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UCC25640EVM-020: Over load protection

Ala Alhajeh
Prodigy 40 points
Part Number: UCC25640EVM-020
Other Parts Discussed in Thread: UCC25600EVM, UCC25600, UCC256402

Dear Sir, 

I am working on UCC25640EVM-020 and UCC25600EVM evaluation boards, to design wide input range dc to dc convertor using LLC topology. Input voltage ( 70 V to 140 V ) : 24V output voltage  

And I was successful on achieving around ~ 96% efficient system at the nominal input voltage (120V).

However i have a problem, with over load protection and short circuit protection... due to the wide input range, at low input line the over load  protection  is well programmed, but at high input line the current limit is way to high, due to the sensing method, in both ICs: UCC25600 and UCC25402.

would you be able to help me to find a solution to this issue ?  a way to control the output current at full input range ?  

Thanks a lot 

Looking forward to hear back from you. 

  • 0
    TI__Expert 7080 points

    Hi Ala,

    An expert will get back to you soon !

  • 0
    TI__Genius 9790 points

    Hi, Ala

    First, what you observed is very normal to UCC256402 and UCC25600. 

    Second, UCC256402 and UCC25600 as LLC controller, which is recommended to be used in the application with PFC stage. 

    So I just to suggest to add a boost stage before LLC stage.

    Yunsheng

  • 0 in reply to Yunsheng Qu
    Prodigy 40 points

    Hi Yunsheng, 

    Thanks a lot for your reply, I was trying to avoid adding another conversion stage to save power and keep the efficiency high. 

    Also i found difficulty in controlling the output noise specially at very light load, highline input voltage, and when the unit go into burst mode, at the moment i get around 1.5 V peak to peak worst case,

    I tried to change (BMTL/BMTH Ratio Programming), but i was not successful. if i disable the burst mode at light load the unit overshoot, 

    would you be able please  to give me some suggestion and leads 

    Thanks a lot 

    Ala

  • 0 in reply to Ala Alhajeh
    TI__Genius 9790 points

    Hi, Ala

    1. BMTL is defined by LLSS resistor divider, i suggest you use pull high resistor ad 470kohm and pull low resistor as 200kohm. and short BW pin to GND.

    2. other element affect burst mode threshold is VCR pin capacitor divider, you should decrease pull down capacitor to keep peak-peak voltage of VCR pin at around 5.5V at full load condition.

    BR,

    Yunsheng