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LM5045, LM5046 / to limit the maximum on time

K.Yaita
Guru 29720 points
Other Parts Discussed in Thread: LM5045, LM5046, LM5025A
Hi,
 
My customer wants to limit the maximum on time of LM5045(LM5046).
Is there any method to limit that?
I think the function like "Volt Second Clamp" of LM5045A is satisfied.
Is it possible to achieve by using external circuit?
 
Best Regards,
Yaita / Japan disty
  • 0
    TI__Mastermind 34465 points

    The LM5045 and LM5046 both have a maximum  on time set by the period of the oscillator. The oscillator frequency is set by the value of the resistor on the RT pin on.

  • 0 in reply to John Griffin1
    Guru 29720 points
    John-san,
     
    I'm sorry for lacking of talk.
    My customer want to set maximum on time to the desired duration.
    So, I think the function like "Volt Second Clamp" of LM5045A is satisfied.
    Is it possible to achieve by using external circuit?
     
    Best Regards,
    Yaita
  • 0 in reply to K.Yaita
    TI__Mastermind 34465 points

    Hi Yaita-san,

    I cannot find the LM5045A in the TI website. The LM5045 is a full bridge PWM controller and does not require a volt second clamp. Can you please check on the TI part number you refer to ?

    Thanks

    John

  • 0 in reply to John Griffin1
    Guru 29720 points
    Hi, John-san,
     
    I'm very sorry to make a typo. LM5025A is correct.
    Could you explain why LM5045(full bridge PWM controller) doesn't require a volt second clamp?
    I'm sorry if it is basic question.
     
    Best Regards,
    Yaita
  • 0 in reply to K.Yaita
    TI__Mastermind 34465 points

    Hi Yaita-san,

    I understand. The LM5025A is a controller for a "forward" type power supply. When the main power FET is switched off, the leakage energy in the transformer has nowhere to go and is usually dissipated in a clamp circuit. The LM5025A minimizes this energy loss with an active clamp. The LM5045 is a "full bridge" type power supply. When one set of power FETs are switched off, the leakage energy of the transformer is returned to the primary through the body diodes of the FETs.This is a natural result of the full bridge topology and a significant advantage over the forward topology.

    Regards,

    John

  • 0 in reply to John Griffin1
    Guru 29720 points
    Hi John-san,
     
    Thank you for your comment.
    I found description about volt-second clamp below.
    ------------------------------
    (UCC2884 datasheet)
    It is during a load transient (a fault such as a momentary short circuit) as the error amplifier increases the duty cycle, that when the volt-second clamp accurately limits the maximum volt-seconds. This ensures that the transformer does not saturate during a fault which can fail the power supply.After the fault is removed the converter resumes closed loop control.
    ------------------------------
    It is necessary for me to study more, so I will do.
    Thanks.
     
    Best Regards,
    Yaita
  • 0 in reply to K.Yaita
    Guru 29720 points
    John-san,
     
    Thanks to your support, I understood why LM5045 doesn't require a volt-second clamp.
    My understanding is below.
     
    Forward type(LM5025A) is “Single Ended”, so core needs to be reset each switching cycle to prevent core saturation (VoltSeconds).
    On the other hand, full bridge type is "Double Ended", so no reset circuit needed as core is operated symmetrically.
     
    I really appreciate for your support.
     
    Best Regards,
    Yaita
  • 0 in reply to K.Yaita
    TI__Mastermind 34465 points

    Yaita-san,

    I am glad that I was of help to you.

    best of luck

    John