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UCC28951: The relationship of Transfomer designed and duty cycle

IKUTETSU KOU
Prodigy 70 points
Part Number: UCC28951
Other Parts Discussed in Thread: UCC28950

Hi Expert

We are designing a 10kW CCM+PSFB CC&CV battery charger.

We are facing a problem that we can`t get full output.

Because of the hardware design ,we are not able to measure the transformer current.

But we can probe the voltage of transformer voltage instead.

In 100khz Fsw condition We can only output about 3kW even dutycycle is almost 100%. 

Also ZVS is quite easy to achieve when the power is at low.(about  2%)

 

We did not use shim inductor.All resonant tank is rely on transformer leak inductance.

We think what cause this problem is we got too large leak inductance,and this make the available duty cycle be come small.

Since we can`t probe transformer current , for make sure what it is going on as we thought.

We try to reduce Fsw to 50kHz let current get enought time to able rise up.

And we are be able raise output power to 8kW at 100% dutycycle.

 

Before we going to redesigned transformer to reduce the leak inductance.

We just want to check how you think of it.

 

  • 0
    TI__Guru* 86266 points

    Hello,

    You should be able to probe the CS pin, comp, transformer input and SS pins.

    Please note that if the CS signal reaches 2 V the design will go into OCP/hiccup mode.  The data sheet has a section that goes into this in detail.

    I would start by double checking your transformer turns ratio.  If you are at 100% duty cycle it may need to be adjusted.  It should be at 50% if you selected the turns ratio correctly.  The following link will bring you to an application note on the UCC28950 which is also applicable for the UCC28951.  It has a section no how to select the transformer turns ratio and gives some design tips that you might find helpful.  https://www.ti.com/lit/ds/symlink/ucc28950.pdf

    Regards,

  • 0 in reply to Mike O'
    Prodigy 70 points

    Mr.Mike

    Thanks for your reply.

    We know the UCC28951 is not going to OCP/hiccup mode by monitor MOSFET controll signal phase shift.

    Control signal is very stable at max level.

    And we also try to take off CS and test under open loop by setting op-amp as voltage follower.

    Indeed we set the turn ratio higher then the datasheet suggest.(We also checked the calculate excel tool)

    We thought we can get full output at 87% dutycycle by datasheet fomula.

    Now we are going to decrease turn ratio & leak inductance both.

    If you know anything we are miss please tell us.

    Just in case,We still want to comfirm one more thing.

    We designed magnetizing inductance quite large (about 10 time of suggested Lmeg in calculate excel tool).

    Because we want make shure magnetizing inductance is always larger then the suggestion value even the transforner is getting hot.

    Will large Lmeg bring any disvantage or down side to us?

  • +1 in reply to IKUTETSU KOU
    TI__Guru* 86266 points

    Hello,

    From your reply it seems that your transformer turns ratio is too large.  The controller is asking for 100% duty cycle and the output voltage is not maintained. I would recommend adjusting the turns ratio and setting a target duty cycle for less than 75%.

    Regardsm