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UCC28070

ron f
Prodigy 130 points
Other Parts Discussed in Thread: UCC28070

I have a 4 phase pfc and I am having a problem. One of the channels from the same IC has a different duty ratio then the other so it is not sharing current (the current is discontinuous in that phase). All other 3 phase have continuous current. It is not that the inductors are different since the on time of that particular channel is much less than the other 3. In other words, the on time of GDA is different then GDB. So much so that the current is discontinuous

 

Any ideas why this may happen? All the component seem correct and there is no sign of instability

Please advise

  • 0
    TI__Guru* 85806 points

    If one channels duty cycle is off there is something causing that gate drive signal to terminate early.  I would check the current amplifier outputs to make sure they are all identical. 

    Discontinuouse inductor current has caused the current amplifier to see a false current sense signal.  This could cause a misbehavior in the single phase.  To remove this issue just requires a PWM offset and a PWM ramp at the CSX inputs.  This is covered in pages 8 through 10 of the application note.  http://www.ti.com/litv/pdf/slua479b

     

  • 0 in reply to Mike O'
    Expert 1260 points

    Mike,

    we plan designing 2 phase interleave PFC using UCC28070( let say phase A and phase B), we disconect phase A when 50% load detect, if load increase imedieatly from 50% to 100%, how fast phase A come back to operate? I worry that if phase A come back slowly then phase B take over load until phase A fully opration.  

  • 0 in reply to Knighty Nimp
    TI__Guru* 85806 points

    The current loops generally cross over at 1/10 of the switching frequency.  The voltage loop generally crosses over between 10 to 20 Hz.

    However the size of the inductor (V=Ldi/dt) is the major limiting factor on how far each phase will change current.  If all you do is turn off a single phase and know the current you can calculate how long it will take to de-energize.  The approximation should be the same for energizing.  You will find this time to be quite large.

    The multiplier output is controlled by the voltage amplifier and this signal is fed into the current amplifiers.  This voltage moves slowly and the fastest it can respond to a small signal change is 50 to 100ms.  However, one phase could take more current while the second phase is energizing or de-energizing than you want it to.   To over come this some engineers modified the Rimo impendence.  They switch in a second Rimo resistor equal in value to the first.  In single phase operation they switch the second resistor out.  What this does is if you turn off one phase for a given power level the Rimo voltage doubles and it should because it is now taking all of the power instead of half.  When you go into two phase operation and you switch in the second Rimo resistor it half’s the voltage across Rimo.  This gives time for the second phase to catch up to the first phase that has been handling all of the power.

     Regards,

    Mike