• State TI Thinks Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 2 answers
  • Subscribers 64 subscribers
  • Views 84 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM5157: Flyback Transformer Selection

c p
Prodigy 20 points
Part Number: LM5157
Other Parts Discussed in Thread: TPS22950, LM5155, LM5156

Tool/software:

Hello, I would like to use the LM5157, with primary side regulation for lower part count, at ~1.5A-2A and 5.5-6V output. I would like galvanic isolation as a safety precaution, while also allowing interaction between digital signals on this isolated reference to high-z inputs on a non-isolated reference. I have a few questions:

1) Webbench doesn't recommend a specific transformer, just a winding ratio. Using the design app note for this part, I found these target specifications:

  • Ns1:Np ~ 0.813 turn ratio (pri to isolated output)
  • Ns2:Np ~ 1.317 turn ratio (pri to AUX supply)
  • L primary ~4.7uH to 15uH
  • Isat primary >3A

Can you comment on using any the following off the shelf parts? Caveats, warnings, recommendations would be appreciated:

  • Wurth 750311342
  • Wurth 750311567
  • Pulse Elec. PAT6261.005NLT  

2) Is there a TI TINA or SPICE model for this part? I found a 2-year old post on E2E saying the part model doesn't exist and found no other useful information yet.

3) Regarding interaction between MCU and parts on this flyback reference, an example is an MCU digital output on non-isolated system ground sending a 5V enable signal to a load switch (like the TPS22950) connected to the flyback's isolated ground and output voltage. Can I tie the isolated GND to non-isolated GND through a large resistance (499kOhm for example) to maintain correct logic levels as well as isolation, avoid the references drifting too far apart (0V to 0V)? I don't expect a large amount of current between GNDs, as all interaction is through very high impedance. I am trying to avoid adding many digital isolators, opto, cpacitive or SiO2 to the design. Any strong recommendations here?

Thank you

  • 0
    TI__Genius 11095 points

    Hi cp,

    Thanks for reaching out.

    1. In general they are all suitable most likely. You can check which one has the best coupling, as this is an important parameter for flyback transformers.
    If you already have your transformer specs, you can reach out directly to the vendor to help you with the decision.

    2. https://www.ti.com/product/de-de/LM5157#design-tools-simulation

    3. If you use a resistor between the 2 grounds, the isolation is gone.
    I think you will have to put one of the mentioned parts if you want to keep the isolation. If not, you can use a resistor between the grounds.

    Best regards

    Moritz

  • 0 in reply to Moritz
    Prodigy 20 points

    Hello again,

    I'm looking at using the Wurth 750311342 with 2:1:2 winding ratio, 15uH transformer, and following this design guide for PSR with following targets:

    • 20V input (Vin min supply = 12V)
    • 6V output (only 1 output)
    • 2A abs max, 1.5A realistic max, nominal or standby = 0.6A
    • 250kHz Fsw

    Moritz: any comment on coupling of this transformer?

    Web bench recommends a transformer with:

    • Ns1 to Np = 0.813
    • Ns2 to Np = 1.317 (aux to VCC)
    • Lp = 5.13uH

    Which produces ~10V on the VCC pin

    However, I have calculated the following assuming I can use the above mentioned Wurth part, using the 2:2 winding for AUX to VCC and the 2:1 ratio for the output voltage:

    • D=0.375 duty cycle, (0.50 max if Vin min ever drops to 12V)
    • Vaux = VCC = 12V
    • Feedback divider values, Rtop = 22k, Rbot = 2k Ohm for 1.0V at FB pin from 12V at 'aux' net
    • ideal transformer inductance of 26.7uH, but because I'm using 15uH and expect P_out_max=9W:
      • delta ILM = 1.6A
      • IL Peak = 1.5A

    1. Is someone able to quickly sanity check these values to ensure I'm not making incorrect assumptions? 
    2. The datasheet states VCC to AGND abs max = 5.8V and states this pin is an output. Although the design guide and web bench use it as an input with 10V applied (this is not obvious and should be called out somewhere I think), I cannot see anywhere stating the max voltage for this pin. I see this is the supply for the internal FET gate drive, and I assume 12V is okay as this is a pretty standard gate drive voltage? Please let me know where in datasheet to find Vin max for the VCC pin. I assume it's fine with up to 18V because this is the abs max for the PGOOD pin, and VCC is used to pull this up in the web bench template...
    3. I have seen some designs where VCC is supplied by a transformer winding (AUX) and another in the design guide where it is likely only used as an output (looks to be internally regulated from the voltage applied to the BIAS pin): is there a recommendation here to supply VCC or leave it as internally regulated. Assuming this just decreases FET drive voltage and slightly increases losses of internal switch node due to higher Rdson? I could just use a bypass cap/resistor on VCC, and send the AUX voltage to FB pin only for PSR?

    I'm encountering issues getting the PSPICE simulations to run, but will order a flyback eval board for this part to start early testing

  • 0 in reply to c p
    TI__Genius 11095 points

    Hi c p

    The transformer seems to be a good fit for your application.

    c p said:
    any comment on coupling of this transformer?

    For a flyback, the better the coupling, the better. Especially for a PSR flyback. So 99% would be good.

    1. Looks good.

    2. According to the datasheet, the lifetime of the device is reduced if the applied voltage is higher than 5.8V. So i would recommend not to connect the 12V to the VCC and just use the gatedrive voltage providedb the internal VC regulator. In the controllers like LM5155 or LM5156, it is possible to ally voltages up to 16V to this pin.

    3.

    c p said:
    is there a recommendation here to supply VCC or leave it as internally regulated.

    In your case i recommend to leave it internal regulated nd just connect the resistor and capacitor as shown in the datasheet. Connecting a higher voltage to VCC can improve the efficiency for higher input voltages, as the internal LDO is then deactivated.

    Also as you said, you could drive other FETs with a higher voltage.

    c p said:
    I could just use a bypass cap/resistor on VCC, and send the AUX voltage to FB pin only for PSR?

    This is what i would recommend.

    Best regards

    Moritz