FET's blowing up occasionally in solenoid valve mains control circuit

[userx2]

Hello,
I must admit, I have always had problems with Fet's not lasting in many applications I have used them as well as seen used by others.
This ranges from welding machines, light dimmers, motor controllers etc.
I never found the real reasons why this happens.
One other designer told me once that he moved away changed his designs to use IGBTs instead since they do not seem to have this issue.

Now I have a circuit that a former colleague of mine designed into one of our products. This one controls a 240V mains operated solenoid valve.
Now, about a year or so down the line, failures have started to emerge where the Fet's are burned out and there is again no easy explanation.
I am now considering changing the circuit to use a triac instead. They do last but have more power dissipation and heat generation

Perhaps someone here has better experience and can share their thoughts what may be happening here?
The solenoid is 230V 6W

The circuit looks as attached. There are 2 Fets and 2 opto isolators to drive the gate voltage.
Either one or both Fets go faulty and will be cracked open or just black bits remaining.
I personally cannot find anything underrated or wrong with the circuit. The mains has MOVs on it as well at the input of the PCB.
Q2 and or Q6 seem to blow up. So far, it is 5 out of 200 delivered units and it happens after a month or longer. Hard to say what customers do with the unit.
The solenoid itself is usually found open circuit as well (yes open, not shorted).

 

[dick_freebird]

Perhaps there's some marginality in the design / BOM that only blows up when you (say) switch the FETs right at peak voltage or current of the mains cycle? Or happen to "catch a bonus" like a nearby big-iron appliance dumps its load?. Or a load that defeats the turnoff of what's supposed to be turning off while the other's turning on?

You might want to "get one in captivity" modified so you can see what happens at crest voltage (not RMS value of nominal line)?

 

[userx2]

Hi X
I calculated the inductance of the coil to be 27.4 H, which leads to a time constant of 15.3 ms with the wire resistance of 1k8 alone. This means that the voltage clamped by the TVS diode remains at around 420 V for several microseconds. Assuming a slow switch-off due to the 100k resistor and at maximum coil current, there will be a uA-current at an elevated RDSon-value, which results in a short time span with dissipated power at the FET in the watt range (see simplified simulation below). Depending on the frequency the coil is switched, the FET may overheat.
Regards
Mathis

Hi there Mathis,

Thank you for all that effort!
However, the measured inductance of this solenoid is much lower than your calculation. It is around 4.6H@120Hz
and 2.6H @1kHz.

How will that influence your simulation?

Best regards
X

--- Updated Oct 23, 2022 ---

An interesting question to ask is whether the Inductance changes a lot when the solenoid is pulled in.
I have not tested this.

I would expect the airgap in the effective core will become zero or much smaller as the plunger is pulled in.

X

--- Updated Oct 23, 2022 ---

I have checked the inductance again and with plunger fully in, it does indeed go up and becomes
3.8H @1KHz
6.8H @120Hz

 

[designreviewday]

Hi X
Oh, I thought the 6 W to be 6 VA, because otherwise, if the coil is a small one, it will get very hot. Attached my calculations based on varying assumptions, and a modified simulation based on a 4.6 H coil with 8k1 losses (wire+core). There is still a power loss peak visible. And it all depends on the velocity the FETs are turning off.