Hello! I am looking to chat about a power inverter I've designed and built. I say designed but it's really more like adapted working designs for my own purposes. I need some guidance on the failures I'm running into and sites like Electronics Stack Exchange don't seem geared towards a back and forth conversation. I'm not a power electronics pro, just an electrician with electronics knowledge. I also don't have the test equipment necessary to really zero in on the problem(s). I'm hoping that with my descriptions and details that we can talk our way through it.
The Inverter: The design is a "low frequency" pure sine wave type. An Arduino Nano outputs an SPWM signal that feeds a pair of IRF21844 half bridge drivers. Those then control low and high side TLP351 opto gate drivers. The power stage is an N-channel FET H-bridge. I've used both IRFB7545 FETs and IRF3205. Exploding both in spectacular fashion. The output of the FETs goes into a 100 uH choke and then a 3 kVA toroidal transformer. Input is 24 VDC, output is 120/240 VAC 60 Hz. I prototyped it all on perfboard originally to good effect. I wanted it to be modular so I had some PCBs made where each FET is on its own board with an individual TLP351. A 4.7 ohm gate resistor is in parallel with a reverse biased Schottky diode and a 22 kohm gate-source resistor as a pull down. Each high side quadrant has an isolated 12 V power supply for the gate drive. Four 10000 uF capacitors sit atop the power board.
Testing: A 1.5 kVA 24 V transformer with rectifier and filter is the test supply. That gives about 33 VDC out. I'm OK with that. On perfboard everything was good. I put about 1000 watts through it with incandescent lamps, heaters, and brushed motors. Now that I have assembled it with the PCBs, it isn't nearly as stable.
The Problem: A small 1/3 HP induction motor caused instantaneous FET explosion. After repairing that meltdown, a 250 watt lamp turning off and back on blew the low side FETs. My little USB scope isn't fast enough to see what is happening before the destruction. Destruction happens with and without RC snubbers installed.
Solutions: Increase gate resistor value? Use a negative gate bias supply? Schottky diodes from source to drain?
I just need some back-and-forth conversation with people who know more than I do. Thanks for everything!
The Inverter: The design is a "low frequency" pure sine wave type. An Arduino Nano outputs an SPWM signal that feeds a pair of IRF21844 half bridge drivers. Those then control low and high side TLP351 opto gate drivers. The power stage is an N-channel FET H-bridge. I've used both IRFB7545 FETs and IRF3205. Exploding both in spectacular fashion. The output of the FETs goes into a 100 uH choke and then a 3 kVA toroidal transformer. Input is 24 VDC, output is 120/240 VAC 60 Hz. I prototyped it all on perfboard originally to good effect. I wanted it to be modular so I had some PCBs made where each FET is on its own board with an individual TLP351. A 4.7 ohm gate resistor is in parallel with a reverse biased Schottky diode and a 22 kohm gate-source resistor as a pull down. Each high side quadrant has an isolated 12 V power supply for the gate drive. Four 10000 uF capacitors sit atop the power board.
Testing: A 1.5 kVA 24 V transformer with rectifier and filter is the test supply. That gives about 33 VDC out. I'm OK with that. On perfboard everything was good. I put about 1000 watts through it with incandescent lamps, heaters, and brushed motors. Now that I have assembled it with the PCBs, it isn't nearly as stable.
The Problem: A small 1/3 HP induction motor caused instantaneous FET explosion. After repairing that meltdown, a 250 watt lamp turning off and back on blew the low side FETs. My little USB scope isn't fast enough to see what is happening before the destruction. Destruction happens with and without RC snubbers installed.
Solutions: Increase gate resistor value? Use a negative gate bias supply? Schottky diodes from source to drain?
I just need some back-and-forth conversation with people who know more than I do. Thanks for everything!