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| OP Supply Bootstrapping: Substituting the p-Mosfet against an n-Mosfet possible? |
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| Echo88:
Its possible to design a +-1kVout OP-Amp-bootstrapped supplies with series connected BJTs/MOSFETs. See here for a usual design: https://www.analog.com/media/en/technical-documentation/application-notes/an67f.pdf Page 58 Figure 79 and a series connected HV-design: http://bbs.38hot.net/thread-11133-1-1.html or https://www.analog.com/media/en/technical-documentation/application-notes/an87f.pdf Page 58 Figure 84 Since n-MOSFETs are easily available with more than 1kV Breakdownvoltage, while p-MOSFETs are only available up to 600V, theres no chance to achieve +-1kV without using series connected p-MOSFETs or BJTs. IGBTs and SiC-MOSFETs might also be interesting, but they are more difficult to get and p-Channel-variants are rare and therefore thats not a really an alternative. But im interested if theres a solution by using only two high voltage n-MOSFETs, instead of a n- and p-MOSFET. I just cant wrap my head around how to control the n-MOSFET, which regulates the negative supply by using the OP-output. Maybe someone can give a suggestion how that may be doable? Simulation-file from AN67 is attached. Thanks. |
| magic:
You can't easily control a gate which is anywhere between 1V and 1000V below your bootstrapped VEE. Maybe some kind of PNP/P-ch cascode to send a control current down there, but then you need an HV device again. Perhaps an optocoupler. Alternatively, don't control it at all. Set up an N-ch current sink pulling your VEE down to ground and bootstrap yourself by sinking current from HV through a bootstrapped N-ch cascode and dumping it onto VEE to saturate that CCS. |
| David Hess:
Use the n-channel source follower as usual on the positive side but use an n-channel common source current sink on the bottom with a shunt regulator across the operational amplifier circuits to protect them. AC coupling to the current sink may be used to improve performance. Update: I might use optocoupled feedback from the shunt regulator to the n-channel low side current sink to adjust the current for constant current through the shunt regulator even with load changes. |
| Marco:
Do you want a two quadrant DC power supply or a high voltage amplifier? |
| Echo88:
I want a voltage buffer, which enables me to measure +-1kVmax without loading the source (using a pA-bias current OP-Amp) and being able to source (sinking not necessary) a few mA to attach a 8.5 digit DMM for example. Since the DMM would load the source with its 10M input resistance on 100V-range and 1kV-range, it cant be used to directly measure voltages across capacitors for example. At the moment its just a simulation and i was pondering wether the bootstrap-circuit could be build like asked in my first post. Seems the necessity for an isolated mosfet-gate controller/optocoupler + current sink will lead to more necessary parts, than the standard series connected mosfet solution. |
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