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Registered Member #883
Joined: Sat Jul 07 2007, 01:02PM
Location:
Posts: 16
there is a variety of scr called a gto (gate turn off). they are made to be comutated by a strong negative pulse on the gate. i believe they are hard to find(read expensive). as steve said igbts or fets would be the way to go.
Registered Member #883
Joined: Sat Jul 07 2007, 01:02PM
Location:
Posts: 16
i triac wont work because as steve said there are no current zero crossings. an scr or triac need to drop below thier conduction threshold to turn off. he needs something that can be turned off.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
If you want to keep with the SCR to carry the bulk of the current, you could always try shunting its load current with an IGBT at the moment you want to turn it off.
Another effective technique is to discharge a small capacitor across the SCR in the reverse direction. This reverse biases the device and causes it to drop out of conduction. SCR's are pretty tough beasts, so there a few dirty tricks that can be used to force them to turn off.
Probably easier just to use an IGBT or MOSFET to carry the full load current though, and ditch the thyristor altogether.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
By the way, you never mentioned whether you're going to filter the supply. If you're only going to full-wave rectify it, and not filter it, then the SCR should turn off fine as the current drops to zero 120 times per second.
If you're going to use filtering, then it's IGBT time. My first try would be a 1200V TO-247 part. The cathode current you're switching is "only" an amp or two, so there's no need for huge bricks. Unless you screw up the RF bypassing, in which case a bigger device will withstand cooking by RF better.
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How to turn off a quad 833C for stacatto operation.
