dc-ac inverter

Discussion in 'The Projects Forum' started by cade007, Apr 14, 2006.

  1. cade007

    Thread Starter Member

    Apr 14, 2006
    14
    0
    I have a final project to design and build a dc-ac inverter and I was wondering if anyone had any experience with this. I am using a full bridge to create the ac voltage. I need some suggestions on how to regulate the duty cycle from my square wave PWM to obtain the 60 Hz frequency. Any help on this topic would be greatly appreciated.

    here is a link to the circuit diagram

    https://webfiles.colorado.edu/penney/index.mht
     
  2. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
    4
    What are the specifications? Is it a squarewave, stepwave or sinewave output? What is the power level? What is the input voltage? What is the output voltage? How good is your understanding of inverters and the circuit that the link you posted? If you expect help from others, you could at least be more generous with the specifics and make it easier for us to help you.
     
  3. cade007

    Thread Starter Member

    Apr 14, 2006
    14
    0
    Sorry some of the specifications are listed in the subtitle. Anyway, I have a 200 VDC source which I am trying to generate a 120 Vrms 60 Hz sine wave output (as close as possible to a household voltage). The power level I accounted for was about 100 Watt rms and an inductor current ripple of 20%. I understand the process of how the circuit will invert the signal, there are just some sublities that I don't know about when I am actually trying to build the circuit. For example, from my circuit drawing where I am measuring my voltage across the resistor and capacitor in parallel, does the inductor and capacitor create somewhat of a filter? What type of capacitor should I use there? I was testing the circuit today and I could see somewhat of a small sinwave output, but only at small input voltages. When I turned the voltage up, the output voltage look like DC.

    The control circuit works as it should, alternating turning on the top left MOSFET at the same time as the bottom right MOSFET then vise versa with the other two MOSFET's. The sinewave is generate by varing the duty cycle via a sinewave at 60 Hz from a duty cycle of nearly 0% to about 80% and I have tested the PWM and it can go up to the 80% duty cycle. I then have the output of the PWM going to two seperate half bridge gate drivers, IR2111 (I know I could have easily used a full bridge driver, but I was limited on parts so I used two half bridge drivers but it works the same way I just had to invert the signal of the PWM going to one half bridge driver in order for the right MOSFETS to turn on and off at the same time). All these gate signals are fine which have a 650 nS dead time.

    My inductor is about 2.8 mH and the capacitor is 220 uF at the output. Is this ok with a 60 Hz output?
     
  4. dragan733

    Senior Member

    Dec 12, 2004
    152
    0
    These days I test also my DC/AC inverter with full bridge driver. I use IR2110. But, I meet the same problem as you. When I turn the voltage up, the sinusoide distorts. I don't know the cause. Tomorrow I will try to change the schematic to half bridge. Maybe here is the solution, to use a half bridge instead a full bridge. Tomorrow I will see. Or maybe the bootstrap capacitor is selected with wrong value. For the inductance I use 1.5mH and C=33uF
     
  5. cade007

    Thread Starter Member

    Apr 14, 2006
    14
    0


    What kind of capacitor are you using? Is it a polypropylene cap? I would think the two half bridge drivers would work the same as a full bridge driver. I have checked my input signals to the gates of the MOSFETS and it is exactly the signal that I was expecting. I think my problem might lie in the cap or possibly the inductor. What kind of ripple did you account for in your inductor?
     
  6. dragan733

    Senior Member

    Dec 12, 2004
    152
    0


    <!--QuoteBegin-cade007
    @Apr 18 2006, 06:08 AM
    What kind of capacitor are you using? Is it a polypropylene cap? I would think the two half bridge drivers would work the same as a full bridge driver. I have checked my input signals to the gates of the MOSFETS and it is exactly the signal that I was expecting. I think my problem might lie in the cap or possibly the inductor. What kind of ripple did you account for in your inductor?
    [post=16273]Quoted post[/post]​
    [/quote]

    You get DC voltage When you turn the voltage up, instead sinus, because you have not connected the load to the output. When I turn the voltage up, I remarked also the gate PWM signals distorts, and as a result I get a sinusoid distorted to the output.

    However I leave the schematic to drive the full bridge Mosfets with IC driver as like IR2110 or IR2111. On Internet on the forums I read that IR2110 etc do suffer from Vs undershoot and bootstrap capacitor overcharging (=> stuff blows up) if you don't do all of the tricks mentioned in the data sheet and corresponding application notes. Also they recommend to use drive transformers.
    It should be A LOT easier to just forget that bootstrap scheme. I would drive the Mosfets with drive transformers. It is more safely and more cheaper than the IC drivers. On Internet one can find the schematics to drive the Mosfets with drive transformers. I think here is the solution to escape the problems driving the Mosfet bridge.
    I will order some drive transformers and I shall continue with my DC/AC inverter after some weeks.
     
Loading...