Boost converter problem

Discussion in 'The Projects Forum' started by adeel abid, Sep 6, 2012.

  1. adeel abid

    Thread Starter New Member

    Sep 6, 2012
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    Dear All,

    i have designed a boost converter. It is based on IRF740. Driving gate signal is generated by microcontroller which is then pulled up with !k resistor to 12V. So Vgs is either 12V or 0V with ton=190us and min. toff=10us giving about 93.3% duty cycle.The gate driving signal is not continuous but actually based upon monostable multivibrator that is only trigerred if load voltage is less than desired value & simultanoeusly indcutor current is reduced to zero (this is sensed by sensing voltage across 1ohm resistor).
    Problem i am facing is that when MOSFET is on for 190us as per ideal calculation id should be ramp with ID(max)=8A. During this phase only the battery, inductor and IRF740 is in the circuit. But practically after implementation i am getting ID(max)=3.5 to 4A (measured by sensing peak voltage acorss 1 ohm resistor).
    There is not so much heating of IRF740 to increase RDs(on) to limit the drain current to this value because i have tested it with no load i.e. without 500 ohm resistance (and hence multivibrator pulse repetition period is quite large to allow cooling between generation of pulses). This is limiting the input power of boost converter and hence the output power delivered to the load.
    please provide reason for this behaviour.
    Thanks in advance.
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,052
    3,244
    How did you determine that it should ramp with ID(max) = 8A? When you calculated that ramp did you include the effect of the 1 ohm resistor?
     
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    That's pretty weak gate drive for a power FET and will give very poor switching transitions.
     
  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    Put a scope on the source (battery) voltage it is probably drooping under heavy current load.

    Switchers need large value, very low ESR storage caps to deliver switching current when the switch snaps on.
     
  5. adeel abid

    Thread Starter New Member

    Sep 6, 2012
    12
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    1 ohm resistor is not in the circuit when transistor is on. however peak inductor current is same whether transsitor is on or off.

    v=L di/dt

    12=300u*i/190u gives i=8a(approx)
     
  6. adeel abid

    Thread Starter New Member

    Sep 6, 2012
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    in prcatical implementation i have used 470uf 50v polar capacitor. voltage drooping is observed to about 0.2v peak near the terminal of inductor connectd to the battery.

    Any suggession to reduce it further?

    Thanks in advance
     
  7. adeel abid

    Thread Starter New Member

    Sep 6, 2012
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    i have checked the voltage waveform Vds by connecting the 12V 3W bulb as load for transistor and its timing waveform is not much deviated. so i can assume that this may not be a dominant source of problem.

    please advise

    Thanks
     
  8. crutschow

    Expert

    Mar 14, 2008
    13,052
    3,244
    What is the resistance of the inductor?
     
  9. adeel abid

    Thread Starter New Member

    Sep 6, 2012
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    multimeter has shown it to be 0 to 0.1 ohms
     
  10. adeel abid

    Thread Starter New Member

    Sep 6, 2012
    12
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    please find the attached real oscilloscope picture.

    upper trace is of gate drive signal and lower trace is of voltage across 1ohm resistance.

    may be this can help you to find the real source of problem

    thanks
     
  11. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    Given the law of V = L di/dt

    If you are getting the wrong inductor current, it follows one or more of these things must be true:

    1) error in measurement of current

    2) Inductance value is not what you think it is

    3) the voltage being forced across the inductor is not what you think it is

    I suspect the inductance is higher than you think. Inductor values are rated at a specific value of current, at lower current values the inductance is higher.

    BTW: a circuit similar to this is actually what we used to measure true inductance since rated values are not always accurate and it varies with current.
     
  12. vegeta

    New Member

    Sep 7, 2012
    6
    0
    Are you sure the source is capable of providing the high peak currents you are expecting. A scope capture of the source voltage and the inductor current for a few switching cycles would be very useful.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    1N5404 diodes are relatively slow to turn off (Trr), and thus should not be used in a switching supply. I suggest that you use Schottky diodes instead.

    Here is a comparison chart for various types of diodes:
    [​IMG]
     
  14. adeel abid

    Thread Starter New Member

    Sep 6, 2012
    12
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    please find the snaps for inductor voltage(1A per div measured with 0.1ohm in series with inductor) and inductor current waveform.

    please advise

    Thanks in advance
     
  15. adeel abid

    Thread Starter New Member

    Sep 6, 2012
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    i have measured the inductance value with LCR meter and found it to be 300uH. so i think its value is correct.
    please correct if any special condition exists in which this measurement could be wrong

    thanks in advance
     
  16. adeel abid

    Thread Starter New Member

    Sep 6, 2012
    12
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    please advice if this phenomena is visible in the waveforms attached in previous post and tell me how it can create the problem of low peak current.

    thanks in advance
     
  17. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    If the 1 Ohm resistor is in series with the FET, it may be affecting the circuit. The voltage drop wasted across the resistor reduces the Vdrop across the inductor which reduces the di/dt rate.

    You should use a current loop and a current probe to measure the current.
     
    Last edited: Sep 30, 2012
  18. adeel abid

    Thread Starter New Member

    Sep 6, 2012
    12
    0
    1 ohm resistor is not in series with FET as can be seen from my first post. during turn on only battery, FET and inductor are in series.

    Please find the attached pictures of cores i am using to design inductor for boost converter. Please advice about its specs, material. whether it can give incorrect reading with my LCR meter its inductance increases as current through it increases? and whether it is suitable for boost converter application with Imax=8A, ton=190us and toff>=10us. if not suitable then which core should be used, advice about its physical appearence and other details. i am in doubt whether i am using the correct core for this application. color of core is green/blue ans yellow/white

    thanks in advance
     
  19. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    You can't spec a core by it's color, you have to know the manufacturer's part number. It will have a spec sheet or graph showing it's key parameters. I don't even know what core material those are. Ferrite and powdered iron are the most common, but there are many other "trick" materials out there as well.
     
    Last edited: Oct 5, 2012
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