NOTE: I would like to move this thread to a private thread so that I can post schematic images. Once that is done I will add an additional schematic image. How can I do that?
Hello,
In our design, two LMG5200's are used to form an H-bridge based buck-boost converter. Recently, the buck side (U9 below) of the converter failed short, with Vin to SW, SW to GND.
Below is the pin-to-pin resistance measurement I took for the failed IC (U9 in the above schematic):
Black Probe | ||||||||||
1 - Vin | 2 - HB | 3 - HS | 4 - HI | 5 - LI | 6 - VCC | 7 - AGND | 8 - SW | 9 - PGND | ||
Red Probe | 1 - Vin | X | 0.1 | 157k | 170.3k | 169k | 0.1 | 105.8k | 0.2 | 0.4 |
2 - HB | 0.5 | X | 157k | 170k | 168k | 0.5 | 105k | 0.3 | 0.1 | |
3 - HS | 157k | 157k | X | 328k | 325k | 157k | 262k | 157k | OL | |
4 - HI | 170k | 171k | 328k | X | 339k | 171k | 277k | 157k | 157k | |
5 - LI | 168k | 168k | 326k | 339k | X | 169k | 274k | 168k | 168k | |
6 - VCC | 0.2 | 0.1 | 157k | 170k | 168k | X | 106k | 0.1 | 0.1 | |
7 - AGND | 106k | 106k | 263k | 276k | 276k | 106k | X | 106k | 106k | |
8 - SW | 0.2 | 12 | 157k | 170k | 168k | 0.8 | 106k | X | 106k | |
9 - PGND | 24 | 0.7 | 157k | 170k | 170k | 0.1 | 106k | 0.4 | X |
When the failure occurred, the converter was outputting about one amp. The input voltage and output voltage were almost equal (32V) with the buck-boost converter switching in buck-boost mode.
Investigation of the failure so far
1. The initial thought would be shoot through or something of that nature, however we have done analysis on the deadtime of the converter and everything appears to be in order. Additionally, the supply feeding the converter is current limited to 9A, and this is a 10A part. There is some capacitance at the input (under 10uF) where additional shoot through current could have come from, however after analyzing the system I think this not as likely of a cause of the failure.
2. Another thought has been ringing at Vin or the switch node. There is an input LC filter that could certainly be causing severe ringing. I probed the input node (green), and the switch node (blue). The math function (orange) subtracts Vin and SW, for the voltage across the top FET.
It looks like the SW node is ringing low, below the -5V limit called out in the datasheet. It looks like it rings as low as -10V in some cases. Is this a potential failure mechanism? If so, what exactly does this negative voltage ring kill in the IC? I guess I would have expected ringing high (over the 100V limit in the datasheet) to have been the cause if ringing was the issue.
Lastly, is failing short typically the result of too much current/a heat based failure, or a voltage based failure resulting in some sort of dielectric breakdown?
Thanks
Jesse