• State
  • Locked Locked
  • Replies 3 replies
  • Subscribers 63 subscribers
  • Views 1072 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Output capacitor for high switching frequency SMPS-Buck controller (TPS54218)

Bishnu Ban
Intellectual 500 points
Other Parts Discussed in Thread: TPS54218

Hi.!

 I designed a buck converter with TPS 54218 (200k to 2Mhz). Because of very less Space on PCB I have to shift the switching frequency as high as possible to reduce the size of output inductor and capacitors. I have selected 1.67MHz switching frequency and got the same result as calculated by the design tool (Switcherpro provided by TI).

The recommended output capacitor is (by swticherpro ) C3225XR0J336M.

it (C3225XR0J336M) has its self resonance frequency at 723kHz and capacitance goes rapidly downwards after this frequency.  The inductive part will dominate after this frequency. it is not recommended to use capacitors over the self resonance frequency.(refer the attached file for schematics, bom and characteristics of output capacitor.

-Question 1: why switcherpro is not considered this effect ? because the capacitance will be around 5uF @1.67MHz.

There are almost no capacitors with self-resonance frequency higher than 1.67MHz.

How can I overcome with my design (less space)?

7713.bom.pdf4011.Schematic.pdf7607.output capacitor.pdf

  • 0
    TI__Guru**** 191445 points

    Switcherpro is an older software tool that is no longer directly supported.  It was originally developed for dc/dc converters with maximum switching frequency of 700 kHz.  I think that no one ever checked the selection algorithm as switching frequencies increased.  In any case, Switcherpro tries to design with minimum capacitance.  Most designs can be stabilized for that, but in practical terms you usually need more output capacitance.  For TPS54218, I like to use around 100 uF.  And that usually means multiple 22u F.  It will always be a trade off, but so far as I know ceramic capacitors will be your best option.

  • 0 in reply to JohnTucker
    Intellectual 500 points

    Hi. John!

    thanks for the prompt response!

    the ceramic are the best option, but as I have written, the switching frequency of my design is higher than self resonance frequency of the ceramic which are available.

    at my switching frequency these ceramic-co are no more capacitive energy storage but inductive storage. An inductive storage can not supply current at high speed (Ldi/dt=VL) i.e. no bypassing effect more.

    I think the designer at TI had though long enough why the switching frequency of TPS54218 can be upto 2MHz, if there is no capacitor with self frequency over 2MHz.

    This is similar to make a car which has maximum speed 500km/hours but you can get no tire/tyre for this speed.

    regards

    Bishnu

  • 0 in reply to Bishnu Ban
    TI__Guru**** 191445 points

    Well, let me just comment, that dc/dc converters are now available with much higher switching frequencies.  4 MHz is not uncommon.