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Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
I got a problem on my multistage coilgun using 5 SCRs with a common IGBT on the cathodes.
When a stage fires the scr gate is biased positive respect to the cathode and the igbt is driven with 23V on the gate with a 3A mosfet totem pole bringing to ground the scr cathodes. Only the SCR with the gate biased positive turns off, the others remains in the off state even with a strong dv/dt. The pulse is regulated by a PIC16f876a. During turn off the IGBT tail current keeps the SCR on. So if a second pulse activating the igbt comes too early the SCR isnt fully turned off. These SCRs i use have a holding current of 18mA. Is there a way to fix this or i have to distance more the stages , or definitely stick totally to IGBTs (5 igbts for 5 stages)?
Registered Member #85
Joined: Thu Feb 09 2006, 01:22PM
Location:
Posts: 21
I have encountered similar problems with my hybrid switching CG but in my case i believe the problem lies with the SCRs themselves, not the igbts. Usually, the Tq of SCRs will be much higher than the Tt of IGBTs. In my case 110us. So it might be possible that in your case the bottleneck is also the SCRs, and that you could solve your problem with ultra fast SCRs with a lower quench time.
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IGBT tail current keeping SCRs on
