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UCC3800: How to design a UCC3800-based open-loop fixed-cycle flyback power supply

zoujiangyilang
Genius 4560 points
Part Number: UCC3800
Other Parts Discussed in Thread: UCC2801, UCC2805, UCC2804, UCC2802, UCC2800

Dear seniors,

I want to use UCC3800 to make a 2W flyback power supply.

Because the input voltage range is narrow, about 11.5V~12.5V, and the output voltage is 14V~16V, the accuracy is not high.

However, due to the small size requirements of the power supply, the efficiency requirements are high.

Therefore, I want to make a flyback power supply with a fixed duty cycle.

I looked at the UCC3800 application manual and the schematic diagram of the flyback power supply with closed-loop optocoupler feedback.

How can UCC3800 be wired to achieve open loop duty cycle, preferably the maximum bearer ratio output?

Is there a corresponding schematic or application documentation to see?

Thank you.

  • 0
    TI__Genius 13660 points
    Hi zoujiangyiliang,

    I think that UCC3800 can work with a fixed duty cycle as following method.
    1. Disconnect CS pin and MOSFET source current sense resistor.
    2. Add a constant voltage at COMP Pin (viz. constant voltage at non-inverting input of PWM comparator) with two divider resisters from VREF.
    3. Obtain a constant slope voltage from RC Pin with two divider resisters (note that these two resisters must be much lager than the resister RT at RC Pin). Connect this constant slope voltage to CS Pin to replace current sense resister voltage to terminate PWM comparator. Once the slope voltage at CS pin exceeds the constant voltage at non-inverting input of PWM comparator, PWM terminates. So we can regulate the duty cycle of UCC3800 via regulating the slope of CS pin voltage.

    Regards,
    Teng
  • 0
    TI__Mastermind 34465 points
    Hello zoujiangyilang,
    It is easy enough to run the UCC3800 open loop at maximum duty cycle.
    If you connect the FB pin to GND then the COMP pin will go to its maximum value and the duty cycle is maximum.
    For a flyback application you will need to use the UCC2801, UCC2804 or the UCC2805 which have a DMAX = 50%
    The UCC2800 and the UCC2802 have a DMAX= 100%
    If you need a duty cycle less than DMAX then all you need is to adjust the voltage on FB.
    0V corresponds to DMAX (50% or 100% depending on the part number)
    2.5V corresponds to DMIN (0%)

    Regards
    John
  • 0 in reply to Teng Feng
    Genius 4560 points
    Hello, Teng，

    thank you for your advice.
    Must the ramp voltage be introduced at the CS side?
    The following John did not mention the CS side.

    Regards,
    ZJYL
  • 0 in reply to John Griffin1
    Genius 4560 points
    Hi John,

    Thank you for your advice.
    Excuse me, can the flyback power supply work in a fixed duty cycle open loop?
    For example, at 0.9 duty cycle.
    When there is no load, will there be transformer saturation or a surge in output voltage?

    Regards
    ZJYL
  • 0 in reply to zoujiangyilang
    TI__Genius 13660 points

    Hi zoujiangyilang,

    This is my thought to realize a fixed duty cycle in an open loop and I think it can work. For my last response, it may be easier that firstly give a constant slope voltage at CS pin, then regulate the COMP Pin voltage to get the duty cycle that you want.
    Because I never let flyback work with a fixed duty cycle in an open loop. and I think there is a large risk because the load is variable. It cannot stabilize the output voltage with a fixed duty cycle. If the load is light, the output voltage will go high then damage the output capacitors and diode.


    Thanks for John's suggestion, it is a better approach. It not only can realize a fixed duty cycle more easier, but also still has the CBC over current protection function because of the existence of current loop compared with my method.

    Regards,
    Teng