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UCC256303: UCC256303 design and change to UCC256403

Rock Shao
Expert 2995 points
Part Number: UCC256303
Other Parts Discussed in Thread: UCC256403, UCC256402

Hello,

Base on current the design as the following,

LLC Input: 390Vdc (with PFC)

Output:12V / 37.5A

Lr: PQ2016, 50uH (14T)

Lm: PQ3330, 360uH (32T:2T)

Fo: 85kHz

Cr: 0.068uF

LLC mosfet:  1. TK25A60X5  2. IPA60R125P6  3. HCS60R150T

Co(Effective output capacitance) as below:
1. TK25A60X5: can not find
2. IPA60R125P6: 398pF
3. HCS60R150T: can not find

There have two questions as the follow

1. I reference the SLUA923.pdf and used MOSFET IPA60R125P6 to calculation the Lm=360uH, Lr=63uH, Cr=40nF, the Lm was the same as the current design, will it cause missing slew rate?
the schematic and calculation as the below

ATM450S-F120 _ (UCC256303) Main Board.pdf
UCC256303(SLUA923.pdf).xlsx

2. If would like to change from UCC256303 to UCC256403 to improved the missing slew rate issue; How can I adjust the parameter of the UCC256403 made the burst-mode to be between 0.375A~0.75A.
the schematic as the below
ATM450S-F120 _ (UCC256403) Main Board.pdf

    

Best regards

Rock Shao

  • 0
    TI__Guru* 85616 points

    Hello,

    1.  If it is the slew rate being greater than 1V/ns at the switch node you should be able to observed this while monitoring the switch node.  It shows up during large signal transients. 0 to 100 % load steps at 1 kHz at 50 duty cycle.  You can verify this by monitoring the switch node, Vout, Isns and VCR when the shut down occurs.  You have 4 scope probes so why let 2 go to waist.  You could have been shutting down due to OCP as well.

    2.There is an application note at the following link that will explain how to migrate from the UCC256303 to the UCC256403 at the following link that will help modify your design to support the UCC256402 https://www.ti.com/lit/an/slua966a/slua966a.pdf  There is also an excel design calculator that you can use as well to double check your design that can be found at the following link. 

    Regards,