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UCC28600: Feedback loop performance with opto

Devendra Patel
Prodigy 50 points
Part Number: UCC28600
Other Parts Discussed in Thread: LM5021, PMP10215, UCC28730, UCC24650, UCC28610, LM5023

I am trying to develop concept of about 200W power supply using webench.

I have below query on schematic:

1. For the feedback loop with isolation, it is using opto coupler. With this kind of FB how will be regulation under normal load and load switching?

2. Can synchronous rectifier  be used instead of diode to improve efficiency. If yes, please suggest IC and schematic for reference.

  • 0
    TI__Mastermind 40120 points

    Hello Devendra,

     

    Thank you for your interest in the UCC28600 QR-Flyback controller.

     

    From the schematic that you presented, I presume the output voltage is ~15V based on the feedback divider resistors Rfbt and Rfbb and assuming 2.5V for the VR reference. For an example of transient response, the closest reference design that I could find is PMP5313, which is 14V at 90W. This design is found at http://www.ti.com/tool/PMP5315#technicaldocuments, and the test report (http://www.ti.com/lit/ug/slur128/slur128.pdf) shows transient responses on page 9 for 2 levels of input voltage. Given the ringing, this loop is underdamped and could be improved with additional work on the compensation network. However, it does provide an example of the typical kind of response time and bandwidth that you can expect through the opto-coupled feedback.

     

    The output diode can be replaced by an SR-MOSFET, either in the high-side of the transformer winding (where the diode is now) or in the low-side of the winding. The low-side makes it much easier to power the SR controller and bias the SR-Fet. I suggest to use the UCC24612-2 SR controller (see http://www.ti.com/product/UCC24612 for datasheet and other design help).

     

    Regards,

    Ulrich

  • 0 in reply to Ulrich Goerke
    Prodigy 50 points

    Hi Ulrich,

    Thanks for your response.

    Yes right, output voltage is 15V.

    Is there any better flyback controller option for 200W output and input DC range of 60V to 160V.

    Can you please share reference design document for these kind of flybcak.

    Regards,

    Devendra

  • 0 in reply to Devendra Patel
    TI__Mastermind 40120 points

    Hello Devendra,

    The term "better" is relative and dependent on which aspect(s) you wish to emphasize.  Since the original title of this thread mentions "feedback loop performance", I assume you wish for better loop response compared to what was presented in the PMP5313 documentation.  As I mentioned before, additional work on optimizing the feedback components can improve the response.

    In any case, 200W is pushing the top end of the practicable power range for most flyback converters.  There are few flyback reference designs near this power level.  Most controllers are design to directly drive the MOSFET, but have limited capability based on the usual expected power.  At 200W, the size of the MOSFET becomes quite large such that direct drive is usually insufficient.  In this reference design,  http://www.ti.com/tool/PMP10215 the power stage is a 2-transistor arrangement with a high-voltage dual-driver IC to handle the gate-charge load.  There are several other outputs and a PFC stage that you may not be interested in.  Simply disregard them; the point of this reference design is to illustrate what is needed for a flyback controller to manage a 200-W power throughput. The LM5021 is capable of operating in CCM, which helps to reduce the peak current needed to achieve 200W.

    Your DC input range of 60V - 160V is similar to a rectified AC input of 80Vac ~ 133Vac.  Most reference designs are targeted for the "universal" 85Vac~2654Vac range.  It is unlikely that there is a pre-designed circuit very close to your application description, so this reference should be used for just that: a reference for what may be needed, not a direct design that you can copy part for part.  The PMP10215 transformer design, for example, will not be suitable for your input range.  I recommend that you follow the design procedure in the LM5021 datasheet to obtain the proper circuit parameters based on your input and output constraints.  

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Prodigy 50 points

    Hello Ulrich,

    Thanks for your quick response.

    I was focusing on design so forgot about title of thread and asked you for better design.

    By better design I mean in terms of cost, efficiency and transient response.

    If 200W is becoming very high for flyback then I can change the loads and make it 100W.

    Regards,

    Devendra

  • 0 in reply to Devendra Patel
    TI__Mastermind 40120 points

    Hello Devendra,

     

    This is a difficult question to answer since it there are too many variables to consider. All design is based on making tradeoffs between conflicting design targets. Usually, there is a fixed set of input and output constraints that must be met, and one searches for a conversion topology (and a controller for that topology) that works best within those constraints. In this case, you are changing the constraints to best fit the topology (flyback). If you reduce the power level further, a wider array of flyback controller options opens up, mostly based on lower overall cost.

     

    However cost, efficiency, and transient response all tradeoff against each other and you need to find the appropriate balance between them (along with numerous other attributes, such as size, manufacturability, temperature rise, etc.). Primary-side regulation (PSR) controllers reduce cost by eliminating the opto-coupler and secondary-side shunt regulator but generally sacrifice transient response (ref: UCC287xx series), unless a sizable pre-load is maintained. An exception is the UCC28730 controller plus the UCC24650 “wake-up” IC which can achieve very good step response from no-load without significant pre-load. But PSR’s MOSFET drive capabilities are limited and may need a gate-drive buffer for power levels higher than 50~60W.

     

    Opto-coupled feedback regulators, such as the UCC28600, UCC28610, LM5021 and LM5023 can stretch faster response into the higher power levels. But these are generalities and can lead to endless tradeoff considerations with all the possible variables involved. And there can be exceptions to everything.

     

    “Better” is a comparison and value judgment between two alternatives and “Best” between three or more alternatives. Better or Best cannot be decided until two or more alternatives are fully, or at least substantially, designed and fixed numbers for all target goals are needed to do that. This forum is not really intended for philosophical discussions about what is better than something else.


    But to not leave you empty-handed, I suggest that the LM5021 is still most likely to provide decent efficiency and transient response at a reasonable cost for power levels between 100-200W at inputs from 60-160Vdc.  

     

    Regards,
    Ulrich