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TPS40170: Current Limit / Softstart / TRK functionality

AndreasKoester
Prodigy 40 points
Part Number: TPS40170
Other Parts Discussed in Thread: LM5022

We are using the TPS40170 in a flybuck converter (10-60V in, 3.3V 5A out). Our problem is the correct values for the Ilimit resistor: Setting it to the requested current limit (5A) it works fine during normal operation, but does not startup (SS cap 100nF). The current limit resistor has to be set for 12A continued operation to allow the startup. At a lower value several softstart cycles are necessary to achive the stable operation.

Trying to understand we looked at the datasheet SLUS970B. It is recommended the TRK pin is tied to the VDD if not used, but section 7.3.7 says "For proper tracking using the TRK pin, the tracking voltage
should be allowed to rise only after SSEAMP has exceeded VREF, so that there is no possibility of the TRK pin voltage being higher than the SSEAMP voltage". Maybe it is not OK to connect the TRK to VDD and that generates the softstart not working as expected, generating overcurrent switch off?

  • 0
    TI__Mastermind 24075 points
    Hi Andreas,

    The OC problem in your project should be caused by the charging current during soft start, not by TRK pin.
    During softstart, Vout rises from 0V and Cout is charged when Vout keeps rising. The total inductor current is load current plus charging current.
    The charging current needs to be considered when setting OC threshold. The charging current can be calculated as below:
    Icharge=Vout*Cout/Tss, where Tss is the softstart time.

    Can you try to increase SS cap value to see if it can solve the issue.


    Thanks
    Qian
  • 0 in reply to Qian Chen
    Prodigy 40 points

    Hi Qian,

    thank you for your answer. We thought about the SS first, but as we measured the details it looks like the high side switch is turned on for about 95% duty cycle. From the beginning, so triggered at the very first pulse.

    At this time the SS capacitor voltage is quite low, and the low side FET is off for some cycles (We assume that's because to output is pre-biased at that time). So we are finding the reason why the pulsewith is at max. width from the beginning.

    I don't know if it is possible to attach/upload some scope plots here, that would help to understand the problem.

    Thanks, Andreas

  • 0 in reply to AndreasKoester
    TI__Mastermind 24075 points
    Hi Andreas,

    Can you send the schematic in pdf file as well as waveforms to below email?
    Q-CHEN@TI.COM

    Thanks
    Qian
  • 0 in reply to Qian Chen
    Prodigy 10 points
    Hi Qian,
    I sent you the files via email.

    Thanks
    Alex
  • 0 in reply to Alexander Bremmert55
    TI__Mastermind 40020 points
    Hello Alex,

    You could try a larger SS cap as previously recommended, however, I do not recommend using the TPS40170 as a flybuck. Recommend using the LM5022 as a FlyBack. The TPS40170 has a pre Bias Start up feature that does not allow negative current to flow, the negative current in the low side FET will be quite substantial, even in normal operation.
  • 0 in reply to David Baba
    TI__Mastermind 40020 points
    Also,

    Some further discovery on my end. Looking at your schematic (thanks Qian) and the above communications it appears you require 3.5A out? If this is the case the Peak currents in the Transformer will be very large. as such, looking at the PA3855-006 the DC current specs are 2A (2 x 1A in parallel). I suspect that the Transformer maybe saturating. Lift up the primary Side of the Transformer and place a current loop and probe there,to ensure the Transformer is not saturating?
  • 0 in reply to David Baba
    Prodigy 40 points

    The max. primary current of the PA3855.006 is 4.6A (there is a formula at the second page of the Pulse data sheet). I aggree that this is quite low for full power. So we had cuntinued to test the power up with no load.

    I assume we run into problems because of some internal IC functionality we don't understand. May be related to the pre-bias support. At startup the high-side FET turns on several times (about 8) before the low side switch starts to do anything. The low side FET has it's internal diode, but that seems not enough to reset the transformer (The secondary side has 0V at the startup, too - so the transformer output voltage is clamped to ~0V). Using a current probe to measure the primary transformer current we detected the inductor current increased over several cycles (~5) and saturation occured. To prevent this we incresasd the SS capacitor to enforce lower duty cycle during that period.

    But that didn't help: Transformer current is quite low now, but the IC stops after about 10 pulses for ~100us, then does another try...

    SS voltage does not decrease during that time (so no overcurrent turn off), and the output voltage is still so low that it is surly < the 0.6V regulation point. I think Alex sent some waveforms via mail.