transistors - Silicon Controlled Switches: applications&suppliers

I came across this description of what are called "silicon controlled switches." I always look to balance n/p-channel electronics for stability, and using these silicon controlled switches seemed like the perfect way to do so. However suppliers seem few and far between, and the few products I could find were >=$50 a piece!

I could find very little information on these devices online, so I am wondering what technological niche they fill, and if anyone sells them at a reasonable price--i.e. on the order of the price of the components an equivalent circuit made out of regular transistors. If not, why are they so expensive?

I am beginning to suspect that they are more commonly known/sold under a different name.

Answer

An SCS is a specialist device that is now seldom available. The equivalent circuit is shown in the page that Feynman referenced. It's main claim to fame is that it is a TRIAC like device that can be driven off in the middle of a conduction cycle using gate drive alone, unlike a TRIAC.

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  A TRIAC or SCR can be driven off mid conduction by interposing an inductive reverse conduction inducing current spike, but that is unusual* and cheating, as conduction is stopped by stopping conduction.

  
  


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  A number of high power camera flashes use this method to terminate flash cycles.

  
  

You can buy an SCS here for $US17.50/25. Given that it is a 1400 Volt x 112 Amp device the price is "not bad". Datasheet here.

An SCS (Silicon Controlled Switch) is a device whose time has largely passed. It lingers on in very high voltage high current niche applications but would almost certainly not be used for new work.

An equivalent functionality can be provided by using 2 x MOSFETS (2 x P or 2 x N channel) connected in series with sources connected and gates connected (!!!).

You thus get a module where the current path is

---[DS]-[SD]--

Driving the 2 connected gates +ve relative to the two connected sources (for an N channel pair) turns the pair on and connecting the two connected gates to the two connected sources turns the pair off. this then is a true bilateral (bidirectional, 4 quadrant) swith - about the most flexible switching device available. Even better than an SCS if you can but an SCS :-).

To equal the above SCS you could use two of something like the IXYS MOSFET (same brand as the SCS) IXTY06N120P 1200V 90A MOSFET priced at $2.79/25 at Digikey but not in stock. It's a TO252 package which is capable but which may get "somewhat stretched" [tm] at those power levels.

In most cases a more normal topology MOSFET or TRIAC solution will do whatever job you have well enough.
There is little that you cannot do with an H-Bridge :-).

http://search.digikey.com/us/en/products/IXTY06N120P/IXTY06N120P-ND/2117410