Can someone identify what these diodes are for?

[liquibyte]

I found this while searching out something else and I modelled it because I was curious.  I'm having a problem getting good results because of VD5 and VD6.  Regular diodes won't work here, or at least I don't think so.  Are these supposed to be references/zeners or something else altogether?

Edit: OK, figured out it's supposed to be an OR network.  I still can't get it to work right though.

 

[Hero999]

Yes, it's an OR network. The diodes make it so the op-amps can only divert current away from the bases of the driver transistors.

A1 is the voltage error amplifier and A2 is the current error amplifier and the OR network stops them from fighting one another.

The drop-out voltage will be huge, made worse by the ORing diodes. All those transistors are connected together to form a lagre three stage Darlington so the over all voltage drop will be over 3 diode drops, plus the saturation voltage.