555 driving mosfet, both failing - Page 1
Forum section: electronics / projects

author: Circlotron
author_rank: Super Contributor
timestamp: March 01, 2017, 05:52:46 am
content: Did a run of 500 boards and had about 5 fail, 4 on test and 1 in the field.In the relevant part of the circuit there is a 555 driving a BSP100 mosfet with a 1K pullup resistor to +12V on the drain. This then drives an automotive tachometer. Frequency is about 800Hz absolute max.The problem has been the mosfet goes shorted G-D-S and this takes out the 555 too. The bipolar 555 drives the mosfet gate directly with about +4 volts seeing it runs on a +5V supply. Is it possible that the gate-source capacitance charging and discharging is loading the 555 output too much? The output pulse is 1.1mS wide and rise and fall time is not really an issue, so would putting a resistor in series with the gate help things?

author: ScottK
author_rank: Contributor
timestamp: March 01, 2017, 06:20:01 am
content: Inductive kickback might be the cause of your failure, though I'd expect that to happen in the field, not during production test. Is the tachometer load you are driving inductive? Do you test using a tachometer on the bench/factory floor? If the load is inductive you must have a snubber diode from the mosfet drain back to the 12V supply to prevent the inductive kickback at turn off from exceeding the mosfet's drain avalanche voltage. Your BSP100 can withstand no more than 30V and 23mJ of avalanche energy.

author: xavier60
author_rank: Super Contributor
timestamp: March 01, 2017, 10:24:57 am
content: Do the failed boards work initially?There should always be a resistor placed between the driver and the Gate for various reasons. If the MOSFET is failing first, a Gate resistor would have protected the 555 from overloading.

author: SingedFingers
author_rank: Frequent Contributor
timestamp: March 01, 2017, 11:20:44 am
content: I've had this on a switcher I designed a few years ago. Albeit a one off.The hypothesis I came to was that I forget the gate resistor entirely. This causes a large current impulse hitting the gate on switching which blew up the 555 eventually. This then leaves the FET not off or saturated at which point Rds is sinking a lot of power and the MOSFET goes poof after a few seconds.

author: bktemp
author_rank: Super Contributor
timestamp: March 01, 2017, 02:59:32 pm
content: Based on the fact that the old bipolar NE555 suffers from a cross conduction problem on its outputs, drawing >400mA at every transistion even without any load, I doubt a rather small capacitive load will do much damage.A short between D-S and G on a mosfet points to a thermal overload condition, either due to a high avalanche energy or exceeding the drain current/SOA.

author: mikerj
author_rank: Super Contributor
timestamp: March 01, 2017, 03:30:12 pm
content: Possibly ESD damage during assembly, i.e. the one that failed was walking wounded when it left you.Is this supplied complete already wired to the tachometer, or are you relying on people wiring this to their own tachos? If the latter is there any circuit protection when someone inevitably connects the output to +12v?

author: bobaruni
author_rank: Regular Contributor
timestamp: March 01, 2017, 03:45:57 pm
content: I have seen MOSFETS without gate stopper resistors self oscillate when driving an inductive load such as a light bulb, this is another potential cause of failure.