I have built many 240V AC dimmers with back-to-back Infineon SPD04N50C3 N-channel MOSFET transistors without any problems. Unfortunately they are not manufactured anymore.
I found a replacement from ST STD7NM60N. I believed that is a perfect replacement, not seeing any real differences in the specification.
Something must be different, because sometimes when I turn on the 200W bulb, using the same PCB and the same production software, one of the MOSFETS goes permanently short. This happens in about every 100 turn-ons.
Looking with oscilloscope, the gate driving PWM signal looks pretty good. Same is true for the AC wave.
Can anybody tell me, why is that, more precisely, which parameter in the specification is weaker? Pinpointing it would help me to look for a better replacement.
Price is an issue. Both MOSFETs cost about $0.50/pcs, there is no budget to go much higher.
Gate is driven via 22 kohm resistors directly from PIC microprocessor. Because the low budget, and because the desire is to drive the gate with as high voltage as possible, we have PIC powered with 5.5V instead of normal 5V, thus gate voltage is also about 5.5V PWM control. MOSFET is switched ON at every AC zero crossing, and turned off within the half-wave dimming level dependent.
Thank you so much for your time.
Link to ST MOSFET: https://www.dropbox.com/s/9dxv59lpdu3vseg/tran_STD7NM60N.pdf?dl=0 or another https://www.st.com/resource/en/datasheet/std7nm60n.pdf