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UCC28070: Is UCC28070 bridgeless parallel PFC modules a good design strategy ?

Don Williams
Prodigy 140 points
Part Number: UCC28070
Other Parts Discussed in Thread: UCC29002

John Griffin is doing a GREAT job...I am familiar with the conventional application for PFC controlling done by the excellent UCC28070, however I now have to design a failover module running at 3KW+

Last effort was a rush but did the job, BUT all three failover modules ran off the same bridge rectifier, which is not quite "pure failover" because the bridge rectfier might have a fail and the whole parallel structure smokes. Paralleling smaller bridge rectifiers makes for multiple return paths also and current loops which is  not good for EMC.

I am looking at the "bridgeless" config in SLUA517, sneaks another extra 1% or 2% efficiency, and has some virtue in regard parallel frontending to a common EMC filter because the second "passive" inductor during the OFF phase will have an inductor current discharging OUT into the filter and the diodes  Da & Db (to reduce CMode flapping about) are reverse biased due to CC mode continuous current.

SO...

Q1 do you think bridgeless is a good way to go generally?

Q2 do you think bridgeless provides a better "common" front end for paralleling PFC modules ?

Any references would be helpful John, 

Thks Heaps

DonW

  • 0
    TI__Mastermind 34465 points
    Hello Don,
    Thank you for such an excellent commendation for myself and for the UCC28070.
    The advantages of the semi-bridgeless topology are that higher efficiency can be achieved and this can be significant at low line voltages.
    Also there is no capacitor on the DC side of the bridge rectifier so crossover distortion is removed and you can expect to achiever better overall harmonic distortion. The disadvantage is that you will no longer have the benefits of interleaving and ripple reduction is not possible.
    A lot of the designs use this topology and I think you would need to look the requirements on a case by case basis.
    Your second question relates to the issues in regard to paralleling of semi-bridgeless modules.
    If you look at the Figures in the SLUA517 document you can see that the high frequency current is forced through the return diodes Da and Db because the boost inductor of the passive section will act as a high frequency impedance. Therefore the topology is not true "bridgeless" and that is why I emphasize the "semi-bridgeless" nomenclature. So there will be significant low frequency current (and power dissipation ) in the lower diodes.
    I think therefore the "paralelling" properties of a semi-bridgeless pfc will be equivalent to those of the conventional bridge type pfc.
    Also I should point out that the conventional method of bridge rectification will ensure that each section of the pfc delivers one half of the power.
    If you use the semi-bridgeless pfc then each section will need to deliver the full power for each half of the AC cycle. This means that you will get better power density with the conventional (interleaved) pfc.

    It is normal practice to parallel bridge rectifiers in order to achieve higher current capability. I have seen it done in many designs and they all share a common heatsink for the rectifiers. The real problem I think comes if you use two separate controllers in order to divide the power equally between 2 pfc stages. There is always going to be a small voltage difference on the output for each stage and the section with the highest output setting will wind up supplying the full power while the other section remains at standby. In order to implement this type of control you would need to use a 4 phase application as sketched in Figure 18 of the data sheet. You could also use the UCC29002 to force current sharing between modules.
    Those are about all the suggestions I can think of and I dont have any references on this subject.
    Hope this helps
    Regards
    John