High Voltage leaky diode (or any diode like device)

I have a circuit that is essentially a resistor and zener. The zener is a
200V low TC zener (1n4085) and the resistor feeding is is 300K. The supply
voltage will vary in an nominal range of 100 to 300V and nust deal with 5
second transients up to 600V (once/hour). To keep the peak power
dissipation below the ~1/2 the diodes rated power of 2.5W, the current
limiting resistor has to be 32K.

In this circuit I need the zener to be a follower (Vcathode = Vin) when the
device is below 200V and a clamper at 200V when Vin ia above 200V.
Odviously the zener has a knee that is somewhat soft but acceptable. My
problem is at 100V the zener has leakage that is proportional with
temperature. This will cause an error when the sener is in vollower mode
that gets worse with temperature.

I'm thinking that if I can find a 800-1000V diode that leaks in a manner
that more that compensates for the zener leakage below Vz, and hang that in
parallel with the 32K resistor, that would fix this problem.

What I really need is a say a 1mA 800 to 1000V rated current source that is
fairly constant over a wide temperature range and doesn't require more than
1V to turn it on.

any ideas?




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