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TPS92411: Trying to improve the TPS94211 design

Miguel Camus
Prodigy 120 points
Part Number: TPS92411

Morning gents,

I have seen in my TPS94211 design that the voltage on the drain of the mosfet is about 18VDC + 8VAC more or less. So basically there is a DC component that is behaving like a constant resistor and on top of that an AC component to adapt to the "shape". I had the idea that adding 3Vf LEDs between the drain and L1- line the dynamic resistance of the LEDs would act as the DC component and the voltage on the drain would go lower. That would increase the light output power and dissipate less heat in the mosfet. Unfortunately it doesn't work like that, the voltage goes higher. I have tried with other mosfets and even darlington transistors and the result is very similar. I'm not expert in mosfets, I always used them as switches. Any idea of how this can be done?

Many thanks,


Miguel

  • 0
    TI__Mastermind 28655 points

    Hello Miguel,

    I assume you mean the drain of the current regulating MOSFET, not the MOSFET in the TPS92411?  If so are you reading this as RMS on a DMM?  If calculated correct the drain waveform should look like the AC rising until the lower TPS92411 opens, then it will be the AC rising minus the lowest LED stack.  As the AC continues to rise the middle TPS92411 will open and the lower one close, then the drain voltage will be the AC value minus the middle LED stack.  This continues through all the TPS92411 states, going from 000, 001, 010, 011, 100, 101, 110, 111, the first number being the upper TPS92411 LED stack.

    If you had 80/40/20 and the open headroom is set to 6V, close to 4V what you would see is the rectified AC rise until it hits 26V, then the lower TPS92411 would open and the drain would have 6V on it at that instant (26V - 6V.  The AC continues to rise and the middle switch opens, the lower closes when the rectified AC hits 46V, the current regulating drain voltage again would be 6V at that instant (46V - 40V).  The rectified AC continues to rise to 66V when the lower TPS92411opens, again the drain voltage will be 6V (66V - 40V - 20V), this continues (86V - 80V = 6V, 100 state just the upper TPS92411 is open), (106V - 80V - 20V = 6V, the upper and lower TPS92411 are open).

    On the way down the close threshold is lower, 4V (this is to prevent oscillation, multiple switch cycles).

    If you add series LEDs from rectified AC the only difference is the rectified voltage will have an offset but the switch still switch at the same voltage they sense with respect to ground.  The only visual difference on the drain waveform will be a shift of when the switches switch and the last state will not go as high in voltage on the drain waveform (possibly making it more efficient depending on where the switches are switching).

    Regards,

  • 0 in reply to Irwin Nederbragt
    Prodigy 120 points

    Hi Irwin,

    Yes I am reading the RMS DC and AC voltage with a digital multi meter, I didn't check it with the scope to be honest. I understand that the real headroom is much lower than I expected and therefore not worthy to do.

    Thanks for your help once again!

    Kind regards,

    Miguel