Single Transistor Forward Converter Help

Discussion in 'The Projects Forum' started by steve80289, Mar 10, 2012.

  1. steve80289

    Thread Starter New Member

    Mar 10, 2012
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    I am currently working on a project where I am Designing a single transistor forward converter in class.

    Project as of now:

    120VAC rectified (to ~168VDC) then dropped to 5VDC with a max current of 1A via forward converter

    I have a concern that I am going to need electrical isolation for my micro-controller that I am using to control the duty cycle of the converter (I decided to use the boarduino with the Atmega 328p chip). I am not sure of the best way to isolate the micro-controller though. I have seen on mouser some small voltage isolation transformers that seem like they may be promising for such a task, but I would like a second opinion because none of my professors know much about it and I would like to get this project done ASAP. Here is a link to the small voltage transformer I was talking about:

    http://www.murata-ps.com/data/magnetics/kmp_78253j.pdf

    I also wanted to know if there was a good place that sold pre-made three winding toroids for a forward converter. I am winding my own at the moment, but I would like a backup plan if mine does not work for whatever reason it may be.

    Please let me know if you have any other advice for a young Engineer.
    Thanks in advance,
    -Steve
     
  2. panic mode

    Senior Member

    Oct 10, 2011
    1,319
    304
    just to clarify, you want to use MCU (arduino or whatever) to control power transistor to step down 170VDC to 5VDC?

    i see couple problems with that, isolating MCU is not one (you can use optocouplers or small transformer for example) but...

    here we have a young engineer who is in a hurry and dealing with relatively high DC voltage that is not current limited and also not isolated from mains. You may proceed but please be careful. Also make sure your capacitors are rated for this or you may have if explode into your face (they do a lot more than little "pop").

    if you are taking on this project on your own, good idea is to take a look at switching power supplies such as

    http://www.pavouk.org/hw/en_atxps.html

    you can see general layout and isolation between stages. you can also post your circuit for suggestions.
     
  3. steve80289

    Thread Starter New Member

    Mar 10, 2012
    6
    0
    Yes, I will be using an MCU for this project at 62.5kHz switching frequency

    Yeah I understand what you mean by "little more than a pop." It has not happened to be because I have careful to check voltage ratings of every single component I order. Thanks for the concern though. What would be a good way to limit the current?

    Thanks for the source, I will take a look. I will post the circuit as soon as I draw it up.
     
  4. crutschow

    Expert

    Mar 14, 2008
    12,991
    3,227
    It's best to use an isolation transformer when working from the mains. At a minimum use a ground fault interrupter when connecting to the mains.
     
  5. steve80289

    Thread Starter New Member

    Mar 10, 2012
    6
    0
    I assume you mean something such as this:
    http://www.hammondmfg.com/pdf/5c0069.pdf

    The circuit I have attached is basically what I have so far (basically just the textbook single transistor forward converter). I will be adding in the isolation from the 120VAC as recommended and will be looking into things deeper when I get back home. now that I think about it I forgot to put the inductor into the secondary side of the circuit schematic. Like I said, I have not had much guidance on this project, and so I have done things according to what I have read in my textbooks.
     
  6. steve80289

    Thread Starter New Member

    Mar 10, 2012
    6
    0
    I am having trouble getting the circuit working still. I used the same single transistor circuit but used a 4.7M ohm resistor to use as a load to troubleshoot the transistor. I figured out that the transistor was not working and replaced it. When I took out the resistor and put in the toroidal transformer with only the primary winding. The surge resistor burned up quickly when I turned the circuit on. Can someone explain why this happened? I assume that it has to do with the current going to the circuit.
     
  7. steve80289

    Thread Starter New Member

    Mar 10, 2012
    6
    0
    This is a picture of the circuit that I am using. I know I should be isolating the mains and I will at some point, but for now this is what I have. The surge resistor I was using burned up when I replaced the 4.7 Mohm resistor with the transformer (as in the pic). I can tell that there is too much current as evidence of that, but I am not sure what I should do to fix this problem. Any ideas? Thanks in advance.

    I am using a variac for isolation at the moment BTW
     
    Last edited: Mar 20, 2012
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