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SMPS output impedance

Rob D'Entremont
Prodigy 130 points

Other Parts Discussed in Thread: TPS55340

Hi,

With a Webench solution using the TPS55340 in boost configuration with a Vin range of 12 - 24VDC (18V nom.) and 36V output @ 500ma. I assume the output impedance is determined mostly by the output capacitor. How could I further increase the output impedance to aprox. 60K @ 1KHz? Simply adding a series inductor to the output would appear to require a large 6-7H inductor.

The reason for the high impedance is so to supply a network with a DC voltage on which devices can inject a low frequency (500Hz - 20KHz) AC signal.

Regards,

Rob

  • 0
    TI__Intellectual 1370 points

    Hi Rob,

    Thank you for using WEBENCH for Power designs. TPS55340 has user selectable frequency between 100k - 1.2MHz. Are you sure, you are wanting to design for 1kHz?

    In general you are right about Output impedance coming from a combination of Cout and Rload.

    For your case of 12-24V @ 36V &0.5A. Rl=72 Ohms, Cout is 2.2e-6 with an ESR of 2.8mOhms. I designed at 1MHz.

    So Zout = sqrt(Rl*Rl + w*w*Cout*Cout). To be more accurate, you can calculate impedance from capacitor so that it includes ESR.

    Not taking ESR into account, and fixing freq at 1KHz with the current webench design, Cout has to be reduced to somewhere around 2.65nF. Let me confirm if it makes sense to do this with product line folks as well.

    Meanwhile you can try edit design feature in WEBENCH to use a 2.5nF capacitance at Cout and try simulations.

    Regards,

    Pavani Jella

    WEBENCH Applications

  • 0 in reply to Pavani Jella
    Prodigy 130 points

    Hi Pavani,

    Thank you for your reply. My apologies for not making myself clear.

    I am not looking to change the design of the  power supply itself but rather add an output stage that has a low DC impedance and a high AC impedance (aprox. 60K @ 1KHZ) as seen from the load.

    The load in this case is many devices on what is essentially a 1-wire bus. The power supply needs to supply 36VDC onto this bus with as little voltage drop as possible at the rated current of 500ma. These devices will then communicate on this bus by superimposing a 5Vp-p AC signal onto the 36VDC. The bit rate of these devices will be in the range of 500Hz to 5KHz.

    The simplest approach would be to add an inductor in series with the power supply but at these frequencies the inductor would be a rather large and expensive 6H.

    Although this is not directly related to the TPS55340 I was just looking for advice as to another approach. Perhaps an active solution such as a simulated inductor but with the higher current capability?

    Regards,

    Rob

  • 0 in reply to Rob D'Entremont
    TI__Mastermind 35675 points

    Hi Rob,

    Thanks for the details you've given on this design.

    This is quite a unique requirement that I haven't seen before. Do you happen to have an example of a design which already does this or maybe an article explaining this further? I'd like to learn a little more because most designs try to get as low of an impedance as possible instead for low ripple and better transient response.

    Best Regards,
    Anthony