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LM5020: How to calculate the compensation and feedback circuit component values

Devesh Agarwal
Prodigy 90 points
Part Number: LM5020

Greetings.

I am unable to calculate the compensation circuit. R5, C7, C6 in the product datasheet page 9. The local TI FAE was also unable to provide any inputs on how to calculate the values. WEBBench does not seem to support this product. The pSpice model gives an error during simulation pointing to the transformer.

Any help is greatly appreciated. We have an isolated flyback topology with various models. Common input and primary. Secondary output voltages vary by model 5V, 12V, 18V, 24V. Output wattage remains the same.

Thanks in advance.

  • 0
    TI__Mastermind 21720 points

    Hi Devesh,

    The LM5020 is compensated like any flyback controller. If the application is not isolated the internal error amplifier can be used. If the application needs isolation then an opto-coupler will be needed. If that is the case please take a look at this application note. Even though it uses a different controller it goes through all the math needed to come up with the correct compensation values.

    Thanks,

    Garrett

  • 0 in reply to Garrett Roecker
    Prodigy 90 points

    Garrett Roecker said:

    The LM5020 is compensated like any flyback controller. ............

    Thanks for your help Garrett. Will work on this. May I request if you have any recommendations on precautions I must use when designing with an open rank opto? Which would have a CTR from 50% to 600%, when compared to a more expensive Rank A 80% to 160%?

    Thanks in advance

  • 0 in reply to Devesh Agarwal
    TI__Mastermind 21720 points

    Hello Devesh,

    The CTR will proportionally affect the low frequency gain of the error amplifier transfer function (see the application note from my previous post).CTR is also function of bias current running through the LED of the optocoupler. If the bias current is well known then I don't think the CTR variation will be as large as 50% to 600%. Depending on the optocoupler this may or may not be specified.

    The other option is to ensure that stability is still achieved over the entire specified range of the optocoupler. The worst case will be a 600% when the gain is the highest. Compensation values could be selected here to help ensure stability. Selecting a Rank A device will just reduce this variation. 

    Thanks,

    Garrett