Simple 300W variable output switching regulator

Discussion in 'The Projects Forum' started by El3ctroded, Mar 27, 2008.

  1. El3ctroded

    Thread Starter Active Member

    Feb 4, 2008
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    I want to make a switching regulator based upon the LM723. Yes, it's a dated part, but it's simple and in a DIP and those 2 things are key here.

    Straight out of the datasheet we have the attached circuit. Now, what I want is to alter this in the following ways:
    1) capable of 300W output
    2) use FETs instead of transistors for T1 and T2.
    3) Vin can range from 12 - 30V
    4) Vout = variable from a maximum low of 7V to ~Vin.

    Thanks!
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    You can't say "Simple" and "300 Watts" together in one sentence unless you're talking light bulbs :)

    You do know that if you want to use FETs (specifically, power MOSFETS) that you're either going to have to use a boost circuit to control the gate(s), or use BOTH P-Ch and N-Ch for regulation, right?

    Basically for a power MOSFET, in order to fully turn them ON, you need to get the gate 10V higher than the drain (N-Ch) or 10V lower than the drain (P-Ch).

    N-Ch MOSFETS have a lower Rds(ON) than their P-Ch counterparts. You want to stay N-Ch if you can.

    It sure would be a LOT easier to use a more modern IC, that has things like the gate control boost circuit already integrated.

    Take a look at Linear Technology's LTC1775.
    http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1042,P1724
    Look at the S-package version. Yes, it's smaller than a DIP - like half scale.
    But there are pieces explained in the applications section that will do just what you're asking to do.
     
  3. Pich

    Active Member

    Mar 11, 2008
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    Quote:
    It sure would be a LOT easier to use a more modern IC, that has things like the gate control boost circuit already integrated

    I second that motion
     
  4. El3ctroded

    Thread Starter Active Member

    Feb 4, 2008
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    Ok guys, I'll take a look at the LTC1775. I was hoping this could be quick; laying out a pcb for the ltc1775 isn't going to be quick, cause I'm the kind of person who does it right when I have to pay for and wait for a PCB to arrive.

    The SSOP (Edit: I meant the SO) package is quite manageable though. They also have, right there in the datasheet a 20A circuit, so all I need is to make it adjustable (and find and order all the parts and select substitutes for those I don't have sources for).

    Thanks Both of you.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Here's an excerpt from the LTC1775 datasheet, page 6, last two paragraphs on the left side of the page (and the continuation from the top right) - relevant sections bolded.

    There's two additional tidbits you need to design a 20A variable voltage supply.

    However, be sure to read through the rest of the datasheet. ExtVcc will need some extra handling, since you need to keep it between 5V and 7V. See Fig. 6 on page 14 for a simple solution.

    If you haven't downloaded LT's SwitcherCad yet, you should do so. It'll make testing your design a lot easier, as this regulator is already set up - not that exact circuit, but you can model it to see what it will do.
     
  6. El3ctroded

    Thread Starter Active Member

    Feb 4, 2008
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    Thanks. I just finished creating the part in Eagle. Next step: SwitcherCad. I had downloaded it once before but didn't spend any time with it. Guess it's now time to.

    So, how do I make it run longer? And how do I change the resolution? I want to see what happens 1s out and it only gives me 1.4ms. Then, if I could figure out how to get it to do that it would take forever unless I lowered the # of calculations it preforms.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    On the menu bar, click on Simulate, then "Edit Simulation Command".
    Put in the start & stop times you want it to run, and your maximum timestep.

    It may take a good while for it to chug through it, as it's switching around 200kHz, and so has a lot of calculations to do. Might be a good time to clean out your garage ;)
     
  8. El3ctroded

    Thread Starter Active Member

    Feb 4, 2008
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    Ah, that doesn't exist on the menu after running a simulation. You have to close the plots window and that option shows up again. :rolleyes:
     
  9. El3ctroded

    Thread Starter Active Member

    Feb 4, 2008
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    Would you happen to have a switch component I can use in SwitcherCad? I don't see one in the components...
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    I usually don't bother with manually operated switches in SwitcherCad.
    If you want to open a connection, use the scissors tool to cut a wire.
    You could use a resistor, and set it to a low value for ON and a high value for OFF.
    You could also use the "sw" model, which is a voltage-controlled switch - but then you'll need to add a voltage source to turn it on and off.
     
  11. El3ctroded

    Thread Starter Active Member

    Feb 4, 2008
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    Ok, I'll give the sw a try. What I'm trying to do is switch on an additional load after the regulator reaches a steady state output so I could see the response.
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    I haven't played around with that, and I don't know what it takes to turn "sw" on.

    You might consider using a signal generator that starts out at 0v with a delay to turn it on.
     
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