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transformer waveform in fullbridge inverter with LM5045

azadeh vkl
Prodigy 170 points
Other Parts Discussed in Thread: LM5045

I designed an inverter application with LM5045 in full bridge topology and run it at frequency of 200kHz.

Here is a capture which red signal indicates on primary side transformer voltage.
But unfortunately I can't analyse in each part of this diagram which component is activated?
Are the pulses with nearly 500nsec width normal or unusual? Are they made because of body diode in MOSFET switches?
Does any body have a practical experimental figure of transformer voltage which helps me to compare with my figure?
how can I find out from this figure that the designed circuit is working correct or not?

Thanks in advance

  • 0
    TI__Mastermind 34465 points
    The waveform is not normal.
    If you wish to view the switching waveform across the primary then it is important to use an isolated probe here.
    This probe needs to be isolated from the ground of the full bridge and the ground used by the yellow and blue waveforms.
    Its hard to tell anything else without the schematic, transformer information and details of how you did your test
  • 0 in reply to John Griffin1
    Prodigy 170 points
    actually the blue and yellow signals are drain to source for two low side mosfets in full bridge. as the source of both mosfets are the same, the difference between them shows the leads of transformer.
    even if I only check the primary leads of transformer on oscilloscope, I would see a waveform the same as the red signal.
  • 0 in reply to azadeh vkl
    TI__Mastermind 34465 points
    Are you saying CH1,CH2 are on the bridge midpoints and that the red channel is CH1-CH2 ?
    Do you expect the power supply to work with 5V on the bridge?
  • 0 in reply to John Griffin1
    Prodigy 170 points

    yes, I exactly wanted to say this.

    here is my schematic, CH1,CH2 are drain of M301,M302, and the power supply of bridge MOSFETs is 48v

  • 0 in reply to azadeh vkl
    Prodigy 170 points

    sorry, M302 was a writting mistake. I had to write M301 ,  M303.

  • 0 in reply to azadeh vkl
    TI__Mastermind 34465 points
    These are a few things I notice:

    (1)The transformer primary voltage is characterized by M302 and M303 or M304 and M301
    To relate to the data sheet, HO1/LO2 and HO2/LO1 are the MOSFET pairs that apply voltage to the primary.
    You need to move the probes to the correct MOSFET pairs.
    (2)The UVLO threshold for this circuit is set for 38V turn on.
    The waveforms in your original post show switching at about 5V input.
    Something not right here.
  • 0 in reply to John Griffin1
    Prodigy 170 points

    Thanks a ton for your notices.

    As shown in schematic, the drain of M301 and M302 are directly connected to the primary leads of transformer. so the difference between them shows the  primary voltage of transformer.

    Also, I had forgotten to say that CH1 and CH2 probes were multiplied by 10. so the 5v in diagram indicates on 50v in real.  

    In another test I shorted the output inductor to see whether mentioned pulses are related to MOSFET body diodes or not. but the transformer voltage curve didn't change. 

    Hope to hear from you soon.