Current Mode PWM Controller Help

Discussion in 'The Projects Forum' started by gdelarge, Feb 18, 2011.

  1. gdelarge

    Thread Starter New Member

    Feb 14, 2011
    3
    0
    I am working on a flyback DC-DC converter and I am trying to simulate a Texas Instruments UC3845 chip in PSpice. I can't find the PSpice model for that chip anywhere, so I found a current mode PWM controller made by linear technology, LT1619.

    Has anyone worked with these chips before?

    Here are links the data sheets:

    http://www.linear.com/product/LT1619#simulateSection

    http://focus.ti.com/lit/ds/slus223c/slus223c.pdf

    Does anyone know what the Rt/Ct pin on the TI chip is?

    Basically I am trying to figure out how to pins on the TI chip correspond to the LT chip.

    If anyone could provide some insight it would be greatly appreciated. This project is for my senior capstone design project at Embry-Riddle Aeronautical University. I am part of the power team and we are trying to provide a power system for a satellite that will hopefully launch some NASA sponsored projects.
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    The TI chip uses external timing components Rt and Ct to complete its oscillator. The way these components can be chosen to set the oscillator frequency is shown on page 6 of the TI datasheet.

    The LT chip uses an internal fixed frequency 300kHz oscillator. http://cds.linear.com/docs/Datasheet/1619fa.pdf

    As far as I can tell, these ICs are not directly equivalent.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    If you do a Google search for "UC1845 spice", I'm sure you will have a lot more luck in finding a spice model.

    Instead of the UC3845, you should be using the UC1845-SP, which is qualified for space.
     
  4. gdelarge

    Thread Starter New Member

    Feb 14, 2011
    3
    0
    Our test prototype doesn't have to space rated but thank you for the suggestion, I will look into that chip.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Without going into lots of details, your supply will need to be as efficient as possible. It's not just power consumption, it's also the dissipation of power within the converter itself. If you can start with a design where you can achieve the highest efficiency possible, you will reduce your cooling and input power requirements. If you find yourself at under 90% efficiency, you'll need to work harder on it.

    Cooling is a big concern, as there are not a lot of ways to get rid of heat generated within a spacecraft. Heat pipes are quite frequently used, as they operate from the waste heat itself with no external power required.

    I'm attaching a .pdf file from TI on a synchronous rectified bridge DC-DC converter that's 92% efficient as food for thought. You will have a very difficult time trying to approach that with the UCx845.

    Meanwhile, no mention has been made of your input power's voltage, wattage limit, output power requirements, etc.
     
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