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UCC28065: The Ton difference between UCC28065 and UCC28063

Jack Li-SH
Expert 4455 points
Part Number: UCC28065
Other Parts Discussed in Thread: UCC28063, UCC28064A

Tool/software:

Dear sir,

Customer:  Delta Lighting time and  Outside Lighting application.

At the input voltage zero-crossing, there is a significant difference in the Ton time between UCC28065 and UCC28063.

 Figure 1: CH1:Vin CH2:Iin  CH3:Vgs_Q1 CH3:Vgs_Q2   

Figure 2: CH1:Vcomp CH2:Iin  CH3:Vin CH3:Vgs_Q2  

Figure 2 shows that the Ton time of UCC28063 at the zero-crossing point exceeds 20us, while Figure 1 shows the Ton time of UCC28065 at the zero-crossing point is approximately 7us.

Figure 3: CH1:Vin CH2:Iin  CH3:Vgs_Q1 CH3:Vgs_Q2

Figure 3 demonstrates that after changing the external resistor of the TSET pin of UCC28065 from 140Kohm to 75Kohm, the Ton time did not increase significantly.

 This part has a very large impact on THDi.  Whether there are any other peripheral parameters that can influence the Ton time in this section? Or is there inherently a difference in the Ton time at the zero-crossing point between UCC28065 and UCC28063?

Nice day!

Regards,

Jack

  • +1
    TI__Mastermind 40580 points

    Hello Jack, 

    There is a design change between the UCC28063 and the UCC28065 that accounts for the wide "flat-spot" in the current around the zero-crossing. 
     

    In the UCC28063, there is an Additional On-Time feature that widens the on-time when VINAC is near the zero-crossing. 

    The purpose is to reduce distortion of AC current around the zero-crossing. 
    The UCC28064A has a similar feature. 

    This feature has been removed in the UCC28065 controller. 
    Why?  Well the feature worked well for ow-line inputs (85~140Vac), and usually worked okay for high line (170~265Vac) but at higher switching frequencies, it added too much on-time and distortion actually increased, rather than decreased.  AN external circuit was used to clamp the VINAC input voltage above 0V to mitigate this issue, and the UCC28064A datasheet explicitly discusses this technique.

    Since the UCC28065 is intended to allow higher frequencies than the UCC28063 and UCC28064A can achieve, it was decided to remove the feature to avoid causing even more THDi.  For most PFC applications, the THDi that these flat spots introduce is within the requirements. 
    For lighting applications, THDi may be more stringent and it is possible that the UCC28065 may not be suitable for some of these applications.   

    A fixed TSET resistor value should not be used to influence the zero-crossing on time since TSET determines overall output power level. 
    It may be possible to construct a special circuit that varies Rtset value dynamically to allow wider on-time only when VINAC voltage nears 0V.  
    The designer will have to ensure that that circuit will not be susceptible to noise nor inject noise into TSET. 

    Regards,
    Ulrich