• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Subscribers 64 subscribers
  • Views 211 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.

LM5156-Q1: Soft Start Issue

Mustafa Turhan
Prodigy 30 points
Part Number: LM5156-Q1
Other Parts Discussed in Thread: TL431, PMP10666, LM5156

Tool/software:

Dear TI Support Team,

I’m designing a flyback converter that outputs ±150V +-60mA, and while the overall circuit functions well in simulations, I’m having trouble with the soft-start function. It doesn't get activated for the first 1ms ( or untill the output voltage reaches 70V). When it activates it only works untill TL431 cathode voltage reaches 20V. I used a zener to clamp to cathode voltage at 20 volts. Soft-start function works better without the zener but TL431 cathode reaches 120 V so I need to use it. I think the problem is that when the voltage is clamped the optocoupler diode is no longer forward biased and there's no current in the transistor side so the compensation pin voltage is pulled high. I used LM5156EVM-FLY, PMP10666 and SNVC240 as reference. I used the excel document to calculate the feedback loop component values.

Thanks in advance for any advice!

Circuit parameters:

  • Input: 30 V
  • Output: ±150 V ±60 mA
  • Frequency: 300 kHz
  • Transformer turns NP:NS1:NS2  1:4:4
  • Peak Current Limit 2.2 A

Schematic:

Secondary Side Feedback loop:

Output Voltage:

TL431 Reference pin:

TL431 Cathode pin:

LM5156 Compensation pin

  • 0
    TI__Genius 12985 points

    Hi Mustafe,

    thanks for reaching out.

    Please try to use a PNP transistor between SS and COMP pin like shown below:

    Also you can try to increase the capacitor of the secondary side softstart circuit to around 1u.

    Best regards

    Moritz

  • 0 in reply to Moritz
    Prodigy 30 points

    Hi Moritz. Thanks for your reply. Adding a transistor between SS and COMP pins indeed worked as a soft-start circuit. I removed the SS network on the opto-coupler side and it still works. I don't know why SNVC240 and SNVAA44 documents says it wouldn't work. Maybe it works in simulations but won't work on real circuit. I'm kind of confused.

  • +1 in reply to Mustafa Turhan
    TI__Genius 12985 points

    Hi Mustafa,

    The solution with the PNP between SS and COMP is always recommended for flybacks. It avoids, that the device requests max dutycyle right in the beginning, when the FB is 0V. For SSR flybacks it is in addition recommended to use the secondary side softstart circuit. This will only have an impact in the very beginning when Vout starts ramping up. Thats why it can also work withou, but its still recommended. In a real life circuit you will most likely see the difference.

    Best regards

    Moritz

  • 0 in reply to Moritz
    Prodigy 30 points

    Hi Moritz,

    Thanks for the explanation. This cleared up my confusion.

    Have a nice day.