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TPS40210 can not startup with large output capacitance

York
Prodigy 180 points
Other Parts Discussed in Thread: TPS40210, TPS43060, LM5122

I'm using TPS40210 to boost 24V to 58V, but the load is purely capacitive, a 8100uF 100V capacitor. The application is about pulsing a very strong pulse to a coil. 

The circuit works very fine with resistive load and with optimal efficiency up to 97% at 0.3A output.

But when i replace the load with the big capacitor, the device keeps restarting at a period around 1 second (i can measure the voltage on the SS capacitor (pin 2) gets charge and discharge at this rate. and the voltage of the loading capacitor rises slowly, step by step.

Now a started to think that a boost converter like this is not suitable for this job. Could someone give me some advice?

 

Schematic Prints.pdf
  • 0
    TI__Mastermind 35135 points

    Hi York,

    It takes a lot of current to charge up 8100 µF to 58 V. The average current can be estimated by I=C x dV/dt, where dt is the soft-start time set by the cap on the SS pin. As a result of the high average current the hiccup current limit can be triggered which causes the restarting you have been seeing.

    I think you are using a 0.22 uF cap on the SS pin, which sets the soft-start time at 11 ms. An 11 ms soft-start time calculates an average current of 42 A to charge up 8100 µF to 58 V. With a 62 mΩ sense resistor, the peak current limit is only 2.4 A typical.

    There are two things you can try.

    1. Increase the SS time so that the average charging current for the capacitor is lower.
    2. Change to a part without hiccup current limit, such as the TPS43060.
    Best Regards,
    Anthony 
  • 0 in reply to Anthony F
    Prodigy 180 points

    Thank you, Anthony, for the very prompt reply. But repalcing the SS capacitor to a very large value means not only a longer soft-start time, but also a longer delay when the current reaches hiccup limit, whhich is undesiralbe.

    When i suddenly discharge these capacitors to a very low voltage (this is what my application's about), the regulator tries hard to regulate the voltage back to 58V which must trigger the current limit again, then the regulator waits a fairly long time before it tries to soft-start again, again undesireable

    Maybe there's a way to modify the feedback and compensation circuit, so i can make this regulator's gain a bit lower, so the regulator behaves "softer" (it does not tries so hard) ? Because the applcation can tolerate unregulated voltage for a limit amount of time.

    i also think the second option is appealing, but the TPS43060 is hard to solder (QFN).

     

    Thanks again for your answer, and i'm eager to solve this problem.

    Best regards,

    York

  • 0 in reply to York
    Prodigy 180 points

    Hi, Anthony, I've done much experiments, and increasing the SS capacitor did work. But i must deal with extra wait time when OCP occurred. Maybe switching to TPS43060 is a good idea.

     

    My question now is, will TPS43060 work reliably with  58V output voltage? I think pushing the device to the limit is not a good idea.

    What else device can i choose?

  • 0 in reply to York
    TI__Mastermind 35135 points

    For a little more margin you can consider the LM5122. This has hiccup and cycle-by-cycle current limit. The hiccup mode can be disabled by tying the RES pin to AGND.