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UCC28600EVM: DC/DC Flyback step-up w/ no aux winding. How to determine components from Excel calculator

Chipset
Prodigy 70 points

Part Number: UCC28600EVM

Hello TI Community,

I am trying to accomplish the following: I need a QR flyback controller to take a Vin=48VDC and step it up to an isolated Vout=400VDC at 0.5A output current. I am experimenting with the UCC28600 evaluation board to accomplish this by hacking it apart and modifying it accordingly.

If you recommend a better QR controller than the UCC28600 suited for my application, please advise.

Thus far I modified the following on the EVM board to (try to) suit my needs:

- Replace transformer with another flyback transformer with NS/NP = 3.9 Lm=230uH, soldered OVP resistors directly to the primary winding, and changed both OVP resistor values (R27, R28) accordingly to achieve a desirable voltage at pin 7 (OVP) of the UCC28600 controller. This was successful.
- I am feeding the VDD of the UCC28600 straight from a 9V battery instead of aux. This was successful.
- Replaced secondary side feedback circuit components R36, D16, and C18 with a 9V battery, while adjusting R38, R39, and R40 to achieve Vout = 100VDC at open circuit (for now). Also success.

Right now I am having many problems with trying to attain a 100VDC output (eventually to be 400VDC) to maintain this constant voltage for purely resistive loads ranging from say 200ohms to 1kohm. I have had no success at all in this. I am currently experimenting with different values of R_CS and R_PL, which I understand is also a function of the OVP resistors as well. I am currently trying to use the slvc104j Excel calculator to assist in determining such appropriate values.

PROBLEM: The slvc104j calculator makes numerous assumptions including the use of an AC source, and the use of an AUX winding. What can I do to say "cheat" the calculator to match my situation at hand so that I can attain appropriate values R_CS, R_PL, and R_OVP1 and R_OVP2?

SIDE NOTE: Eventually my converter will be the DC input voltage to an isolated H bridge inverter.

Thank you for your help.

  • 0
    TI__Mastermind 27105 points
    Hi,

    Could you help me understand why you decided to use a flyback topology for this design spec? 200W output power is quite big for a QR flyback.

    Best Regards,
    Ben Lough
  • 0 in reply to Benjamin Lough
    Prodigy 70 points

    I need the primary and secondary isolated.

    Here is my critical design requirement : I need to take an input voltage of -48VDC wrt ground and convert it to a maximum of 100Vrms AC output with no DC offset wrt the same ground, with frequencies varying from almost DC to less than 10kHz. The design requires a maximum output AC current of 500mA at the 100Vrms. The load is mostly resistive and constant.

    My initial idea: Convert the -48VDC input to a 100*2*sqrt(2)=283VDC isolated voltage and feed that through an H-bridge to create an AC tone. Through previous trial and error experimentation I discovered I needed about 400V+ to generate a 100Vrms output. I ran into multiple problems leading me to try a QR controller IC with built-in feedback.

  • 0 in reply to Chipset
    TI__Mastermind 27105 points
    Hi,

    Thanks for the additional information and apologies for the long delay for my reply. At 200W, a single QR flyback design will be challenging to handle the peak currents in the power stage without going to great lengths (larger core, hardier FETs, etc). A better approach would be to use an isolated push pull converter to go from 48Vin to 400Vout. Generally, a single push pull converter can support up to 500W depending on the design whereas flyback you tend to be limited to ~100W unless you do interleaved flyback. Take a look at LM25037:www.ti.com/.../lm25037.pdf

    For reference here is some more info about the push pull topology:training.ti.com/topology-tutorial-what-push-pull

    Best Regards,
    Ben Lough
  • 0 in reply to Benjamin Lough
    Prodigy 70 points

    Thank you for your advice. Can you recommend any TI evaluation boards for isolated step-up converters similar to what I'm doing? Evaluation boards for either push-pull, interleaved flyback as you mentioned, but also half bridge, full bridge, and forward converters?

    To add, ideally I would want equal positive and negative rails given my application but this is an easier fix I can deal with later unless you have one ready. I suppose step-down eval boards should be ok too since I can always change out the transformer to my own, and change passive feedback values (assuming I don't run into unexpected problems like I had here..).

  • 0 in reply to Chipset
    TI__Mastermind 27105 points
    Hi,

    Whichever one you choose, it will require extensive rework to support as most of our DC/DC EVMs were not designed to have 400Vout. Here are a few that you might consider.

    Push pull: www.ti.com/.../toolssoftware

    Hard switched half bridge:www.ti.com/.../toolssoftware

    interleaved flyback:www.ti.com/.../lm5032eval (this is technically an interleaved boost EVM but you can use LM5032 as an interleaved flyback)

    Best Regards,
    Ben Lough