Ideas for 120VAC -> 40VDC @ 700mA transformer given limited volume

Discussion in 'The Projects Forum' started by torinwalker, May 30, 2007.

  1. torinwalker

    Thread Starter New Member

    Jan 20, 2005

    I'm in the process of researching (a design for) a transformer that must fit insize a cylindrical volume measuring 1" diam by 1.75" long. I thought I would pass it by this forum because so many of you have experience far beyond me; I thought you might come up with some clever ideas I haven't thought of.

    The supply must convert 120VAC mains to 40VDC, and carry 700mA (minimum) ~ 1A (maximum).

    I am exploring three avenues:

    1. Transformerless - Though the 15uF cap is large, measuring .9" x 1.84", it is well within the limits of the container. I can make this work, but I don't know that I need such a large capacitor, since:

    700mA @ 120V (I = 2 x pi x C x f x Vrms)

    is not the same as 700mA @ 40V. Or is it? I don't know, since I'm getting the voltage across a shunt zener diode. I figured since I'm only using 40V, the power is less for the same current. My gut tells me it's not the power, but the current I should worry about, in which case, the cap value is correct. Comments, please.

    Please do not bug me about the implications of transformerless supplies unless it pertains to the question. The design will be well protected against electrocution. Thank you.

    2. Linear supply - I figured I might be able to get a cylindrical ferrite rod and fabricate a step-down transformer. Size is the big question. If someone has done this before, I'd be interested to hear their opinion.

    3. Switching supply. I'm not at all versed in SMPS theory, but I can imagine that I may be able to reduce the size significantly since I don't have to compenstate so much for variables of input and output that are normally considered in commercial supplies. My input voltage is fixed, and my load is fixed. Again, comments on this matter are appreciated.

    4. Some other idea that I haven't thought of, but you have. Please tell!

    One more note on the transformerless design. I've been simulating multiple loads in series (e.g. If each load normally drops 10V, then I put four of them in series, each with with their own parallel 10V zener), but the simulation is wonky.

    Can you think of any reason why four 10V zeners (in parallel with their loads), in series aren't the same as one 40V zener with four loads in series?

  2. beenthere

    Retired Moderator

    Apr 20, 2004
    1. The idea of limiting the voltage and passing a amp of current through a capacitor makes me cringe. I doubt the capacitor alone could fit in that volume. Not to mention the rectifier diode, filter capacitor and zener diode capable of dissipating 40 watts (where does the heat sink go?).

    2. You might have some luck with a hand wound transformer, assiming you have space for the rectifiers, filter capacitor, and regulator circuit.

    3. While switchers don't have a bulky linear transformer, the have to rectify the line, store the charge on a filter cap, run a FET to drive the high frequency transformer, rectify the low voltage, filter the output, and have a means of sending an error signal to the primary side FET driving circuit for regulation. Don't think you can fold all that up enough to fit.

    4. External power supply.

    You state that the load varies. The zener must have a limiting resistor that will allow the full load current to pass. With the lower load, the zener must be able to dissipate the increased current flowing throug it instead of the load. Zeners makes pretty poor regulators. A three terminal regulator like the 78xx series is far superior, but may need large heat sinks.
  3. cumesoftware

    Senior Member

    Apr 27, 2007
    I think a 1" diam and 1.75" long is a small space for a 60VA transformer (using rought calculations since it depends on how the rectification is made). So a linear design won't satisfy your requesits.

    My best bet is on switching power supplies, since they dissipate less power, thus can occupy a smaller size.
  4. John Luciani

    Active Member

    Apr 3, 2007
    External Supply.

    A 1.4 in^3 40W supply from an 120VAC input is going to be extremely difficult
    (probably not possible).

    1. In your transformer-less you will have a diode and a cap so you will get around
    180VDC. You want 40V at 1A. Some element in the supply will need to dissipate
    a fair amount of power. 140W at the peak of the sine wave.
    2. The linear transformer will be too big.
    3. SMPS is the only option. You still need to get from AC to DC which will
    consume all of your space with hold-up caps. If you get a very wide range
    DC-DC converter you could reduce your capacitor size but it probably will not
    be enough.

    (* jcl *)
  5. Gadget

    Distinguished Member

    Jan 10, 2006
    I can't see any way to fit ANY type of supply into the space you want to occupy.
  6. torinwalker

    Thread Starter New Member

    Jan 20, 2005
    A few points to clarify:

    1. The load will be fixed absolutely, never changing.
    2. External supply, though a good idea, is out of the question. It must be contained in the volume specified. Imagine that this device will be plugged directly into a wall-socket.

    Do you think the SMPS can be made smaller if the load is fixed?
  7. John Luciani

    Active Member

    Apr 3, 2007
    No. The SMPS can be made smaller if you can reduce the power that you require
    or the voltage tolerance that you can withstand.

    Checkout for some SMPS controller
    ideas. Unfortunately the TOPSwitch part only switches in 100KHz range which
    will require larger inductors and capacitors.

    A couple of other parts to checkout ---

    Maxim MAX5052, On-Semi MC34023 and NCP1205, Fairchild FS8S0765RC and

    (* jcl *)