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UCC15241-Q1: Problem with unreliable startup of DCDC Converter

Scott Osborne
Prodigy 100 points
Part Number: UCC15241-Q1
Other Parts Discussed in Thread: UCC21732-Q1

We are using the UQQ15241-Q1 and UCC21732-Q1 in an IGBT Gate Driver application (six channels).

We have a stable input voltage of 24.75V (for 1second) before we switch the ENA pins on the UCC15241 high. Typically only half of the six UCC15241 converters start up correctly and pull the PG pin low.

The VDD-VEE voltage is 23.0V (-8/+15V)

To get all channels to start up we have to toggle the ENA pins a severla times within a 0.3 sec. 

Is there a limit to how much VDD-VEE Capacitance  is safe to use to ensure good startup.

We have 3*10uF plus 0.1uF total at 0V bias which is ~15.1uF at the final 23.0V target voltage.

There is also some VDD-COM and COM-VEE capacitance (12.4uF and 23.2uF at operating bias voltage).

We used the Excel UCC15241 Calculator sheet for the design.

When the converters do start up the rise time is about 6ms to 23V which is well within the soft startup time.

Do we have too much capacitance or is there another issue that causes startup problems?

Regards

Scott

  • 0
    TI__Genius 15616 points

    Hi Scott,

    The startup issue most likely come from an inrush current problem. If you Toggle EN high for all the devices at the same time, the inrush current of you main supply will go very high. That is why toggling EN helps because then it gives chances to the rest of the devices to turn on pulling lower current (lower inrush current).

    I recommend two options:

    A. Connect RC filters between the EN signal source (might be a micro controller) and the EN pins of the six devices. Use different RC components for different groups of devices. For instance, three UCC15241-Q1 will have (R=1kohm, C=10nF) and the other three (R=5kohm, C=10nF). With that, the (R=1kohm, C=10nF) will turn on first and then the (R=5kohm, C=10nF) ill turn on after.

    B. If you EN supply signal comes from a microcontroller and you can use more than one PWM channel, connect three UCC15241-Q1 to one channel and the other three to other PWM channel, programming both PWM signals with certain delay between both.

    Thank you