LM5088: Internal Boot Diode is shorting between Vcc and BOOT pins.

Today I just had an internal Boot diode fail short while the 24V output was working into an 80 ohm resistive load.

I don't see how a trasient could have possibly been introduced into the sytem.

Any thoughts?

Thannks.

Hello Bob 

How much overshoot at V(BOOT-GND), V(SW-GND), V(BOOT-SW) ? Are you able to capture waveforms ? 

- Eric Lee  

I replaced the failed part.

Overshoot appears small, about 12% or about 3 volts, if you believe the display of the Tek MSO2024B. See attached.

Here is the schematic.  Values based upon LM5088_Quick_Start.xls

Vin is 26VDC, Load is 80 ohm resistor, Vcc measures 7.66V.  I don't currently have access to our differential probes, but I would expect Vboot to Vsw to be Vcc.

Hello Bob 

Your schematic looks good. Since the 12VDC is directly connected to the EN pin, I think it is better to supply V+ before supplying 12VDC.

How did you find the internal boot diode is damaged ? This design is under mass production ? 

- Eric Lee

V+ supplies the input to the 12V regulator as well, so V+ leads 12V by a small amount.

The failure manifests itself by the 24V supply repeatedly trying to start and not being able to.

LEDs powerd by the 24V rail flash dimly for a short amount of time.  Sorry, I don't have a scope plot of that.

A lap around the LM5088 pins revealed an inconsistancy on pins 15 & 16 vs. a functional unit.

Further investigation revealed a short (about 5 ohms) between Pins 15 & 16.

A functional unit measures about a 0.53V drop accross the forward biased internal diode.

Hello Bob

Any possibility that the pin is overstressed during start-up or during shutdown ?  

Is it possible to add a 2-5ohm gate resistor at Q4 ? and check if any change ?  

- Eric Lee

That cut and jump would be difficult to unlikely on this board.

I believe that I can squeeze an external diode between Vcc and Boot.  Will something like a BAT46W (100V, 150mA, SOD-123) sufice?

Hello Bob

It is better to populate a small and weak diode, then you can notice easily if any issue

- Eric Lee 

So something like 40-50V and 30-50mA?  Like a BAS81?

Hello Bob

I think BAS40 series is good. 

- Eric Lee

I had another unit fail today.  It had run in the next assembly for sereral days and failed when powered up.

What do you think of adding a 2-6 ohm resitor between the Boot pin and the capacitor?

Hello Bob 

It's a good idea. It will work like as a gate resistor, also the resistor helps to minimize a surge current flowing through the boot diode. 

- Eric Lee