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Full-bridge topology based by UCC28251

user1915949
Prodigy 85 points
Other Parts Discussed in Thread: UCC28251, TL431

Hi friend, i am junior in switch power.

Now, i am use UCC28251 to design full-bridge power, the followings are the parameter:

input voltage: 200Vdc-320Vdc

output : 28V/46A

switch frequency is 100KHz

Transformer: Np=13Ts, Ns=4Ts,L=800uH, the core is EE55

filter inductance: L=20uH

i have some questions:

1st. There is a separate power to supply UCC28251, first, the aux power is switch on, and the D=50%; then the input power is switch on, the  UCC28251 soft-start is just 20ms, after 20ms, the IC is switch off.  i am really do not know why it is off. i check the datasheet, there are four reason that the IC could off, first VDD undervoltage; 2nd is EN undervoltage; 3rd is OVP; 4th is over temperature. But I think my circuit is not the above reason.

2nd. it is  the same as 1st. first the input voltage is switch on, then the aux power is one. but the UCC28251 was notwork at all.

i am really hopeful that u can  reply my questions. i am looking for your answer

best regard

  • 0
    TI__Mastermind 31580 points
    Hi,

    You need to look at the voltage output and on the FB pin during start-up to see if the controller is trying to regulate. What load are you using?

    Regards

    Peter
  • 0 in reply to Peter Meaney
    Prodigy 85 points

    Hi, peter

    here is my text wave.

    text 1, give power first to UCC28251, then give power to input voltage. The result is the output voltage rise to 7V, but then the chip is not work

    Fig 1, CH1: output voltage    CH2: COMP voltage     CH3: SS voltage  

                                                                        Fig 1

    test 2, first give power to input voltage, then give power to UCC28251. the result is the output voltage does not rise, the SS(soft start-up) rise to 0.7V, then the 

    chip does not work. During soft start-up process, the COMP  is following SS. when COMP is more than 0.4V, the PWM is valid.

    Fig2, CH1: output voltage    CH2: COMP voltage     CH3: SS voltage

                                                                                                  Fig 2

    i am really do not know the reason why  the chip is not work. would u like to help me . thanks very very very much

     best regards

    alec

  • 0 in reply to Peter Meaney
    Prodigy 85 points
    Hi friend,
    the load is electrical resistor. what's more, the RAMP is Saw tooth wave， when the RAMP resistor is 360KOhm and the RAMP capacity is 100pF, the circuit can work, but when loads more, the output is decrease. others, the RAMP resistor is 360KOhm and the RAMP capacity is 470pF, the circuit can not work at all. Following are the text wave
  • 0 in reply to user1915949
    TI__Mastermind 31580 points
    Hi,

    Can you look at the CS pin voltage and see how lage the current signal is during the start up. Also in test 2 why is the COMP signal collapsing when the output voltage is way below its regulstion level. Do you have a schematic that you can share?

    Regards

    Peter
  • 0 in reply to Peter Meaney
    Prodigy 85 points

    7701.Schematic Prints.pdf

    Hi, friend

    here is my schematic

  • 0 in reply to Peter Meaney
    Prodigy 85 points
    HI,
    the CS pin (UCC28251 is ILIM pin) is put to GND, so the chip is not in protection mode. during soft start-up, the COMP is following SS(soft start-up pin), the reason why COMP is collapsing is the SS signal is collapsing. But i check the datasheet, the chip is not in protection mode, and the SS should not to collapse. however, in my schematic, i can not find the real reason.
    i hope u can give my some suggest
    best regards
    alec
  • 0 in reply to user1915949
    TI__Mastermind 31580 points
    Alec,

    what feedback circuit are you using to measure the ouput voltage, compare it to a reference level and then generate an error signal to drive the COMP pin? The COMP pin needs to be driven, held a voltage greater than 0 to gnerate gate drive pulses. See figure 33 in the datasheet.
  • 0 in reply to Peter Meaney
    Prodigy 85 points

    Hi Peter,

    the feedback loop is TL431 and photoelectric coupler, like in the schematic. would u find something wrong in schematic? there is a test i tried, use a voltage in output, and change the voltage, so i can see the PWM D is changing as output voltage

  • 0 in reply to user1915949
    TI__Mastermind 31580 points

    Hi Alec

    If you can control the D in the way you describe you can set D to a  short time width and then increase Vin and ensure that the power stage will run in this condition.

    After that the problem may be due to an issue with the SS time been too long and the Vdd on the IC not be supplied from the auxillary supply for example.

     

    Regards

     

    Peter

  • 0 in reply to Peter Meaney
    Prodigy 85 points

    Hi Peter,

    i will try the way u told to me, thanks a lot

    what's more. i think the PCB is low quality that due to this issue.

    thanks very much

     best regards

    alec

  • 0 in reply to Peter Meaney
    Prodigy 85 points

    HI, Peter

    i have a new question, i am very happy if u can answer to u me.

    it is about UCC28251 HICC pin, in my actual circuit, if the pin float. the circuit can work, others,  it can not work. i am really do not know why it is?

    is it the reason that PCB problem? 

    Best Regards

    alec