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TPS40210 output cap question

maxthestork
Intellectual 390 points
Other Parts Discussed in Thread: TPS40210

Hi All,

 

 I am using the TPS40210 in a boost app that takes a 12V battery supply and generates 30V at 2.5A to run a motor. 

 For the output cap, it will have to abosrb some energy dumping from the motor when the motor returns energy to the power supply and the output cap also has to handle the AC component from the boost topology.

Question :

- Is the ESR for the output cap in the example calculation a maximum amount or a target ESR that is required.  Can I just use caps in parallel to increase the ripple current rating and also to lower the ESR.

- This is probably a really newbie question but what is the AC current that the caps have to handle.  Is it equal to the worst case ripple current in the inductor?

 

cheers

Peter

  • 0
    TI__Guru 51705 points

    Peter,

    Let me address your questions:

    Question :

    - Is the ESR for the output cap in the example calculation a maximum amount or a target ESR that is required.  Can I just use caps in parallel to increase the ripple current rating and also to lower the ESR.

    The ESR calculation is a maximum ESR based on the assumptions made in the calculation. 

    This perticular calculation is based on an assumption that since boost-converters have very high output voltages, they will not be able to use ultra-low ESR ceramic capacitors and will favor higher ESR electrolytic capacitors.  As a result, the ripple voltage is divided as 1/8 of the ouptut ripple from capacitive charging and discharging of the capacitor and 7/8th from ESR.  This ratio will result in an output ripple waveform which is nearly triangular.

    The two leading multipliers of the equation:  8/1 for capacitor and 7/8 for ESR are based on this ratio.  The product of the inverse of the capacitor muliplier and the ESR mulitpier needs to be 1, but the values can be adjusted.  For example, for 1/4 Capacitive Ripple and 3/4 resistive ripple, the capacitance multiplier would be 4 and the ESR multiplier would be 3/4

    - This is probably a really newbie question but what is the AC current that the caps have to handle.  Is it equal to the worst case ripple current in the inductor?

    It looks like we omitted the output capacitor AC ripple current from the datasheet calculation.  The ouptut capacitor will absorb pulses of current from the power stage and deliver these over time to the load and will experience similar RMS currents to the input capacitor of a BUCK.

    The RMS output capacitor current is the RMS diode current [ Iout / (1-D) * sqrt (1-D) ] minus the load current (Iout)

     

  • 0 in reply to Peter James Miller
    Intellectual 390 points

    Hi Peter,

     thanks for taking the time to answer my questions. I am going to have to go away and read a lttle more to understand some of the principles.

     

    best regards

    Peter

  • 0 in reply to maxthestork
    Mastermind 9620 points

    Hello Peter-san,

    Please answer following question for your post dated Jan 29 2010.

    Please advise how to decide two multiplier of the equation, for example 8/1 for capacitor and 7/8 for ESR or 4/1 for capacitor and 3/4 for ESR or ………..

    Best Regards.

  • 0 in reply to Toshio Ushikubo
    TI__Guru 51705 points

    Toshio,

     

    Those ratios are based on engineering experience with the types of capacitors typically used on BOOST outputs, which generally have higher ESR per unit capacitance than the low-voltage capacitors generally used on BUCK converters.

    What's critical is that the sum of the two fractions is 1.  It's generally reasonable to generate capacitance and ESR values for a couple of different ratios:  1/8 to 7/8, 1/4 to 3/4, 1/3 to 2/3 and 1/2 to 1/2 and compare the required ESR and Capacitance against the type of capacitors that you want to use.

    If you have a spreadsheet set up, calculating a couple of capacitor sets is fairly easy. 

  • 0 in reply to Peter James Miller
    Mastermind 9620 points

    Hello Peter-san,

    Thank you for your answer.

    I could understand the equations.

    Thanks and Best Regards.