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Opto Coupler usage in TPS61175 flyback SMPS

Neil Yu
Prodigy 70 points
Other Parts Discussed in Thread: TPS61175, TL431, LM5021-Q1

Hello,

I found a typical application about TPS61175 in flyback SMPS. Here is the link and schematic.

www.ti.com/.../getliterature.tsp

My question is that:

1) According to the datasheet of TCMT1107, it's CTR is 0.8~1.6. However, this typical application uses CTR as 0.1. How does this work ?

2) According to the application note, the source of the BJT in TCMT1107 is from TPS61175, a current source of 130uA. I am wondering how this source drive the BJT in TCMT1107. In other words, how does the BJT work with TPS61175?

3) I found something wrong with the pole and zero calculated in the application note. For example, the pole designed to help negate the limited bandwidth of the optocoupler. I calculated it as follows:

fz=1/(2*pi*R2*C7)=14.5k Hz

The result is different from that calculated in the application note, which is 30k Hz ! I don't know why.

4) Is there any other application note about the application, where the optocoupler is connected directly to the "COMP" pin without a pullup resistor and a voltage source?

  • 0
    TI__Mastermind 40080 points

    Dear Sir,


    I'm not the writer of this application note. But here I'll give my reply from my point of view: This kind of connection is not a good connection for both opto-coupler and TL431
    1) Its hard to say the 130uA source can't drive the BJT in TCMT1107. If the primary current flowing through the TCMT1107's diode is small, then the seconday BJT can still work quite well.
    2) You are right, fz=1/(2*pi*R2*C7)=14.5k Hz instead of 30KHz,. I think the author may adjust the parameter finally;
    3) Another application is SLVA496, it delete TL431 by adding one more windiing.  You can also check  the lm5021-q1 datasheet, it has a pull up resistor internally.

    Could you please share your  application inforamtion with us? Like in what kind of equiment? the input and output requirement?

    My email addresss is helen-chen@ti.com


    BR,Helen

  • 0 in reply to Helen chen
    TI__Mastermind 40080 points

    Neil,

    Be sure the input voltage is ≤18V, then this IC is OK for this usage.

     

    The connection in the www.ti.com/.../getliterature.tsp is not good. The TPS61175’s COM pin has very weak current ability. So you need to add a pull up voltage at the BJT side to ensure the system works well under all conditions.

     

    Since this is an Aux power, so I think the voltage adjustment is not a problem. 1-2V tolerance doesn’t matter, right?

    If you didn’t want to add a pull up voltage at the BJT side, then add more one winding is a better solution. Making the output winding has a good coupling with this winding .

    BR,

    Helen