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Original question:

UCC28742: UC28742 Start-Up

UCC28742: Flyback DC-DC Converter doesn't shutdown when Iout=Iocc + feedback loop resistor values

Alexandru Dalin Dragoi
Prodigy 20 points
Part Number: UCC28742
Other Parts Discussed in Thread: TL431

Hello Ti,

As the title says, the converter I'm working on (HIGH VOLTAGE Flyback DC-DC Converter) doesn't stop regulating when the output load reaches a chosen Iocc value.

Target values: 

  • Vin = 60 - 1000V
  • Vout = 100V @ Iout = 0.1A (Rload = 1k)
  • Iocc = 0.16A @ Vocc = 80V (Rload = 500 ohm)
  • Vov = 120 V

The values I need to work with for the transformer (I can't change them):

  • Np = 36, Na = 12, Ns = 64
  • Nps = 0.5625, Npa = 3, Nas = 0.1875
  • Lp = 1.07mH (measured with RLC meter)

I managed to calculate the values of the components using the formulas in the datasheet, EXCEL provided by you ("sluc652b") to make sure I got the values right, and using the Simplis model provided.

Initially, my target frequency was fmax_target = 67kHz, but following a personal mistake (I chose the wrong transformer core, instead of being gapped, it is gapless), I increased the inductance in the primary from Lp_target = ~300uH to Lp_actual = 1.07uH. I think the actual maximum frequency at which the converter can operate is fmax_real = ~20kHz (based on what the sluc652b EXCEL says). 

Also, the values for R_TL and R_opt are chosen by trial and error, since initially the converter did not work with the values from EXCEL.  Also, I didn't change the values for R_fb3, R_fb4 and C_fb because I didn't know what values I should put in their place.

I'll attach some screenshots from the simulator with a normal load, and one where I exceed the Iocc threshold (Rload = 100 ohm) + the EXCEL I used to calculate those values.

I kindly ask you from the bottom of my heart to help me solve the problem if possible.

Rload = 1k (the converter works even with 50k or 100k load)

    

Rload = 100 ohms (it is not working even with 1ohm). 

 

Schematic

Kicad schematic

sluc652b_flyback_High_voltage.xlsx

Best regards,

Alexandru

  • 0
    Prodigy 20 points

    Forgot to mention: 

    • shunt regulator used : TL431I
    • optocoupler : PS2811-1
  • 0 in reply to Alexandru Dalin Dragoi
    TI__Expert 8301 points

    Hi Alexandru,

    I am taking care of this thread and I will be back to you tomorrow morning.

    Thank you

  • 0 in reply to Manuel Alva Hernandez
    TI__Expert 8301 points

    Alexandru,

    -Your Flyback will not shutdown because UCC28742 works is working on CCM (Constant Current Mode). CCM occurs when an overload situation is happening. The controller will lose regulation of the Output Voltage in order to keep switching and provide the overloaded current.

    -From your simulation results, Vout is going up and down (out of control) in order to keep Iocc at the load.

    -Section 7.3.4 of Datasheet explain this mode detailly.

    Thank you

  • 0 in reply to Manuel Alva Hernandez
    Prodigy 20 points

    Hi Manuel,

    Thank you for taking the time to answer my question!


    So my converter is behaving normally? Nothing wrong with it? Then why won't the IC shutdown after 120ms after Io=Iocc?

    And, what about the values for R_TL, R_opt, Rfb4, Rfb3 and Cfb. Are they chosen right? For example R_TL I chose by TRIAL AND ERROR by simulation until the converter worked. But Rfb4, Rfb3 and Cfb I left at default values, I didn't change them. Should I use other values?

    • shunt regulator used : TL431I
    • optocoupler : PS2811-1

    Thank you,
    Alexandru

  • 0 in reply to Alexandru Dalin Dragoi
    TI__Expert 8301 points

    Alexandru,

    -Nothing wrong with the converter. 

    -Ropt typical value is 1kohm in order to keep the minimum bias current for TL431 of 1mA at full load conditions.

    -Rfb4 is selected to improve the no load to full load transient events. Rfb3 is to improve the full load to no laod transient event and prevent the optocoupler transistor to be saturated. See below typical values.

    -Rtl can be calculated with the equation below:

    -I_CENL is the opto-transistor current at no load. This can be calculated by (VFB_max/Rfb4 + I_FB at no load (22uA)). VFB_max can be taken from Datasheet (see below).

    -CTR_NL can be estimated as slightly higher than 10%.

    -VOPT_NL can be taken from Datasheet of Optocoupler knowing the forward current of the optocoupler diode (I_CENL/CTR_NL).

    -Cfb is part of the Type II compensator and can be estimated by try ans error.

    Thank you