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Original question:

TPS92518: Dimming ration conditions

TPS92518: ShuntFET Dimming

Terry Wu2
Expert 2110 points
Part Number: TPS92518


Hello Sirs,

We use ShuntFET dimming try tps92518 EVM and find low current will have flick issue .

Find the first current cycle waveform will be different . Detail as attach file .

May I have your comment ?  Is it reasonable ?

We need to implement 10,000 : 1 ratio and won't have flick issue .

Many thanks !Flicker issue.xlsx

  • 0
    TI__Mastermind 25500 points

    Hello Terry,

    Have you adjusted your max toff to ensure that the inductor current ripple is the same when the shunt FET is on as it is when the LEDs are on? This needs to be tuned to ensure each cycle starts at the same inductor current. The best way to do that is to match the two inductor ripple currents.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Expert 2110 points

    6507.Flicker issue.xlsxHello Clint,

    Thanks your suggestion !

    Try to let optimize inductor peak current same as Hysteretic operation current .

    Also have flicker problem . The LED current waveform still have random fluctuation .

    Waveform as attach file "Optimum Maxoff" sheet.

    Is there anyother suggestions ? TKS !

  • 0 in reply to Terry Wu2
    TI__Mastermind 25500 points

    Hello Terry,

    Are you using the EVM and its BOM? Do you have any output capacitance? Also, what is the length of wire between the dimming FET and the LED load? Parasitics can cause this type of thing if there are long leads between the FET and the LEDs. In that case an RC snubber across the FET may be required. Have you tried a snubber?

    I will have somebody else take a look at this and see if they have seen it before. But if you do have leads between the FET and the LEDs it can cause serious voltage spikes that can break down the FET or cause it to turn on/off briefly when it isn't supposed to during transients.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Expert 2110 points

    Hello Clint,

    Many thanks your comment !

    We use tps92518 EVM and follow EVM user guide Fig.12 to add shuntFET components. (No output Cap for LED )

    We could try to add RC snubber for around 15 cm wiring between EVM to LED .

    Vin is 12V , LED configuration is  6V

    We'll try to reduce ripple current and increase PWM frequency .

    Pls also provide suggestions if any. Many thanks ~

  • 0 in reply to Terry Wu2
    TI__Mastermind 25500 points

    Hello Terry,

    I have learned a bit more and it seems like this is a possible scenario. If the shunt FET turns off during the 515 switch off time there may be some delay in terminating the off time (which is why the inductor current sloped increases drastically downward). However in most cases it is cycle by cycle and doesn't result in flicker.

    However parasitics can cause it to behave differently. I still think the RC snubber is good because it will clean it up and make it cycle by cycle again. It is also useful for protecting the shunt FET. I know 15cm doesn't sound like much, but even nHs of inductance can cause huge voltage spikes due to the very fast di/dt. So I would recommend a snubber no matter what.

    If that doesn't get you where you would like to be then reducing the ripple current will help minimize the effect as you mentioned.

    Regards,

    Clint