Switch-Mode Power Supply

Discussion in 'The Projects Forum' started by Andy Larson, Apr 18, 2015.

  1. Andy Larson

    Thread Starter Member

    Mar 26, 2015
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    I had posted a thread pertaining to a linear PSU a little bit ago. One came forward and said that i should make a Switch-Mode Power Supply. I just want confirmation that I have correct info on what I need. This is a hobby project btw.

    Circuits I need:
    1. Power Factor Correction
    2. Insulated Gate Bipolar Transistor
    3. PWM Controller
    4. Isolated error Amplifier
    5. Output rectifier
    6. Input and Output Filter
    7. Optocoupler
    Specs I'm aiming for:
    adjustable 1-30v at 1.5A DC <5% ripple
    Current adjustment would be nice but isn't needed
    Budget:
    $300 max

    As a side note, I'm a highschool student with intermediate electronics and electrical knowledge. I do have basic tools and a soldering station. I do know how to solder and test for voltage, amps, resistance via a multimeter I own.
     
  2. Reloadron

    Active Member

    Jan 15, 2015
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    Hi Andy

    Looking at your requirements:
    Adjustable 1-30v at 1.5A DC <5% ripple

    If you can live with a low end of 1.2 volts I would not bother trying to build a SMPS but just build a very simple Linear Power Supply around a LM317 Regulator. The data sheep provides several simple circuit examples. You should be able to find a line stepdown transformer easy enough that will take mains down to around 30 Volts and a few rectifiers to comprise a full wave bridge. For example TRANSFORMER, LOW POWER, 117V, 50/60HZ, 28.0VCT @ 2.000A, LEADS. That gets you a transformer rated for 2 Amps. Any number of simple bridge rectifiers can be used, they typically cost about $4.00 USD. You are looking at under $50 USD and if you want to be cool you can snag a nice little current / voltage meter on Ebay for literally a few bucks to compliment the project.
    Seriously since you only want a few amps of adjustable supply in a 1 to 30 volt range I would go Linear and use a LM317 based design. For your needs I haven't any idea why building a SMPS from scratch was suggested, I am sure there was a reason, I just don't know what it was. If you still want to go the SMPS route have at it and hopefully someone will be along with some suggestions. Building a SMPS from scratch is not for the faint of heart. :)

    Example of current / voltage meter from Amazon for literally $12 USD.

    Ron
     
    Last edited: Apr 18, 2015
  3. Andy Larson

    Thread Starter Member

    Mar 26, 2015
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    I meant it would adjust between 1v and 30v via a pot
     
  4. Reloadron

    Active Member

    Jan 15, 2015
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    Yes, I understand that, which is why I suggested a LM317. I asked if for the low end you could live with 1.2 rather than 1.0 volts. A simple LM317 circuit as shown in the LM317 daa sheet I linked to has a low end output of 1.2 volts verse the 1.0 you mention. The LM317 is an adjustable regulator between 1.2 and 30 volts which will handle the 1.5 amps you mentioned.

    Ron
     
  5. Andy Larson

    Thread Starter Member

    Mar 26, 2015
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    I was told that deduction from 30v (was going to use a 30V transformer) would melt the Lm317. (30v-Vout) * 1.5A
     
  6. tcmtech

    Well-Known Member

    Nov 4, 2013
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    At a peak of ~45 watts PF correction is not really necessary and mostly just a waste of parts and money with no real gain to show for it.

    As for the 1 - 30 VDC @ up to 1.5 amps to be honest for a $300 budget you would be money and quality ahead to buy one unless this is primarily for the learning experience.

    Now as for a linear power supply given your budget you could go with a basic two stage system where you use a variac for the primary voltage regulation followed by a step down transformer followed by a basic linear linear circuit.

    Given a basic 100 VA variac driving a 100 VA 26 - 30 VAC transformer you could get an initial open circuit voltage of around 35 - 40 VDC at which dong a linear regulation down to ~30 VDC is not such a hard drop to handle with basic linear regulator components plus with 100 VA sized transformers you would have a roughly 2.5 - 3 amp continuous working capacity and being all the primary power is transformer based you would also have substantial short term safe overload capacity that is very hard to build into a beginners level SMPS.

    Total cost would still fall well under $300 and you would have a pretty robust power supply that would be easy to set up with extra unregulated DC and AC outputs you couldn't get from any SMPS.
     
    Reloadron likes this.
  7. Andy Larson

    Thread Starter Member

    Mar 26, 2015
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    If you looked on my other thread on a DC power supplies. One gentleman claims that I shouldn't do a linear regulator, but go for a smps
     
  8. Reloadron

    Active Member

    Jan 15, 2015
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    You are not going to melt or cook a LM317 regulator. Yes, at higher output voltages the LM317 will have limited current out (you will not get 1.5 Amps) but as the data sheet shows in application examples this is easily overcome. For example the Texas Instruments data sheet Figure 24. High-Current Adjustable Regulator Circuit shows how to work around and get much higher current out. This involves only adding a few simple inexpensive transistors. The LM317 like any regulator requires adequate heat sinking which is not a problem. Even a SMPS design will require the driver transistors have adequate heat sinking.
    The LM317 in a TO3 or TO220 package is hard to kill. It has very strong survival instincts. You can still pursue building a SMPS but building one is not easy, including finding the parts like a transformer. Then parts layout is important also, there is quite a bit to it where a linear supply is simple. Finally another option is simply buy an off the shelf power supply solution. The stuff pouring off the boats from China is very inexpensive compared to what power supplies cost years ago. A Mastech HY3005D Variable Linear Lab Power Supply 0-30V 0-5A can be had through Amazon for under $120 USD.
    A Google of LM317 circuits will get you dozens of LM317 based circuits which will do what you want and need also. Beyond the data sheets.


