• State Resolved
  • Locked Locked
  • Replies 6 replies
  • Subscribers 63 subscribers
  • Views 822 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM5088-1 fails on output short

Georgi Krastev
Prodigy 40 points
Other Parts Discussed in Thread: LM5088

Hi,

I'm designing a non-isolated DC/DC buck converter based on LM5088-1. The input voltage can vary from 32 to 56VDC and the output is set to 30VDC with maximum load of 8A. The output current and voltage are monitored by microcontroller. Also there is a crowbar OVP circuit comprised of Q301, U302 and some passive components. 

The schematic:  4505.LM5088_30Vpsu.pdf

The entire design works great. It has excellent regulation and transient response and just one majour flaw - it cannot survive output shorts. It cannot even tolerate charging a 1000 uF alluminum capacitor if it is connected after the soft start period is out.

Investigation of failed LM5088 shows a short between BOOT and SW pins. On the other hand bootstrap capacitor is intact.

Here are some captures of failing converter. Yellow traces are BOOT to GND and green are the output.

The converter fails less than 800 microseconds after conecting fully discharged 1000 uF alluminum capacitor.

Y cursors show what happens with the output voltage after connecting the cap. 

I'm running out of ideas ... and ICs :-)

Your suggestions are much appreciated.

Thanks in advance,

Georgi

  • 0
    TI__Guru 58171 points

    Hi

    I guess it is better to monitor inductor current and SW node spike after connecting 1000uF capacitor.

    This time you can connect a zener diode in parallel with C309 to not damage BOOT-SW.

    What's the part name of your inductor ?

    Regards,

  • 0 in reply to EricLee
    Prodigy 40 points

    Hi, Eric.

    The inductor is Wurth Elektronik WE-HCF series 47uH. Here is the datasheet: 3652.7443634700.pdf

    I cannot monitor the inductor current as I don't have differential probe. But I can snapshot a SW node spike and I'll get back to you later.

  • 0 in reply to Georgi Krastev
    TI__Guru 58171 points

    Hi

    I think you have to monitor the inductor current. There is a possibility inductor current being saturated.

    Regards,

     

  • 0 in reply to EricLee
    Prodigy 40 points

    I'll try to think something out for monitoring inductor current.

    I have 2 scenarios with SW node voltage under examination. The first one is connecting a 1000 uF capacitor:

    The negative spikes can be easily seen on the yellow trace. Here is a close up:

    The second scenario is connecting 1.2 ohm resistor. At 30V this should yeld 25 Amps.

    After 8 ms in the overload the microcontroller shuts down the converter. And the close up of the spikes at SW node:

    Following your advice I've added zener diode (BZX55C15) paralel to the bootstrap capacitor and this saved the soul of LM5088. In fact this solves my issue with controller not surviving output short circuit.

    Is it a good practice to leave this diode in the production design? 

    BR,

    Georgi 

  • 0 in reply to Georgi Krastev
    TI__Guru 58171 points

    Hi

    It is good if your issue resolved with a zener diode, but finding the root cause is better.

    Regards

  • 0 in reply to EricLee
    Prodigy 40 points

    Hi, Eric.

    Acquiring differential scope probe, which will enable me to trace the inductor current, proved to be pretty expensive thus impossible at this stage.  So I guess I'm sticking to the workaround with the zener diode which is working flawlessly so far.

    Thank you for your valuable advice.

    Best regards,