Current transformer with phase cut rectifier

Discussion in 'The Projects Forum' started by eem2am, Jan 8, 2013.

  1. eem2am

    Thread Starter New Member

    Jul 22, 2012
    9
    0
    Hello,

    How can I implement a variable phase-cutting rectifier on a 50KHz current transformer secondary?

    I have a current transformer with a split secondary which is charging a battery from its secondary. The primary current is a fixed sine wave.

    Schematic: Full wave rectifier on current transformer output………
    http://i46.tinypic.com/28jxsgg.jpg

    …As you see, there is a full wave rectifier which puts DC into the battery from the secondary as follows…..

    Full Wave Current waveform into battery:….
    http://i46.tinypic.com/ipb989.jpg


    The primary current is fixed and is a sinusoidal 50KHz.



    I now wish to reduce this current by phase cutting the rectified sinusoidal output. I can do this by periodically shorting out the rectifier diodes.

    Here is the waveform that I now require into the battery……(0.3 duty cycle)………
    http://i48.tinypic.com/qq4ksi.jpg

    …as you can see its phase cut version of the initial waveform and has a 0.3 duty cycle.

    I also want to be able to do any duty cycle from 0.1 to 0.9…….so for example, I may sometimes want the following waveform to charge the battery………

    Current waveform into battery (0.7=duty cycle)
    http://i49.tinypic.com/2wo9e7d.jpg


    Here is a rough version of the phase cutting circuit…..
    http://i48.tinypic.com/2vhyq04.jpg

    …as you can see, the controlled voltage source switchs the FETs on and off periodically via the FET drivers, shorting out the rectifier diodes as it does so.



    Though my question is, how can I most simply provide this “phase cut” waveform?
    What circuitry can most simply do this job?

    …on any particular product I only need one duty cycle…and it might be anywhere between 0.1 to 0.9…..(so that makes things easier)
     
  2. crutschow

    Expert

    Mar 14, 2008
    12,977
    3,221
    I don't understand how "shorting" the diodes will reduce the current. :confused: You want to open the diode circuit, not short across it.
     
  3. eem2am

    Thread Starter New Member

    Jul 22, 2012
    9
    0
    I understand what youre meaning...but shorting the diodes makes the current flow in a loop through the secondary , and through the diodes, and thus it does not go into the battery.

    ....so, shorting the diodes "diverts" the current away from the battery, which reduces the current into the battery.

    This is part of a system which is a waterproof lighting system with an emergency battery..............instead of a voltage source, it uses a current source (50KHz) which is coupled into the luminaires via current transformers, which are waterproof as they are not like normal connectors.
    ..the battery charging is also done by a coupler, as you see represented in my pics
     
  4. crutschow

    Expert

    Mar 14, 2008
    12,977
    3,221
    OK, that makes sense. I forgot that is was a current transformer. :p

    Since it's a battery load do you really need to "phase cut" the charge current. Perhaps you could just do a ON/OFF duty-cycle type of control with the ON and OFF periods being several cycles. All you need for that is a low frequency PWM type of circuit. Then you wouldn't have to synchronize the switching with the incoming waveform. I don't think the battery would care. It's the average current the battery sees over a long period that is important.
     
  5. MKCheruvu

    Member

    Nov 20, 2012
    30
    6
    can you provide the IC Part Number(I assume it is from Linear technology)
     
  6. crutschow

    Expert

    Mar 14, 2008
    12,977
    3,221
    As a suggestion, do not post schematics as JPEGs since it makes them blurry and hard to read. Better to use a GIF, PNG, or other format designed for graphics, not pictures.
     
  7. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    Put a single SCR ink your circuit in place of D3. Phase control the gate at 100cps rate.
     
  8. crutschow

    Expert

    Mar 14, 2008
    12,977
    3,221
    The AC frequency is 50kHz. How will phase control at 100Hz work? :confused:
     
  9. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    NO. It will need 100KHz, I missed the K on first read. I ASSUMED (yes, I have experience in that field:p) that we were talking about 50HZ line frequency. Strange to charge a battery with rectified 50KHZ
     
  10. eem2am

    Thread Starter New Member

    Jul 22, 2012
    9
    0
    This method is used for the battery charging, but its mostly used with the LED luminaires.........ie the inductive coupler drives a chain of LEDs.

    Its 50KHz because the high frequency makes the coupler smaller.

    Do you think the cap C1, in the first schematic above, corrects the power factor?
     
Loading...