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Thread: 12v 250w Car SMPS based off SG3525

  1. #1

    12v 250w Car SMPS based off SG3525

    Hey all! I'm wanting to build this car smps (see link below), but it's not going to be use in a car. In fact, it's probably just going to end up powering an audio amp I built. Mostly though, the entire point of this project is for me to learn more about some higher powered switching supplies building techniques (while at a relatively "safer" voltage level) before going on and building one that can be plugged directly into the wall.


    This is the site where I am getting the schematic from: http://sound.westhost.com/project89.htm
    And these are some quick links to the two schematics i'm looking at:

    http://sound.westhost.com/p89-f9.gif
    http://sound.westhost.com/p89-f1.gif

    I prefer to combine the two so that there is the output voltage adjust circuit as well as leaving out the Q1-Q4 driver transistors as they (according to the site) are apparently not essential because the SG3525 can source/sink enough current to drive the mosfets.


    Now here is where i'm running into some problems. I have been breadboarding the driver circuit from Figure 9 for two days now trying to make it work and it just won't budge! Here are some of the changes I made. Keep in mind that when I finish this thing, all the part values will be correct. Just for initial breadboarding purposes are some of the components changed. I am powering this right now through a variable bench supply @15vdc.

    -IC is a UC3525AN...according to the datasheets, there is no difference between the SG and UC version besides who manufactures it, but it was worth mentioning just cause i didn't use the "exact" IC
    -R7,9,10: They are all 15K instead of 12k..I couldn't find for the life of me any 12k's in my parts bin.
    -R8: 25 ohm...I know it's lower but it will still put the dead time on the output
    -C11: 47uF but that shouldn't matter as it's a start up timing capacitor
    -I didn't want the remote turn on/off feature so i removed R11, R12, C18, Q1, and grounded the shutdown pin.



    I double checked every single connection with my ohm meter when I placed the parts to make sure that things were connected correctly. Everything seemed perfect but when I applied power, no output showed on the oscilloscope for either out A or out B! I checked all my connections once again and yep...still nothing on the output. So i tried removing the ground connection on pin 10...same result. I tried a 15k pull up resistor to VCC on pin 10 and still no pulsing output. Then I thought it might be the chip that was broken so I replaced that too and tried the same things with pin 10 as before. Same result. I know this thing works as Rod Elliot does NOT post circuits on his website that he has not already tested himself. Does anybody have some suggestions for me?

    Thanks,
    Codex653

  2. #2
    if anyone would mind helping that would be wonderful

  3. #3
    diysmps Member
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    Hi Codex,

    this is not easy to analyze. From my point of view you MUST add the external drivers as done in the "f1" schematics: according to the 3525 data sheet the absolute maximum source/sink current on the driver section is 500mA but with 2x22ohm gate resistor (resulting in a 11ohm resistor) and 15V you are sourcing/sinking a peak of more than 1A (15V/11ohm) which is more the double the "recommended operating conditions (400mA source/sink).
    Another small thing: in the same "f1" schematic you would need an anti-parallel diode on the BD139 driving the relay coil...
    Last edited by McMax; 03-04-2012 at 08:13 PM.

  4. #4
    Darn...I was hoping to not have to use Q1-Q4. Oh well, it's only 4 more parts. As for the relay coil, I'm not wanting to have any sort of remote turn on/off thing at all. It serves no purpose for what I would use the power supply for and it's extra parts I don't need.

    Hey also for anyone reading this, if you happen to have a MULTISIM model for the SG3525 it would make it alot simpler to figure out what is going on here
    Until then, does anyone else have ideas of what could be going on??

  5. #5
    If you have 5v on the ref pin (16) of the 3525 it is working, check if you have frequency from +12 to the gate of the mosfets, if there is 30k or so you probably have damaged or false transistors. Check this and tell me whats going on.

    Saludos!

  6. #6
    Quote Originally Posted by e1000 View Post
    If you have 5v on the ref pin (16) of the 3525 it is working, check if you have frequency from +12 to the gate of the mosfets, if there is 30k or so you probably have damaged or false transistors. Check this and tell me whats going on.
    Ahhh ok I will check that then when I have the time. I have some baseball games tonight and tomorrow that are going to go VERY late, so I might not get the chance to look at it until wednesday.

    As for the mosfets, I do not have them hooked up yet. I have only breadboarded the control circuitry to make sure it would work before I tried connecting the rest of the circuit. As of right now there is no output on either out A or out B.

  7. #7
    diysmps Member* giololucas's Avatar
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    Quote Originally Posted by codex653 View Post
    Ahhh ok I will check that then when I have the time. I have some baseball games tonight and tomorrow that are going to go VERY late, so I might not get the chance to look at it until wednesday.

    As for the mosfets, I do not have them hooked up yet. I have only breadboarded the control circuitry to make sure it would work before I tried connecting the rest of the circuit. As of right now there is no output on either out A or out B.
    This site you mentioned it´s very good (rod elliot).
    I built an amplifier with ideias of this one. If you want to check it, search for JBL based amplifier on this blog.
    More than 4 years working with a 15 inch subwoofer on heavy load, and never failed.

    Best regards.
    " Se você quer ser bem sucedido, precisa ter dedicação total,
    buscar seu último limite e dar o melhor de si mesmo
    ."

  8. #8
    I found time this morning before classes to check the breadboard! I guess it helps to take a second look at a project after sleeping on it, cause I fixed it! Now I have both outputs pulsing, it's time to start tweaking the circuit to get the duty cycle and frequency correct! I'll see if i can figure out how to post images of the waveforms from my scope...I'm not sure how to do that yet :P

    Got a question for you all. The transformer I am planning on using came from a generic 300W ATX computer power supply, nothing fancy at all. What typical frequency do those transformers run at? Yeah I know that depends on the type of ferrite, power level, core size and a whole bunch of other factors but I'm just wondering so I can have a base frequency I can start testing my transformer around. What do you think? 30Khz? 50Khz?? maybe even 80Khz?

  9. #9
    diysmps Member*
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    I find these transformers are run from 40-70kHz. You can go to 100kHz but the losses go up drastically in the core, the windings (skin-effect unless you use Litz), the mosfet switching and lastly your secondary rectifier diodes. It's a case of speed kills.
    30kHz-50kHz is a good start. If you are using the 4-transistor mosfet driver scheme, you can push it past 75kHz, but not with slower (non-Schottky) rectifier diodes. My $0.02

  10. #10
    Quote Originally Posted by Redwire View Post
    I find these transformers are run from 40-70kHz. You can go to 100kHz but the losses go up drastically in the core, the windings (skin-effect unless you use Litz), the mosfet switching and lastly your secondary rectifier diodes. It's a case of speed kills.
    30kHz-50kHz is a good start. If you are using the 4-transistor mosfet driver scheme, you can push it past 75kHz, but not with slower (non-Schottky) rectifier diodes. My $0.02
    Ok thanks Redwire! I have my breadboarded circuit at 32kHz right now, so I'll change around a few components to try and get close to 55kHz when I get the chance. In the mean time, I took a few pictures of the waveform from the circuit. I'll post em in just a min here.

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