LM5106: erratic switching, killing MOSFETs

Hello Frank,

I am an applications engineer at TI supporting this device, and I will work to help you resolve your concerns.

Thank you for the plots showing the HO output pulse of concern, interesting that it is sensitive to slight change in VDD voltage.

From the plots shown, I cannot determine what may be the cause of this behavior. So I do have some questions to ask.

The schematic shows VDD at 12V, but in the scope plots it is mentioned there is a problem when HS is > VCC +0.5V. Is VCC the input voltage to the power train (or VBus)? Or is VDD lower than 12V. Please confirm VDD voltage level.

Conditions to replicate are EN low, IN Low, boot capacitor must be completely discharged. Then the sequence is EN from low to high. With IN still low I assume, please confirm if IN is still low.

If in the beginning of the plot, EN is low, both gate drive outputs should be low, I am not sure why HS is high at that time.

Can you record plots of the above sequence of the unexpected HO pulse with some additional signals?

Please provide some additional plots showing the following: Plot 1) EN, IN, HO, HS, Plot 2) EN, HO, LO, HS,  and Plot 3) EN, HO, HS, HB.

I would like to see the events of enable, low side FET turn on, HB bias turn on, and HO behavior.

Regards

Richard Herring

Hi Richard,

thanks for your quick response!

- slight VDD change: absolutely, it is almost digital. Not present at <10V, intermittent between 10.0V and 10.5V, always present above 10.5V.

- VDD: it is 10V despite showing 12V in the circuit diagram, sorry for the confusion. I actually did mean VCC (10V) and not VBUS (48V), as strange as this sounds.

- condition to replicate: yes, the sequence is EN=low, IN=low, then wait for a couple of seconds (discharge bootstrap), then apply voltage to HS greater than VCC (done through lab supply and 4k7 resistor, to simulate motor BEMF), then EN=high while keeping IN=low.

- HS is high because I am applying external voltage to HS as described. In the application, the externally turned motor is doing that with it's back EMF. We first noticed power stage damages when it is enabled while spinning the motor, before tracking this down to this weird LM5106 behavior.

Here are the requested plots. It is clear to see that the driver turns on both MOSFETs (!).

1) A=EN / B=IN / yellow=HO / blue=HS

2) A=EN / B=LO / yellow=HO / blue=HS

3) A=EN / B=HB / yellow=HO / blue=HS

Cheers

Frank

Hi Richard,

I may have a clue to the problem. Adding a 1 Ohm resistor in series with the bootstrap charging diode D401 seems to fix it. Is it possible that the LM5106 has a problem with its high side level shifter when it sees high dV/dt at the HS node? How could that be related with the fact that the problem only shows up when V(HS) > VCC?

Cheers
Frank