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Original question:

LM5156: Is it possible to design a single output using PSR method in LM5156?

LM5156: Average Current Mode Control Design

Bailey Sauter
Prodigy 30 points
Part Number: LM5156
Other Parts Discussed in Thread: AMC3302, , PMP22339

Tool/software:

I am designing a flyback converter and would like for the control loop to primarily regulate a constant output current to a pre-biased load bus. Therefore, my design idea is to place a sense resistor between the converter output and the load, sense the drop across this resistor with an isolated differential amplifier (to maintain isolation across the flyback, such as the AMC3302), and feed this voltage back to the feedback pin of the LM5156 using a voltage divider to tune the desired setpoint and an op-amp to make the differential output single-ended. 

I have a few questions based on this: 

1. Are there any obvious issues with this design idea? 

2. Perhaps there is a better IC for this application? I am aware that there exist some CC/CV PSR ICs, but this design seems like it would generate more accurate regulation at light load and eliminate the need for an auxiliary winding on the flyback transformer.

3. Is there a way to combine the ACM control loop with a voltage control loop that shuts off the converter in the case of overvoltage on the output? 

3. Why does the LM5156 datasheet show the compensator components connected from COMP to GND rather than COMP to FB as I am accustomed to seeing? 

Thanks for any help that can be provided.

  • 0
    TI__Genius 15855 points

    Hi Bailey,

    Thanks for reaching out.

    1. When using external amplifiers for the regulation loop, you should connect the FB pin to GND and directly connect your external circuit to to the COMP pin. With this you disable the internal error amplifier. As reference you can have a look at the design, that use LM5156 in flyback configuration with an optocoupler. The EVM for example. https://www.ti.com/tool/LM5156HEVM-FLY

    2. There are some dedicated RSR controllers, however i am not sure if they have any advantage in terms of CC. Yes, with the design you plan to do, there would be no need for an AUX winding. But the same is the case when you go for the traditional optocoupler SSR solution.

    3. You can combine voltage and current loop with a Or-ing circuit. As a reference, please have a look at PMP22339 reference design https://www.ti.com/tool/PMP22339 It is a SEPIC charger and uses this kind of regulation loops.

    4. The reason for this is, that the internal error amplifier is an OTA (current output). This makes it possible to connect the COMP components simply to GND. Another advantage is, that the absoulte vaules of the FB resistor divider does not play a role anymore in the compensation (only the ratio).

    When the internal error amplifier is a normal opamp, COMP is connected to FB.

    Please keep in mind, that when you feed the signal directly into COMP pin, the compensation has to be done at your external error amplifiers.

    Best regards

    Moritz