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I am testing Toshiba SSM6L14FE parts in my application and they are failing 100% of the time. This is quite odd since I am not putting any known stresses on the parts…no high frequency DC switching, no overvoltage, etc… The application is a low dropout full wave bridge rectifier running between 2.5V and 12.5V DC. Failure happens at 2.5V and 10V after say 15-30 seconds. The output is only a few mA at most which makes the failure even more strange. The only thing I can think of is this application runs the current in reverse in parallel with the protection diodes which is not a standard operation. But I’ve seen many application notes for full wave bridge rectifiers using MOSFETs.
A 100% failure rate is a pretty good indication that something’s not working as expected. Some manner of deficiency in the gate drive leading to shoot-through would seem a likely culprit. Take a close look at current waveforms, looking particularly for large flows during state transition events.
If this doesn’t point you toward the problem, a schematic of the circuit in question would be helpful.
Here’s my schematic. VA-VB is the input DC voltage which can vary from 2.5V to 12.5V. Since it’s DC there should definitely be no problem with overdrive. I’ve also done testing with the resistors shorted and the zener diodes removed with a lower voltage so that the gate isn’t overdriven, and the result is the same.
The only thing I could guess is that perhaps since the SSM6L14FE package is so small, it’s difficult for me to solder my prototypes correctly. I had to solder several times to make sure everything is connected. I’m guessing the solution is simply lengthen the PC board footprint pads to guarantee reliable soldering.
You understood what I meant, that is what I was expecting: one input being 0V and the other having the right voltage input. Otherwise, you’d have a short situation. Is there any point in operation where there is potential that you get 2.5 to 12.5V on both inputs at the same time?
Yes of course, but the odd thing is the circuit seems to short itself out. I’m not sure if there is some problem with an intermittent connection, or these parts are simply not designed to run DC in reverse. The problem happens after applying power for several seconds or sometimes even minutes.
Your circuit looks OK, so the problem is strange indeed.
Do you get +3.3V at the output while the circuit is “working”?
One thing you could do is to route one supply input via a current meter, and
start stepping the voltage supply from zero, say in 0.2V increments and measuring
the current at each step. That would give a hint of the problem.
The datasheet does not mention the current rating for the bulk diodes (only pulsed test), thus there is a chance that the FETs do not go ON at all and all the current flows through the bulk diodes until they die.
That can happen for example, when the output of the bridge is shorted. Try removing the LDO for a start.
Also please check that the schematic symbol is correctly mapped to the geometry, i.e. on the layout
the source and drain are not swapped for either FETs.
Seems you and others aren’t understanding the issue. The circuit seems to work then suddenly stops working.
The LDO regulator on the output draws a steady 3mA of current. I highly doubt the problem is here. When I bypass the Mosfet full bridge rectifier everything works cleanly.
