SMPS with variable output voltage design issues

Discussion in 'The Projects Forum' started by d183, Mar 23, 2009.

  1. d183

    Thread Starter New Member

    Mar 23, 2009
    Hi all, I'm new to this forum and so far looks like a great community.

    Here's my constraints:
    I want to design a SMPS with the following parameters:
    Vin = 24VDC
    Vout = 2 to 22 VDC (continuously adjustable via pot)
    Load: A mechanical oscillating relay. It will draw anywhere from 0.25A - 2A peak in an approximate square wave (0A being the lowest value) at a frequency of about 80-250Hz.

    Here's what I've done so far:
    I've looked through for ICs that will offer something along these lines and I've decided on using either the LM2576 or the LM2596 since they offer good efficiency and simple design. I have ordered the LM2596 since it's more efficient and will reduce part size slightly with the higher switching frequency. Using the design guide in the data sheets I've found that there is a large range of inductor sizes given the operating range of my design. I hoping that a potential divider with a potentiometer to the feedback pin will offer a variable output voltage. I will bipass the pot with a capacitor to reduce noise.

    Here are my questions:
    With such a large output voltage and output current design I've had some trouble choosing inductor and output capacitor values. I imagine that I will want to be running in continuous mode for as much of the time as possible.

    - How can I find the ideal inductor given the large range of operation?
    - Should I choose the largest inductor that the design parameters offer (IE design for continuous mode for worst case scenerio)?
    - Are there drawbacks to using a large inductor under low or high loading (load current will switch to 0, or sometimes full load will be around 2A and output voltage will vary greatly)?
    - How would I match an output capacitor?
    - Is the potential divider with the potentiometer adequate to offer a variable output power supply?

    Thanks everyone and I hope that you can shed some light on these issues. I'm still new to SMPS, so forgive my greenness.