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UCC28742: Removing Auxiliary winding

reza kamalzadeh
Prodigy 50 points
Part Number: UCC28742


Hi
hope you be well

i want  to design a 10w DC-DC converter (Flyback topology) using UCC28742. since my input voltage is only 24v (for further information output is 5v) i was wondering if i could remove auxiliary winding and instead connect VDD pin directly to the input voltage. 

also instead of connecting VS pin to Auxiliary winding i want to connect it to Drain of the mosfet (of course through a resistor divider).is it possible to do that? is it practical?

  • 0
    TI__Mastermind 40580 points

    Hello Reza,

    Thank you for your interest in the UCC28742 flyback controller.

    Your proposed scheme has some concerns.

    1.  You may connect VDD directly to the 24-V input bus, however the maximum VDD turn-on threshold of the UCC28742 is 24.5V, which I exceeds the nominal bus voltage.  The typical value is lower, at 21.6V, but the low end of the 24-V bus may not exceed that. 
    To ensure start-up, you will need to add a small boost circuit to temporarily boost the bus voltage to 25V and apply that to the VDD.  After start-up, it can be disabled and bypassed with a diode. 
    The minimum level of the 24-V bus should always be greater than the maximum VDD turn-off threshold (8.3V). 
    The maximum level of the 24-V bus should not exceed the VDD absolute max rating of 38V.

    2. You cannot connect the primary drain voltage through a divider to VS input. The signal is in the wrong polarity.  The correct polarity is on the secondary output winding.  However, connecting to the secondary will circumvent input-to-output isolation.  If such isolation is not necessary, then you may connect the VS divider to the secondary winding.  Because Vout is so close to the internal OVP threshold voltage, the Rvs2 resistor value will be much lower than the Rvs1 value (instead of the typical ~4:1 ratio).    You can also feedback directly from the shunt regulator without an optocoupler.
    If input-to-output isolation is required, then you will need to keep the auxiliary winding and optocoupler.  

    Regards,
    Ulrich