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UC3854 PFC problems

Discussion in 'Misc Electronics' started by Spirit, Nov 8, 2005.

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  1. Spirit

    Spirit Guest

    I am working on UC3854 PFC. Input is 80-250V, 50Hz, output is 385V dc.
    Everything is made like on 250W pfc preregulator on Unitrode UC3854
    datasheet, except inductor and power supply for UC3854 (for testing I use
    external stabilised 18V dc).

    For inductor I use ETD35G ferrite core with 250 turns of wire. Core
    specifications are on this site:

    Problem is that pfc preregulator works ok without a load and I have 385V on
    output, but when i put any load, even very small load like 20k resistor, I
    get very large input current, like 2A and more, and duty cycle about 90%,
    and very small output voltage, just few volts.

    I checked, and rechecked all components and connections many times but
    everything is like on Unitrode schematic...

    Now I think that problem is in inductor.

    Does anybady have experience with UC3854 inductors?

    I am very interesting in if that inductor is very critical or pfc can work
    with several inductors with more or less worse performance?
  2. Shouldn't there be a lot of smoke?

    Graham Holloway
  3. Spirit

    Spirit Guest

    Boost diode is ok polarised.

    I think inductor is the problem.

    What kind of ferite corres are suggested for this type of application?
    Torus, EE,...?
  4. CWatters

    CWatters Guest

    Faulty/wrong capacitor?

    Not Low ESR type?
  5. Phil Allison

    Phil Allison Guest


    ** Did you create an air gap ?

    ........ Phil
  6. Spirit

    Spirit Guest

    Yes I tryed with varyous and without air gap, but always the same :-(

    For protection I use 10 ohm resistor in serie with input AC voltage.

    When I connect UC3854 to 18V, and disconnect input voltage of power part,
    duty cycle is about 90%

    Then when I connect 60V to input, without a load, duty cycle is about 70%,
    and I get 380V at output, that is ok.

    When I connect load ( 5.6k ohm resistor) on output, duty cycle is about 90%,
    output voltage drops to cca 100V and mosfet comes very hot.

    In the moment when I disconnect load, duty cycle is about 5% for the moment,
    and then goes to 80% without load.

    I tyed with one coil, with another coil, and with 2 coils in serie, and
    always the same thing.

    I really don't know where can be the problem...:-(
  7. Without any load, and a DC input voltage, after the start up transient the
    output voltage should be correct, and the duty cycle zero. Because of some
    losses (resistor divider for measuring the output voltage, ...), the duty
    cycle will be very low.

    Did you made any calculations on the value of the coil?? saturation
    current?? Duty cycles that should be normal?? Continiuous/discontinous boost

    PFC's are generally not the most easy things to design, if you want to go on
    with the project, some guidelines (for these, an oscilloscope is needed.
    Without oscilloscope, I give you very little chance to make it work

    - start from scratch with verifying basic UC3854 operation on your board :
    1. oscillator running correct frequency

    2. try a circuit that you can generate variable duty cycles at
    the output with a variable resistor --> you can use the current error
    amplifier with a simple feedback resistor in a basic opamp amplifier. This
    sets the level where the saw tooth from the oscillator is compared, and
    generates a certain duty cycle at the output.

    - when you can generate veriable duty cycles, you can start playing around
    with the power stage. Use only DC input at the moment.

    1. apply small input voltage (from a current limited power supply),
    voltage with a certain duty cycle (use also this duty cycle to start
    testing), the output voltage will be equal or less than the calculated
    voltage (it will be less if the boost converter works in discontinous mode).

    2. Monitor the solenoid current!! Put a small ressitor in series
    with the soleoid (for example 1 Ohm for first low power tests, later on 0.1
    Ohm or less). The resistor must have LOW PARASITIC IMPEDANCE otherwise, the
    voltage measured on it will not be equal to the actual current in the coil
    (it will show steps).

    3. play around a bit with the duty cycle, load resistor and input

    4. evaluate the inducatnce and saturation of the inductor used. The
    inductance can be calculated measuring the slope of the current and the
    voltage across the inductor. Saturation is the point where the current
    starts to rise faster than linear. The soleoid should always be used below

    5. At the point where you reached the full input voltage and load
    conditions ( = output voltage set correctly with the potentiometer, and full
    load applied), evaluate output ripple, thermal behaviour of the MOSFET,
    solenoid, ... Also some early measurements about efficiency can be done.

    When the tests above are finished with success, a first milestone is
    reached. Remember you are still working in a DC environment, not with AC
    input, so the final result will be different (according to efficiency,
    heating, output ripple ...)

    - now it is time for the control loop. This is not a piece of cake. The
    control loop settings depends highly on the inductor value and output
    capacitance. This chip uses current mode control, wich is a big advantage
    for boost converters. Some interesting documentation is written by THE man
    of switching power supply control loops : Lloyd Dixon (the unitrode days).

    There should be a document available for this control loop type used in PFC
    devices, but I can not find it right away.

    Have fun.
  8. Mark

    Mark Guest

    I agree you need a scope and a current probe to even begin thinking
    about troubleshooting these.

    Also lok at the layout re grounding,,, a small amount of ground
    coupling from the power circuits back into the control circuits can
    casue unpredicable behavior. Use single point grounding for the power
    and another single point ground for the low level stuff.

  9. Indeed, grounding can be very imortand. Also when doing measurements, don't
    be supprised to see a lot of switching noise.
    I'm always wondering how they generate the nice and clean waveforms in
    datasheets or application notes, ...
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