Help with LT1339 test circuit

Discussion in 'The Projects Forum' started by shaunAES, Jan 26, 2011.

  1. shaunAES

    Thread Starter New Member

    Jan 26, 2011
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    0
    Hi

    I need help with a Linear Technology LT1339CNPBF test circuit. I have built the circuit but used similar N Channel MOSFETS to the ones on the test circuit because the mosfets on the test circuit are obsolete. My circuit does not switch the MOSFETS ON AND OFF. Can you please help me get the circuit working. I used the IRFIZ34G MOSFET instead of the Si7884DP MOSFET that was supposed to be used in the circuit. Attached is the circuit diagram.

    Thanks.
     
  2. Gundalf

    New Member

    Jun 22, 2010
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    Are you driving the new FET with sufficient voltage? The IRZ line have a fairly high capacitance and need a reasonable Vgs to switch on.
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    As a suggestion, you would get more members looking if your schematic were in a graphics format, preferably PNG. Zip files can have extra content.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    You posted the Linear Technology test model except you've changed V2 to 12v from 28v, and R2 to 5k from the original 3k. The resistor change would make the output 7.44v instead of 4.96v. Is this what you intended? The MOSFETs in the .bmp are still the originals.

    I had an IRFZ34n model that I used to run the simulation with, along with your changes, and it worked, but the output current (30A) due to the .25 Ohm resistor is simply far too much for the IRFZ34n, which is rated for 29A maximum. Power dissipation in the upper MOSFET is 24 Watts, which means a melt-down.

    [eta] I then changed the MOSFETs to IRF1404's (these are in the supplied library), and the power dissipation in the upper MOSFET went down to 1.7W, the lower 1.54W; this is acceptable.

    What output voltage and current were you really expecting this circuit to output?

    Instead of zipping up .bmp files, it's preferable to create .png files. .png files are far more compact, are not lossy like .jpg files are, and need no other software to view.
     
    Last edited: Jan 26, 2011
  5. SgtWookie

    Expert

    Jul 17, 2007
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    I've made a couple more minor changes from the last one; added in provisions for calculating efficiency, saved it with the IRF1404's, removed V2 and sourced M1's drain current from V1, and changed Rload to be a current source so that efficiency can be calculated automatically by LTSpice (the program requires ONE voltage source and ONE current source in order to calculate efficiency).

    Many of the components are still ideal models (particularly the inductor and the capacitors) so until "real" models are used, the efficiency calculation won't be very accurate. The LTSpice library does not include a 10uH inductor that is capable of 30A current. Further, modeling saturation of an inductor is somewhat complex. Google "SPICE inductor saturation" for lots of documentation.
     
  6. shaunAES

    Thread Starter New Member

    Jan 26, 2011
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    0
    I attached a .png file. Thanks for your help SgtWookie. I will try what you have done on your circuit. I need an output of 3.3V. LT1339 test circuit.PNG
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    I have already uploaded the modified LTSpice circuit in my previous post.
    To get 3.3v out:
    Change R2 to 3.3k, and change R3 to 2k.

    This works, because the LT1339's FB input wants to see 1.24v; it adjusts the output current to meet that goal.
    1.24v/2k Ohms = 0.62mA.
    0.62mA * (2k+3.3k) = 0.62 * 5.3k = 3.286 Volts. This is 99.6% of 3.3v.
     
    Last edited: Jan 27, 2011
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