Single phase half bridge converter working help

[electricallov]

why are we using two diodes instead of two scrs in a single phase half bridge converter? What difference does it make? (say in my attachment s2 and s3 are diodes instead of scrs?)
Wont we get the same operation with two scrs itself (only thing you have to fire them together with s1 and s4). I am confused. Please help.

 

[NascentOxygen]

why are we using two diodes instead of two scrs in a single phase half bridge converter? What difference does it make? (say in my attachment s2 and s3 are diodes instead of scrs?)

electricallov, your attachment is missing.

 

[electricallov]

Im sorry. Now I've resubmitted.

 

[NascentOxygen]

Hi electricallov. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif

Offhand, I can't see a reason for not using 2 SCRs and 2 diodes here, at least for resistive loads. (Highly reactive loads can pose problems for SCRs, so I'd need to examine this closely to see whether that may be a consideration, especially when the L element is shown so prominently in the schematic here. )

 

[GingerLee]

In my humble opinion,
if you use all four SCRs then you would end up with negative voltage output for a short duration before s2 and s4 are triggered.

 

[electricallov]

GingerLee, this negative voltage concept solves everything. I did not notice the freewheeling path created when the two scrs are replaced by two diodes. This solves my question. Thanks