Problem with MOSFET dc/ac converter

Discussion in 'The Projects Forum' started by jeka616, Jan 15, 2009.

  1. jeka616

    Thread Starter Active Member

    Jan 14, 2009
    121
    1
    Dear colleagues,

    Apologize if this is asked twice, used search, didn't find anything related.....

    I've been trying to make DC/AC converter. I used CD 4047, LM 358, 2 transistors and then Hexfet Power Mosfet output stage. See attached diagram.

    I used center tap transformer from old UPS, which is ~18/18 V primary, and 220v secondary.

    Circuit runs ok with 12V, with 150V output. The problem is, I want to run it on 24v to get full 220v output. But mosfet are blown every time when I try to connect to 24v.

    I've used snubber resistors, diode , regulator...all sort of stuff..

    My thought is in modulation, can I have pulses overlaping, so excessive current flows thorugh mosfet??

    Today I will increase snubber capacitor to 100/200/500 mf. And maybe put 2 mosfets per rail. See what happens..

    Appreciate if you can suggest possible solution.
     
    Last edited: Jan 27, 2009
  2. duffy

    Active Member

    Dec 29, 2008
    44
    0
    I think you forgot the attachment.
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    What frequency are you trying to run? The transformer is only going to couple flux efficiently at 50/60 Hz.
     
  4. jeka616

    Thread Starter Active Member

    Jan 14, 2009
    121
    1
    The cd 4047 gives square waves ~50 HZ.

    Duffy, the circuit should be seen in message, as I do read it now. Maybe it is not visibile when others open it....
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Maybe the transformer core saturates when the voltage causes the current to be too high for it.
     
  6. jeka616

    Thread Starter Active Member

    Jan 14, 2009
    121
    1
    I took this transformer from old UPS supply, where 24v was used. And I copied power stage almost exactly. So this has worked before...Thats the strange part.

    The only thought point is the mosfet driving circuit..
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I can't see the schematic either. :confused:

    Please edit your 1st post, and click the "Go Advanced" button on the bottom.
    Then click the "Manage Attachments" button, and select your schematic's image from your computer to upload.

    .PNG format images are preferred. .JPG images are simply too lossy to be useful.
     
  8. jeka616

    Thread Starter Active Member

    Jan 14, 2009
    121
    1
    Thanks for tutorial. Hope this time it will be possible to see.

    I'll ask some more question about TIna Ti later on. Trying to model there.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    I'm sorry to sound critical, but the schematic you posted is really awful. :(

    Could you please try to re-draw it in a more readable form?
     
  10. jeka616

    Thread Starter Active Member

    Jan 14, 2009
    121
    1
    Ye..kind of drawn on fast hand..

    I got the picture from Tina TI. Hope this describes pretty much what I created.

    Thakns for reply guys, really feel there are people on earth with same hobbies and are ready to help
     
  11. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    On one drawing you are driving the MOSFET gate with 12V after a 7812 while the other directly with 24V. Which is which?

    The Vgs spec for IRF3205 is max. 20V and with 24V drive, you are doomed.
     
  12. jeka616

    Thread Starter Active Member

    Jan 14, 2009
    121
    1
    No, I drive it thorugh transistors through LM 7812. I just forgot to draw it in Tina file. The hand drawn one is correct.
     
  13. jeka616

    Thread Starter Active Member

    Jan 14, 2009
    121
    1
    Gents,

    Good news. Circuit worked with 24 V, after I increased snubber caps to 220mkf. But now I have 290V at the output :) Strange that this power circuit is almost same as the ols UPS supply, and the transformer is same.

    I will probably need to re-wire the transformer. Any other suggestions? some how to control voltage at mosfet out put?
     
  14. jeka616

    Thread Starter Active Member

    Jan 14, 2009
    121
    1
    Today I tried various combinations of caps. Currently I have 100 mkf as a snubbers. They seem to get warm a bit..and also the 5w resistors which shunts the diodes in snubbing circuit get really warm.

    I used another transformer, which secondary is 23-0-23. With that, when I hooked up 100W bulb, the voltage dropped to 180 V..

    So..need some more experiments to control voltage at output. All suggestions are welcome.
     
  15. jeka616

    Thread Starter Active Member

    Jan 14, 2009
    121
    1
    Folks,

    anybody willing to share ideas about it? Helooooo....
     
  16. jeka616

    Thread Starter Active Member

    Jan 14, 2009
    121
    1
    Either nobody knows the answer..or my system is not working to post the reply..anyways.

    Circuit seems to work ok. But the snubber cap and resistor get really hot..i think mosfets are not conducting fully..I measure the amplitude of voltage at gate of transistor driver, it shows 5.6 v. The output shows the same..no amplification, or digital multimeter can';t read square wave fully?

    Need a help guys.
     
  17. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The outputs of the CD4047 switch at exactly the same time. There is no dead time to allow one Mosfet to turn off before the other Mosfet turns on. So there are moments when both Mosfets are turned on and then they short the supply.

    An extremely simple square-wave inverter will not power many electronic products properly and has very poor voltage regulation.
     
  18. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Along with AudioGuru's assertions (which I agree with), you have not provided an updated and corrected schematic that shows your circuit in it's current configuration.

    You have provided no updates since the first two postings; one hand-drawn schematic that's practically illegible, and the Tina-TI version that omits significant details (such as the 7812 regulator).
     
  19. jeka616

    Thread Starter Active Member

    Jan 14, 2009
    121
    1
    Hi folks.

    I've updated the drawing, this currently shows what I have hooked up..

    I was concerned about the dead time. I don't have an OSC, so it is impossible to see the output waveform. But the PWM circuit was taken from existing inverter circuit, modified little. But it was driving 2N3055 , rather than mosfets..

    If the outputs are clashing, how can we explain that the circuit works? I do have 225v at output. It draws about 5 amps when I hook up 100W light bulb, which tells me I have roughly 84% efficiency? After about 5 min working, snubbers get hot..this tells me that mosfets are not conducting well, so current flows thorught the snubber circuit. As I said earlier, I have about 6v at driver transistors output(as multimeter reads.

    HOw can I add dead timing details to CD4047? which pins should it go to? Need some theory behind.


    Thanks for reply. Really value this.
     
  20. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    The circuit is not properly designed and will not work satisfactorily:

    1. CD4047 is used as a astable, its Q and /Q changes simultaneously and as other users have pointed out, there is no dead time for two drive legs of the inverter transformer. There is no way to make the chip output a dead time.

    2. LM358 is an opamp and not a comparator. You are using it as a voltage follower but has exceeded its allowable common mode input voltage range of Vcc-1.5 maximum. This is not good. Why it is needed in the first place is questionable.

    3. The gate drive to each MOSFET is effectively through 2.4K+82Ω so the gate charge current is very limited. The MOSFET turns ON slowly.

    4. I have no idea of how the existing snubber works as it just charges up the 100uF when the leg is not conducting but discharge all its charge through the 0.5Ω into the MOSFETs to 0V, wasting energy.
     
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