In the schematic attached below, what's the purpose of C5 and C6 in conjunction with D7 and D8?
D7 and D8 along with C5 and C6 form a voltage reference of roughly 600V for the gate of Q1.
Q1 is a cascode device which buffers the drain voltage of Q2.
The combined "switch" of Q1 and Q2 are then capable of withstanding a much higher voltage than the 800V rating of a single device.
When Q2 turns on, D17 clamps the Vgs of Q1 to 15V, and Q1 turns on as well.
When Q2 turns off, Q1 acts as a source follower, and its source (and therefore the drain of Q2) will sit at roughly one Vth below the gate, which is at the ~600V reference created by D7 and D8.
The drain current of Q2 when off is only the leakage current, so the dissipation in Q1 is small.