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UCD3138: Multiple Interleaved Dual Active Bridge Converters with Phase Shedding

Neal Malackowski
Prodigy 40 points
Part Number: UCD3138

Tool/software:

Hello,

I'm looking for the "proper" digital controller to use for the application listed in the title (and described below) and was hoping to get more insight into whether or not the UCD3138 would be an adequate solution or if a C2000 series controller would be more appropriate. 

The converter would consist of interleaved, bidirectional dual active bridge converters each controlled by an individual UCD3138 or C2000 MCU and operating with SPS modulation. The converters will operate with a phase shift between them in order to reduce ripple current seen by the common load. Further, the converters should be able to intelligently "shed" phases as a function of load in order to keep system efficiency high.

Also, does the UCD3138 have the capability to implement Extended Phase Shift modulation?

Thanks

  • 0
    TI__Expert 8680 points

    Hello Neal,

    Thank you for your interest in the UCD3138x digital power controllers. By Dual Active Bridge (DAB), do you mean a topology like the diagram below? 

    The UCD3138x controllers can be a good fit for this design. There are 8 DPWM outputs available to drive each MOSFET (external gate drivers required). The UCD3138x controllers also have an Arm7 core that allows you to program any additional control algorithms that you want to implement. The UCD PSFB Firmware (https://www.ti.com/tool/UCD3138FW-PFC) and UCD FB-LLC Firmware (https://www.ti.com/tool/UCD3138FW-FBLLC) shows the complex control scheme that you can implement.  

    If by interleaved you mean twice as many MOSFETs, then you may need multiple UCD3138x controllers. These UCD3138x controllers can be synced up via UART, I2C or PMBus.

    I am not familiar with SPS modulation or Extended Phase Shift modulation. However, if you already know how to implement these algorithms in C code, then I do not think the UCD3138x controllers will have any limitations. 

    TI does have an existing DAB reference design w/ C2000 controllers: https://www.ti.com/tool/TIDA-010054 

    Regards,

    Jonathan Wong

  • 0 in reply to Jonathan Wong
    Prodigy 40 points

    Yes, that's exactly what I'm referring to.

    By interleaved, I'm referring to Parallel-In Parallel-Out (PIPO) connecting up to four of these individual converters with their own UCD3138x. Can they be synced up and then phase shifted with respect to one another? For example, if the primary controller is operating at zero degrees, the secondaries would be operating at 90 degrees, 180 degrees, and 270 degrees, lead/lag with respect to the primary.

    Single Phase Shift (SPS) modulation only has one degree of control freedom and that is the phase shift between the full bridges on either side of the DAB. Extended Phase Shift (EPS) modulation adds the duty cycles of the primary full bridge into the equation, so now there are two degrees of control freedom.

    Lastly, could you briefly explain when a design would necessitate using the C2000 as opposed to the UCD3138?

    Thanks Jonathan!

  • 0 in reply to Neal Malackowski
    TI__Expert 8680 points

    Hello Neal,

    I do not see any problems with interleaved connection. You can customize the phase with respect to each other as long as you use a communication bus to sync them together.

    For the Single Phase Shift, I think that is possible but it may depend on the number of Error ADCs (EAP) you need. The Error ADCs are the primary inputs into your PID filter. For example, in the UCD PSFB EVM (https://www.ti.com/tool/UCD3138PSFBEVM-027), there is a voltage loop, which is measured by EAP0, and there is a current loop, which is measured by EAP1. Each UCD3138x controller only has 3 EADC pins. If you are adding more loops into your control, then you may run out of EAP pins. I do not know exactly how the SPS will be implemented so the number of control loops may be a consideration. See the block diagram on page 25 of the UCD3138PSFBEVM-027 User's Guide:

    I will get back to you on the differences between UCD and C2x.

    Regards,

    Jonathan Wong

  • 0 in reply to Jonathan Wong
    TI__Expert 8680 points

    Hello Neal,

    Please see the Introduction to UCD3138x Devices (https://www.ti.com/lit/an/sluaas7/sluaas7.pdf). This answers the questions: What is the UCD and Why should you use the UCD?

    In summary, the UCD3138x controllers are digital controllers that are specifically optimized for power supplies. This is opposed to the C2x controllers, which are general purpose MCUs that are not limited to just power supply design. For example, look at Figure 7-6 on page 41 in the UCD3138A datasheet (https://www.ti.com/lit/ds/symlink/ucd3138a.pdf). Figure 7-6 depicts the UCD's PID filter, which is designed for power supply control.

    If you use a C2x controller, then you would need to write all the code that implements a PID filter. This includes manually writing code that does the multiplication, addition and clamping that you see in the diagram.

    The UCD controllers have all of the paths etched into the silicon itself. So, you do not need to code a PID filter since that is inherent in the UCD's internal hardware. The only things you can change are the values highlighted in blue, pink and green, such as the Kp coefficients. So, designing with the UCD controllers is much simpler since you do not need to program much of the control, and the response time can be faster since the signals travel through the silicon automatically.

    These values can also be changed live via the Fusion Studio GUI. The Fusion Studio GUI allows you to view the control loop and monitor status while your power supply is running. You can also change values, like the Kp coefficients, in real-time to see how changing the values affect the performance of your power supply. See the below screenshot from one of the GUI tabs in the PSFB topology:

    If you require a more general purpose MCU, then the C2x would be a better fit. The C2x also generally has more pin count and pin functionality, so you are not limited by the hardware. For example, the UCD controllers all have 3 Error ADC pins. If you are only using one Error ADC pin, then the other two Error ADC pins will be unused since their functionality cannot be changed. The C2x controllers can have much more customization with their pins so a pin is not limited to one function.

    Regards,

    Jonathan Wong

  • +1 in reply to Jonathan Wong
    TI__Expert 8680 points

    Hello Neal,

    We just published a new Technical Article "How real-time control technologies enable reliable, scalable high-voltage designs" (https://www.ti.com/lit/ta/ssztd36/ssztd36.pdf). This article provides a comparison between UCD vs. C2x and when to use each product.

    Regards,

    Jonathan Wong

  • 0 in reply to Jonathan Wong
    Prodigy 40 points

    Jonathan,

    Thanks so much for your detailed responses and the link to the new article, I greatly appreciate it!