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UC3856: uc3856

SHIVA LEDALLA
Prodigy 10 points
Part Number: UC3856
Other Parts Discussed in Thread: UC3846

We are using UC3856 current mode control IC in  ac to dc converter.

a)      This IC is not able to drive the MOSFET, it can able source 120mA only, whereas  datasheet indicates peak 1.5amp. With external full bridge driver it can able to drive MOSFETs.

 

b)      Dead time and frequency are not matching with theoretical formula as per datasheet and interrelated. At switching frequency of 80kHz,

 

1 usec dead time is our requirement. Please suggest.

 

c)       When converter loaded,observed skipping of pluses. Provide design excel sheet of UC3856.

 

d)      Provide detailed application note / reference designs of UC3856 as these are not available on TI web.

  • 0
    TI__Genius 13660 points

    Hi  SHIVA LEDALLA,

    Please find the answer for your questions:

    a) This IC  has two 1.5 mA( peak) totem-pole output. it can driver the switching of power MOSFET directly. 

    b) Output dead time is determined by size of the external capacitor, CT on 8 pin, according to the formula: Td (us) = 2000* CT(uF) / ( 8 - 3.6 / RT(kohm)), For larger values of RT: Td (us)=250 * CT (uF).    RT is the resister on the 9 pin.         Oscillator frequency is approximated by the formula: FT (kHz)= 2 / (RT(kohm) * CT(uF)).

    For the required spec,  FT= 80 kHz, Td= 1us,  I recommend to use CT= 0.0039 uF,   RT= 6.34 k  (6.41 k).

    c) and d) Unfortunately, there is no calculator of UC3856 and reference design available.

       But we can find the typical application in UC3846 datasheet (Refer http://www.ti.com/lit/ds/symlink/uc3846.pdf).

    The UC3856 is a pin for pin compatible high performance bipolar version of the industry standard UC3846. It can replace the UC3846 with few circuit modifications and features increased speed, higher gate drive current and reduced propagation delays. The difference is located here for your reference.  

     http://www.ti.com/lit/an/slua178/slua178.pdf  

    Regards,

    Teng