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    Buck switching power supply output and input capacitance change ?

    Hi,

    I'm using TI TPS62745 buck converter for reducing about 7 Volts to 2 Volts. in datasheet and TI calculator software, it's recommended to use a fixed value capacitance for input or output.
    My question is: the capacitance which they recommend is the least value and can be more than that ? wouldn't it make problem for Buck converter switching function ?
    for example, for the converting i said, TI calculator software recommended 10uF capacitor in input and output too, now there are two problems :

    1- in output line, connecting to MCU, i used some different value capacitors for bypass and surge current sources ... now their value will be added to output of Buck converter circuit !
    this is the sample schematic:
    Click image for larger version. 

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    2- i used the same battery source for another buck converter, 7 to 3.8V for GSM module, that circuit also has input capacitor so it's value will be added to other buck circuit input capacitance ...
    or even i have to use some high value capacitors near GSM module to supply transient current consumption and those will be added to output of 3.8V buck circuit ...

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    Re: Buck switching power supply output and input capacitance change ?

    Quote Originally Posted by hm_fa_da View Post
    My question is: the capacitance which they recommend is the least value and can be more than that ? wouldn't it make problem for Buck converter switching function ?
    Yes, can be more than that. Input capacitor is simply another bypass cap. Output capacitor is 2nd order low pass filter i.e. you will get even lower ripple if no control circuit is involved. However, in this case one should take care about the stability of the system.

    - - - Updated - - -

    Table 3 of the specified datasheet shows 22 uF maximum output cap in order to work welll for the specified conditions, but no max for input cap.


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    Re: Buck switching power supply output and input capacitance change ?

    Hi,

    My question is: the capacitance which they recommend is the least value and can be more than that ?
    --> read the datasheet, it gives very detailed iformation about capacitor selection.
    It begins with chapter 9.2.2.2

    If you have additional capacitors, then you may seperate them by using a series R or series L.

    Klaus


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    Re: Buck switching power supply output and input capacitance change ?

    Quote Originally Posted by KlausST View Post
    Hi,


    --> read the datasheet, it gives very detailed iformation about capacitor selection.
    It begins with chapter 9.2.2.2

    If you have additional capacitors, then you may seperate them by using a series R or series L.

    Klaus
    Thanks, in the datasheet it's written how to select true L and C for Buck converter but i didn't find a formula for the series R or L after the output. just saw "Super-capacitors can be used in parallel to the ceramic capacitors when it is made sure that the super-capacitors series resistance is large enough to
    provide a valid feedback signal to the error amplifier which is in phase with the inductor current."

    How much is high enough ?!
    I have 2 buck converters and one booster in my whole circuit, one buck is for MCU which is always on, another buck for GSM module which is Enabled/Disabled by MCU, and one booster to provide higher voltage for an output device which it's input is connected to output of GSM buck.
    As datasheet says, input capacitors can be higher.
    For MCU buck, i can use R series after the TPS output because the whole current consumption is low, maximum around 1-2mA in working mode and small uA in sleep times. (i don't know how much R is enough Exactly ?!)
    But for GSM module which has up to 1.8A surge current consumption, i can't use resistor (because of high drop voltage), as you said, i can use series L too, i need to use some high value capacitors near GSM module to provide the source for surge currents.
    Now how should i calculate that series L value ?! (in datasheet only L value formula for Buck or Boost converter circuit is written).
    Won't that series L in output make instability like more C ? (in datasheet just said about more R for output C)

    Thanks again



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    Re: Buck switching power supply output and input capacitance change ?

    I'm using TI TPS62745 buck converter for reducing about 7 Volts to 2 Volts. in datasheet and TI calculator software
    Can this buck be simulated with the TI software with the output capacitors you have? The only problem the output capacitors would give is for the control circuit. If there was no control circuit, it would have been very O.K.


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    Re: Buck switching power supply output and input capacitance change ?

    Quote Originally Posted by CataM View Post
    Can this buck be simulated with the TI software with the output capacitors you have? The only problem the output capacitors would give is for the control circuit. If there was no control circuit, it would have been very O.K.
    Thanks, Good idea, i didn't notice it before.
    I tried TI simulator as you said, gave different values for C manually, just when i decrease the output C (100uF to 1uF for test), output got unstable. but when i increased output C even to 1F , all worked fine and even better output quality ! (less ripple and better load transient handling).
    I tried it both on boost and buck converters, same result.



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    Re: Buck switching power supply output and input capacitance change ?

    Quote Originally Posted by hm_fa_da View Post
    I tried TI simulator as you said, gave different values for C manually, just when i decrease the output C (100uF to 1uF for test), output got unstable. but when i increased output C even to 1F , all worked fine and even better output quality ! (less ripple and better load transient handling).
    I tried it both on boost and buck converters, same result.
    So in conclusion, increasing the output capacitor does not cause stability problems in contrast to decreasing it which it gives.
    The only thing to fear was the stability problems.. I guess one can rely on the SW provided by the manufacturer itself.


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    Re: Buck switching power supply output and input capacitance change ?

    dont increase the input capacitor to higher value.This will lead to more inrush current.Outputs lead to stability issue.



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