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Switched power supplies - 0V with load.

Discussion in 'Power Electronics' started by JohnJohnCat, Jan 10, 2019 at 8:20 PM.

  1. JohnJohnCat


    Dear all,
    I am facing a strange case that hope you can give me some keys.

    When I use a linear power supply, all is ok.
    But when I use up to 4x different commercial adapter-type power supplies, the voltage I read on the terminals of the load is zero. These are the nominal values of them: PSU1: 5V&4A, PSU2: 12V&1A, PSU3: 24V&2A, PSU4: 5V&2A. I do work with them normally so I guess the problem is on the load. There is not a shortircuit, the load seems reasonable for the output power (very reasonable,...)

    The load is an output stage of a JFET transistor for a UHF amplifier. Regardless of its state (OFF, LINEAR, SAT), I think I should read the nominal value of the PSUi on the terminals but this does not happen.

    As attachment, there is a picture detail of the PCB and the behaviour of one PSU. Should anyone have some interest in checking the others just let me know but are "similar" waveforms, dropping to zero.

    Viaje Corriente Estado JFET-Desconocido_Detail.png


    Thanks for your support.
    Receive best regards.

    [Mod edit: tidied up picture mess]
  2. davenn

    davenn Moderator

    Sep 5, 2009

    Many switching PSU's require a minimum load ( current being drawn) before they will "come to life"

    OFF position wouldn't do that
    Linear position wouldn't either ( if you are referring to a linear amplifier is. class AB next to no current drawn till there is RF applied to the input and the amp is actually operating
    SAT ?? what is that ? Satellite ( doesn't make sense ?) Saturation ? doesn't really make sense either
    Maybe it is supposed to be Class C mode ?

    SMPS for transmitters are generally specifically designed for the job to overcome the
    zero load issue

  3. JohnJohnCat


    Dear Dave.

    Thanks for your reply.

    I have also thought about this possibility, but did not guess it could affect. With the linear PSU i read a demand of ~30mA (by memory).

    I meant SATuration, one of the possible states of the JFET.
    In this circuit, I expect Vds=12Vdc working in the linear zone. But it is the point that I am adjuting right now.
    Entering in the power amplifier classes issues, it is a Class A-type.

    These PSU are commercial ones. I think not much adjusted for low demand of power. It was curious any of the tested four probablisty speaking could help. My objective is the RF circuit, not the PSU.

    I will post the results, after increasing the load with some resistor.

    Thank you.
  4. davenn

    davenn Moderator

    Sep 5, 2009
    for some types of SMPS .... that will not be enough

    that will decrease the load .... less resistance = higher load
    BobK likes this.
  5. BobK


    Jan 5, 2010
    Unless ot is put in parallel.
    davenn likes this.
  6. davenn

    davenn Moderator

    Sep 5, 2009
    going by his posts, I doubt he understands that
    He appeared to equate increasing load with increasing resistance ;)
  7. JohnJohnCat


    Hello all.

    As promised, here I com with the results.

    I have increased the load (of the power supply) by adding some resistors in paralel at the input of the DC feeding connectors.
    Again, the same results. Two different PSU have been used.

    The funny thing is that these switched PSU can feed another device with less Amps demand. With my DUT, the linear PSU indicates now a demand of [email protected] and for this other device, [email protected] So I have been able to increase the power demand of the swithed PSUs. Attached the behaviour of them, FYR.

    I start thinking that I need to have also in consideration the impedance and not only the resistance of my circuit.
    I will also double check all the solderings.

    Do you have any other constructive advise?

    Thanks and regards.

    Attached Files:

  8. davenn

    davenn Moderator

    Sep 5, 2009

    well it's a DC supply so impedance is irrelevant .... resistance is the thing :)

    OK, lets go back to the start ....
    1) you haven't stated the make or model of any of these supplies
    2) you haven't stated if any of these supplies have worked in the past and it is a problem with a newly purchased unit ( same make and model)
    3) have you actually got / looked for datasheets for the supplies ?
    4) I still suspect a possibility of not enough load to switch the supplies on ... they may require much more than your 130 - 160mA ?
    Again check the datasheets for the PSU's
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