    Ron
     
  9. Reloadron

    Active Member

    Jan 15, 2015
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    Got a link to the other thread and did he provide his reasoning? See Andy I don't know what was covered in the other thread or what the logic was that dictated SMPS. I personally for your needs and based on your experience would not suggest SMPS but again, that's just me and my thinking.

    I also like tcmtech's thinking as to using a simple Variac.

    Ron
     
  10. Andy Larson

    Thread Starter Member

    Mar 26, 2015
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    http://forum.allaboutcircuits.com/threads/dc-power-supply.109640 my other DC PSU post
     
  11. Papabravo

    Expert

    Feb 24, 2006
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    What was suggested also in the other thread was a transformer with multiple taps to reduce the input voltage to the LM317 so it and any bypass components should not have so much work to do. To use any of the other techniques mentioned would requires some design assistance for a beginner. If somebody want's to do this for him fine and dandy. If he wants to scour datasheets and application notes that's also fine. But I think is disingenuous to tell him he wont have problems with power dissipation and not give him an example of a circuit that does just that. I have no doubt that others can come up with such circuits, but can he quickly find the answers he seeks? I suggested he might achieve his goal quicker if he lowers his expectations to something more manageable to get his feet wet, before swinging for the fences in his first at bat. The point is to do something and learn something.
     
  12. Andy Larson

    Thread Starter Member

    Mar 26, 2015
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    On the multi taps: wouldn't the voltage remain main the same? Or would I use a lower Volt transformer? Would a lower volt transformer w/ a reflector and smoothing wired to a DC-DC covertor work? Like a Buck coverter
     
  13. ScottWang

    Moderator

    Aug 23, 2012
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    You may try LM2576HV-adj, 1.2V~50V/3A, check page 15.
    I'm not sure the 1V used for, maybe you can in series with a 1N5400~5408/3A to reducing the voltage, but the voltage will according to the current and a little change.
     
  14. Papabravo

    Expert

    Feb 24, 2006
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    You can have multiple taps on either the input or the output, but lets start with a 24 VCT transformer. It has three wires on the secondary (output) winding, one on each end and one in the center Now the idea would be to use one side and the center tap for a low voltage range, and the same side and and the other end for the high voltage tap. You could arrange for a relay to switch the input of the bridge rectifier from the center tap to the opposite end depending on the output voltage being in the high range or the low range.

    12 VACrms * 1.414 = 16.96
    24 VACrms * 1.414 = 33.93

    Allow for some drop in the bridge rectifier and we have an input of ≈ 31.5 VDC for the high range 15.3 VDC for the low range.
    This method would reduce your power management tasks to more manageable ranges.

    One word of warning. Be careful of the temptation to hookup multiple bridge rectifiers to the same secondary. I vaguely remember trying that once in my youth with spectacularly bad results. I didn't know enough at the time about why it was bad and I've forgotten what I actually did. I'm tempted to say it was a ±12 VDC supply with the center tap grounded and a bridge rectifier on each side of the grounded center tap or something along those lines.
     
    Last edited: Apr 18, 2015
  15. Andy Larson

    Thread Starter Member

    Mar 26, 2015
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    Ohhh that makes sense. I could do that. But there is no way to go up? I mean start a low voltage and boost up to a higher?
     
  16. Reloadron

    Active Member

    Jan 15, 2015
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    Another very good suggestion as to a regulator.That and using taps on a transformer. There is no shortage of viable options which will work.

    Ron
     
  17. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Back in the days before the wide range SMPS designs came into cheap mass production putting a variable AC system ahead of a typical linear regulator was how most any high capacity regulated power supply would have been built.

    To keep power waste to a minimum the front DC rail voltage is only kept a few volts over what ever the linear regulator needs for a minimum forward voltage drop across the power handing devices and to do that they common design was to have the variac control and the pot for the linear regulator ganged together so the forward voltage drop across the power handing devices is fairly constant from minimum to maximum being both voltages would always be getting adjusted simultaneously.

    The other advantage to that design was that by using a center tapped transformer plus and a minus regulated outputs could be made just by making a negative mirror circuit of the positive power supply side and liner a regulator circuits.

    If done right you would essentially have a single power supply that can provide a center tapped AC output plus dual unregulated and regulated DC outputs all with considerable overload capability that a SMPS cant do.

    I don't know what the other persons reasoning was for not going linear but to be honest from a realistic electronics hobby use application a properly designed multi output wide range adjustable linear power supply is way more useful that a s single output SMPS setup and can be built for way less as well. ;)
     
  18. Andy Larson

    Thread Starter Member

    Mar 26, 2015
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    Thank you guys so much. But I have one final question. Would a DC-DC converter work as a regulator? Like a Buck, Boost of a Buck-Boost
     
  19. ScottWang

    Moderator

    Aug 23, 2012
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    A DC-DC converter is a power source may not a voltage regulator, but some are the regulator.
     
  20. ScottWang

    Moderator

    Aug 23, 2012
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    As these are regulators.
     
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