The reason for failure is the high dV/dt of the "modified sine wave". It will cause self triggering of the MOC3023, and possibly even the Triac. Placing a capacitor in parallel to the Triac isn't the recommended means, you should rather follow the suggestions in the MOC302x datasheet, see https://www.edaboard.com/threads/162207/#post686270
Yes, that's a clear result. I also wrote about possible self triggering of the triac before I realized the 1500 Hz issue. But be sure, that a standard MOC3023 can be easily self triggered due to it's specified maximum dV/dt of only 10V/µs. Vishay has special types that are capable of 10 kV/µs.I am not convinced the MOC3023 was self-triggering. By removing the MOC3023 and using the circuit (below) without any trigger (or capacitor), the TRIAC remained on anyways.
...I also wrote about possible self triggering of the triac before I realized the 1500 Hz issue. But be sure, that a standard MOC3023 can be easily self triggered due to it's specified maximum dV/dt of only 10V/µs.
Vishay has special types that are capable of 10 kV/µs.
P.S.: Assuming, that dV/dt can be managed with suitable devices, commutation time may be a more critical problem. It depends however on the duration of the modified sine zero gap. 50 us is below the specified turn-off time even of many SCRs.
1500 Hz is a bit fast for a triac (generally these are even slower than SCR's), the capacitive load and the reasonably steep edges of the AC source are the reason the triac stays on, as the current does not decay to low enough values for long enough for the triac to completely turn off, and then it is hit by re-applied dv/dt which triggers it on again for the next (reverse) cycle and so on, .... Back to back high speed SCR's will perform better
Total voltage seen by triac, don't forget that the triac sees the 2 x the source volts every time the source switches (unless the source is a short during the 0V region) due to the voltage from the charge stored on the load cap adding to the newly reversed source voltage, so possibly 300V in a small fraction of a uS applied to the triac every switch to rail of the inverter source.
Regards, Orson Cart,
I would be inclined to add a resistor between the gate and MT1 so when the opto turns off the gate isn't left floating.
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