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UCC29950: PFC +LLC Combo controller paralleling capability

chaitanyab
Intellectual 620 points
Part Number: UCC29950
Other Parts Discussed in Thread: UCC29002

..

Hi,

I am seeing UCC29950 is a very good PFC + LLC controller that can give > 90% AC to DC peak efficiency in some case even 94% and single IC operation. I am designing a power supply module for universal- AC input and  100W, 20V output. I have a paralleling requirement which means if needed user can parallel these 2 to 3 modules together to increase the overall power output. 

Now I am afraid that the LLC converter may not be easy to parallel. By literature, in general for LLC paralleling , it seems it needs sophisticated DSP to do paralleling for LLC, I don't know if the small analog IC can work with another converter in parallel. Can we parallel the UCC29950  based design without any major trouble? like Forward or flyback topologies?

  • 0
    TI__Genius 16155 points

    Hi Chaitanya,

    Thanks for reaching out to us.

    Please correct me if I misunderstood you.

    First of all, you want to connect multiple PFC+LLC converters in parallel. Am I correct?  If this is the case, you can use our load share controller at the output of each module such as UCC2900x, UCC39002 Advanced 8-Pin Load-Share Controller datasheet (Rev. I) (ti.com) to connect them in parallel.

    Second option you are looking for is, using one PFC converter and multiple LLC converters. If this is the case, you can use any of our CCM controllers (for a PFC power rating less than 300W, TM controllers are preferred. For more than 300W, CCM controllers are preferred ) (Power factor correction controllers | Products | TI.com) and a single LLC controller such as UCC25640x (UCC256402 data sheet, product information and support | TI.com) for every LLC converter. And then you can use load share controller at the output.

    You can not use a single controller for driving multiple converters.

    You can always connect other converters across the PFC DC bus. But make sure you are sizing the components according to the power delivered. Also, while paralleling different controllers, you also need to think about the auxiliary power for powering up the each controller.

    Please let me know if you have any further questions.

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    Intellectual 620 points

    ..

    Hi Manikanta,

    I think the first option is closer to what I am doing, I need to connect multiple PFC+LLC in parallel. Let's assume there are 2 blocks (like bricks) of PFC+LLC converters in parallel. Now each block has 1 PFC +LLC stage, this PFC +LLC single block can be further simplified using combo controller such as UCC29950 which can do PFC +LLC both together in a single chip option. Now the LLC's are inherently complicated to parallel as they are frequency controlled instead PWM or Phase shift. You can see the issue is described in Parallel LLC issues

    It states that: 

    "Unlike a conventional PWM topology, based on a fixed frequency and variable duty cycle control, a resonant converter uses the switching frequency (Fsw) as the main control parameter. The typical high gain of the resonant tank makes the resonant current (Ires) very sensitive to the operating frequency.

    When two resonant converters operate in parallel at the same frequency, any mis-match between the components of the two resonant tanks can generate a current imbalance in the two converters, which in the worst case can force all the current into one of the two phases, thus generating over-stress and most likely shut-down due to the Over Current Protection (OCP) mechanism."

    Considering this do you still recommend the load share controller approach? I think it suits for PWM based controllers and not frequency based resonant converter.

    Is there any way to synchronize the frequencies for UCC29950 or any other combo controller  ?

    Is there any application note or product showing paralleling of LLC resonant converters without microcontroller?

    PS: Apologies for putting literature from other semiconductor company, however it was relevant here.

  • 0 in reply to chaitanyab
    TI__Genius 9140 points

    Hi Chaitanyab,

    it is possible to use two UCC29950  controllers in parallel. 

    and you should add a diode on each output branch and sample the voltage before the diode. The aim is to make sure each UCC29950 can start up at any load with pre bias voltage on the system output(terminal connecting point of the two modules).

    since each UCC29950 works separately with different frequency, so there is no output ripple current cancellation.

  • 0 in reply to David Huang33
    Intellectual 620 points

    Thanks David perfect ! 

    Do you think the output paralleling diode can be replaced by an synchronous rectifier to avoid losses or to still meet my efficiency target ?

    Do you think adding the load share IC post this output paralleling diode will help further? or it is probably not needed anymore?

    And I assume feedback sampling points to go to its individual LLC controller IC / optocoupler respectively,  basically I don't want to share any internal signals outside as I in my case 2 power supplies(PFC+LLC) are essentially 2 products in parallel use.

    Do you think I still need to synchronize the switching frequency to avoid EMC issues?

  • 0 in reply to chaitanyab
    TI__Genius 9140 points

    Hi Chaitanyab,

    the output parallel diode can be changed to SR mosfet but please make sure the SR fets should be disabled at start up that means we should be able to have UCC29950 at any pre bias load startup capability.  

    of course, you need to take efficiency into consideration whether the output diode can meet your system's efficiency requirement.

    for a load share controller with UCC29002 system. you should also have load share bus(LS) line in the system. it is not possible without any other connection then two controllers can do  the load share.

    for two LLC controllers, if you need load share feature with UCC29002, it is not possible that you can do synchronize frequency. That is because for LLC you need to change the frequency to adjust the output voltage and achieve the load share.

    hope this helps you.

  • 0 in reply to David Huang33
    Intellectual 620 points

    Thanks David really helpful !!

    Just to be clear "for a load share controller with UCC29002 system. you should also have load share bus(LS) line in the system. it is not possible without any other connection then two controllers can do  the load share." here you mean they can't do a load share if there is no LS line correct?

    Do you know any evaluation kits for combo PFC + LLC controller? which I can buy? I am not seeing any for UCC29950, however kits for other part numbers closely matching will do too.

  • 0 in reply to chaitanyab
    TI__Genius 9140 points

    Hi Chaitanyab,

    yes, in the load share system the Load share bus line should connect from one module(UCC29950 output voltage) to another module(UCC29950 output voltage)  through the LS pin on UCC29002 controller.

    I suggest you can buy a UCC29950EVM for a design reference through TI.com.

    here I attached the guideline for the EVM.

    www.ti.com/.../sluub69a.pdf

  • 0 in reply to David Huang33
    Intellectual 620 points

    Hi David,

    I see the efficiency drops to 88% for 115V. I need > 90% efficiency, is it because of CCM PFC stage? Normally LLC have more than 94% efficiency so I think the loss is in the PFC stage. Why is this part using CCM PFC? 

    Is there any other controller that uses CrCM PFC and combo LLC? or >90% total eff at 115V?

  • 0 in reply to chaitanyab
    TI__Genius 9140 points

    Hi Chaitanyab,

    this is just a demo board a few years ago when UCC29950 released. There is not special care about the efficiency. it is easy to improve the efficiency from 88% to 90% by selecting some lower Rdson mosfets both on LLC and PFC.

    unfortunately TI still do not CRM+LLC combo chip at present.

    UCC29950 was designed to cover more power level , so at that moment CCM is selected.