, 500mA step-down SWIFT DC/DC Converter to step down to 5V. This 5V power rail supplies directly to a Push-Pull Transformer Driver, SN6501-Q1, to create an isolated line for the CAN transceiver. The isolated 5V supply rail directly connects to a bqSWITCHER battery Charger, bq24105-Q1, which is a switch-mode battery charger for a Li-ion Backup cell.

Since all of these switching supplies will be operating at much different switching frequency, I would like to ask if there are any precautions I should be taking ? I had initially thought of putting LDOs in between 2 switching supplies to reduce the output ripple to the next input, but that seems to be a very costly affair. Please feel free to give me any solution that seem fit.

- Tapajit.

  • 0
    TI__Mastermind 20580 points

    Hi Tapajit,

    The key will be to make sure you have followed the filter design for each of the DC/DC converters per the datasheets of the devices.  The TPS57040 is extremely flexible in switching frequency and gives alot of information on how to set up the device to optimize for different requirements, however that will be you challenge: how to adapt its use best to your system requirements.  You will likely want to choose a switching frequency that doesn't match the other two devices to ensure that output voltage ripples do not easily add as you cascade the devices.  Also to try and spread the emissions spectrum to frequency bands that don't have requirements or the requirements are lower.  Make sure you match your filter design per the recommendations for the switching frequency you choose to make sure to minimize the output voltage ripple.  Make sure to pay attention to component choice impacts like ESR of a capacitor, effective capacitance (de-rated at the use voltage) instead of the "datasheet" capacitance, aging over life time, etc.

    - Scott     

  • 0 in reply to Scott Monroe
    Prodigy 160 points

    "Also to try and spread the emissions spectrum to frequency bands that don't have requirements or the requirements are lower."    --    Please clarify what you meant.

    Kindly also explain how to derate a capacitor at the use voltage ? You can direct me to some Application note.

    - Tapajit

  • 0 in reply to Tapajit Mondal
    Prodigy 160 points

    Hi Scott

    Could you give a reply to my query please. Thank you.

    - Tapajit

  • 0 in reply to Tapajit Mondal
    TI__Mastermind 20580 points

    By spreading the emissions spectrum to frequency bands that don't have requirements I mean that depending on the region of the world or the specific car, bus or truck maker they have varying EMC emissions requirements.  Usually those requirements have some gaps in emissions requirements between long wave, AM, FM, TV bands.  You can choose to put the fundamental switching frequency in one of the gaps and then try to get the 2nd, 3rd harmonics either in the gap as well or choose it so that the fundamental is in one gap and the harmonics are in another gap.  This minimized the theoretical chance to have an emissions failure during EMC testing.  It depends on the end use case of your system.

    Derating of the capacitor comes from the capacitor vendor is is normally a chart in the datasheet similiar to aging, tollerance, etc.  Below is a derating example from a capacitor datasheet that is a 22uF ceramic capacitor rated for 16V.  As you can see if you put 5V DC on the capacitor the derated capacitance value is acutally lower by about 30%.  So if you are supposed to have 22uF as a filter capacitor on a 5V switcher, you would need to either add in parallel or make a different capacitor choice either higher value that derates to 22uF or possibly looking at say 35V or 50V capacitors which will derate less at 5V DC.

    - Scott