PWM Generator and Control

Discussion in 'The Projects Forum' started by Ibrahim Ogbachi, Dec 28, 2013.

  1. Ibrahim Ogbachi

    Thread Starter New Member

    Dec 6, 2013
    4
    0
    All,

    I am working on 2-phase interleaved bidirectional dc-dc converter as shown in the circuit below. I have not been able to find answers to the following questions running through my mind. The questions are:


    1. 1. How do I generate two independent PWMs of 180 degrees out of phase with dead time? In this case considering 50 kHz switching frequency and 50% duty cycle.

    1. 2. Which PWM generator can suite my specifications-PIC microcontroller, IC or 555 timers? I have PIC16F88 in the lab, can it do the job?

    1. 3. What is the best switching technique I can use for my design? I am using IR2101 as my gate driver.
    Mate, please assist me on this; I am new on converter design.


    Many Thanks,
    Ibrahim.
     
  2. DickCappels

    Moderator

    Aug 21, 2008
    2,647
    632
    An NE555 or the LMC555 would be a poor choice. The 16 pin Texas Instruments TL494 is a good choice because it has adjustable dead time.

    If you wan to use a microcontroller, you might want to have a look at the 8 pin Atmel ATTINY25 which has dual PWM outputs with adjustable dead time.

    My experience with PIC is very limited but I see that the PIC18R14K50, the PIC18F25K22, and PIC18F2520 all have programmable dead time, and so might be suitable.

    The PIC16F88 might work but you would have to bit-bang or "semi-bit-bang with timers" the waveform. Doing push-pull PWM by bit-banging it with a microcontroller is a little bit risky because any difference in the duty cycles of the two drive signals can cause excessive dissipation in the output devices, so you should try real hard to make that perfectly symmetric.
     
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  3. Ibrahim Ogbachi

    Thread Starter New Member

    Dec 6, 2013
    4
    0
    DickCappels,

    Thanks for your response. The TL494 has maximum duty cycle of 45%, this will be lower than my duty cycle of 50%.

    Someone suggested that since i am using interleaved phases
    that i should
    use a current mode control scheme, meaning i should not generate PWM with variable duty cycle, but rather just provide a clock, and the current control loop will settle on a duty cycle based on the peak current setpoint. So i only need to create phase interleaved clocks (not interleaved PWM) using special ICs that are specialized for making phase interleaved clocks (look up LTC6902 and LTC6909).

    Based on the recommendations above, i now consider using LTC6902 & IR2101 and IXFK170N20P as my Power MOSFET with 170A and 200V. I am trying to reduce the number of phases to two due to complexity.

    Please look at the attachment below, i am not clear on how to connect the LTC6902 to the driver or the clocking sequence. For 2-Phase, my OUT1 & OUT2 of LTC6902 will go to the gate driver HIN & LIN, am i right? While the output of the gate driver HO & LO will go to the upper and lower Power MOSFET of the same phase, am i right?

    Do i repeat the same sequence for the second phase? I am scared of the two upper MOSFET will be ON at thesame time.

    Please advice.

    Many thanks,
    Ibrahim.
     
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,773
    931
    If you include 'dead time', then your duty cycle cannot be 50%. You will need to subtract the dead time percentage for the actual duty cycle calculation.

    EX: positive wave + neg. wave = 10 uS

    Period of waveform = 12 uS

    Duty cycle would be 4/5 of period, or 80%. OR as you have interpreted duty cycle:: 40% for each half(pos. & neg.)
     
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  5. Ibrahim Ogbachi

    Thread Starter New Member

    Dec 6, 2013
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    0
    Kermit2,

    Many thanks for your inputs, the dead time consideration well noted. Please, can you comment on my clocking sequence as illustrated in my previous post and attachment. Please bear with me, i am new to this design.

    Thanks,
    Ibrahim.
     
  6. Ibrahim Ogbachi

    Thread Starter New Member

    Dec 6, 2013
    4
    0
    All,

    Please, i need your help on the clock sequence as in the previous post and attachment.

    Thanks,
    Ibrahim.
     
  7. John P

    AAC Fanatic!

    Oct 14, 2008
    1,632
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    Some PIC processors in the 16F series also have the dual PWM with off time feature. The PIC16F690 is one example.
     
  8. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,773
    931
    I don't see any 'dead time' between the waveforms you have drawn.

    read this
    http://www.edaboard.com/thread238874.html

    the last or near last response has a link to a pic chip calculator for making a look up table in memory to drive the outputs.
     
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