High voltage constant current circuit

[DanioIO]

Hi,
How can i change a standard constant current circuit so it will work with 500V supply which exceeds the maximum supply voltage of a op amp?

[rs20]

I'm not sure if that op-amp circuit is necessarily the right way to go there, others may have other suggestions. But that aside, it seems to me that all the voltages the op-amp needs to see there are near 500V, so just use 480V or whatever as the op-amp's ground and you'll be fine. That way, the op amp has a 20V supply. Maybe a 5V supply would be more appropriate. Or, equivalently, re-draw your circuit with ground at the top, and negative 500V at the bottom, and work that way.

Be careful with 500V! What are you actually doing? How much current are you hoping to deliver? Do you really need constant, regulated current, or would a resistor suffice?

[Zero999]

That circuit uses a PNP output transistor which has a lot of gain so will need some additional frequency compensation, otherwise it'll oscillate.

I was wrong. Looking at it again, the feedback is taken from the emitter, so it's an emitter follower, rather than a common emitter amplifier as I'd previously thought. A common emitter amplifier is fairly immune from oscillation and can safely included in the feedback loop of an op-amp without worrying about introducing another pole or oscillation.

The sense resistor is connected to +V so the op-amp needs to float near the +V and both of the inputs must be within the common mode range.

How much current do you need?

[richard.cs]

You'll have to be really careful with the transistor selection there of you want to avoid problems with second breakdown. Scan the datasheets for the dc safe operating area.

[f5r5e5d]

how much current, accuracy, noise, Zout?

there a (very) few depletion mode MOSFET made today that can be used in 2 terminal CCS to 500V, Supertex(now Microchip?) and Ixys

[Marco]

Ignoring the wisdom for a moment, the smallest change so that particular circuit can still work is to use a simple zener+resistor+PNP regulator to make another voltage rail for the opamp, 490V for instance.