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PMP7760: Design Questions

Maor Matityahu
Intellectual 340 points

Part Number: PMP7760

Hello,

I have some questions regarding the PMP7760 development circuit:

1) How do i choose the maximum duty cycle? the maximum duty cycle of LM5022 is 0.90, can i choose for the minimum input voltage (lets say 12V in my design) duty cycle of 0.89? or there is an equistion to calcultae the maximum duty cycle? what will be than the minimum duty cycle at the maximum input voltage?

2) How do i calculate the secondary widing turns? can i use the formula (Vo+Vdiode)*Np/Vinmin ?

3) The design is CCM or DCM?

4) How do i calculate R106 if i want minimum input voltage of 12V?

Thank you!

  • 0
    TI__Genius 17700 points

    Hi Maor,

    1) The maximum duty cycle is determined by the designer. I recommend you choose a duty cycle less than 50%, because it will remove the need to add slope compensation to account for subharmonic oscillation, which will happen at duty cycles higher than 50% in peak current mode control topologies.

    http://www.ti.com/lit/an/snva866/snva866.pdf is a good application note that will explain more.

    2) You can use the equation: 

    Vo/Vin_min = (N_sec)/(N_pri) * (D_max/(1-D_max)), and solve for N_sec. The note above also utilizes this method to solve for the turns ratio between the secondary and primary.

    3) CCM or DCM is decided by the designer. This article explains the difference well: http://e2e.ti.com/cfs-file/__key/communityserver-discussions-components-files/188/5635.Under-the-Hood-of-Flyback-SMPS-Designs.pdf

    4) R106 is used as a way to make sure the VIN pin does not exceed its maximum rated voltage. If your input is 12V, what will happen is that the auxiliary winding will be supplying a voltage to VIN, which will supply the internal regulator. If the input is higher than D100, Q100 and Q101 would both turn on, which would pull VIN down to an appropriate voltage level until the auxiliary winding will reverse bias Q100.

    Thanks,

    Richard

  • 0 in reply to Richard Fu22
    Intellectual 340 points

    Hello Richard,

    Thank you for the answers!

    i have some questions regarding to your answers:

    2) This formula is for CCM only? can i use this formula if i work in Transition mode at the maximum output current and in DCM for lower output currents?

    if i work in DCM mode (not Transition Mode) in the maximum load i cannot use this formula?

    3) Can i plan my design to work in DCM mode with all the input voltages and output current range?

    4) A) Why R106 limits the voltage at Vin and not D100?

        B) In th PMP7760 design, how to calculate R106? Do i need to calculate him so at the minimum input voltage (30V) the bias current to the IC will be above the minimum start up current of the IC? what is the minimum bias current to start the IC?

    Thank you!

  • 0 in reply to Maor Matityahu
    TI__Genius 17700 points

    Hi Maor,

    Sorry for the delay.

    The above equation is for CCM only. You can use this formula as long as your average inductor current is larger than half of the peak-peak ripple.

    The form is different for DCM.

    Please refer to this thread for the transfer function: https://e2e.ti.com/support/power-management/f/196/t/633998?TINA-Spice-UC2843AQ-DC-DC-DCM-Flyback-converter-power-stage-transfer-function

    DCM is simply a operating region for a dc-dc converter. A converter can both be in CCM and DCM. It enters DCM mode when the average inductor current is less than half the peak-peak inductor current. In this case, you can certainty plan your design to work in DCM mode. 

    I think you would need to calculate it such that the current going through Q100 does not damage the FET upon startup. After the output hits the regulation target, Q100 should turn off. 

    Thanks,

    Richard

  • 0 in reply to Richard Fu22
    Intellectual 340 points

    Hello Richard,

    So if i work in maximum load at the boundary between CCM and DCM i can use this formula to find the secondary windings?

    Thank you

  • 0 in reply to Maor Matityahu
    TI__Genius 17700 points

    Hi Maor,

    Usually you would pick one operation mode or another and design your power stage around that operation mode for all input and output conditions. 

    As you change the load, the converter will change in operating mode from CCM to DCM (or the other direction). 

    Is there a specific reason you would like to work in maximum load at boundary between CCM and DCM? There usually is not a big difference in load that will cause a converter to go from operating in CCM to DCM. 

    I'm afraid it will result in a larger design effort then to simply design in one area because you would need to select the worse-case scenario by checking the turns ratio for both CCM and DCM. In DCM, the transfer function is load dependent; which will add another worse-case factor for you. In CCM, you will not need to account for the load, but there are other trade-offs such as the inductor being bigger and presence of the RHPZ.  

    Thanks,

    Richard