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High voltage safety and design considerations
OM222O:
Hello
I have to build a current source that drives +-25mA through a coil with a resistance of 1.5k\$\Omega\$ and inductance of 2H. Since the resistance and inductance are huge, I have to use a high voltage op amp to get a quick response. The best choice was the ADHV4702-1 which can do +-110V (basically rectified mains). I haven't designed anything with high voltages before, so I'm not sure what safety considerations I should use. I'm putting 40mA fuses on both rails, directly at the input but I'm not sure if that is enough.
Also since it's used in a robot, I'm not sure what power source to use. It can't be mains since it has to move and can't be plugged in, but I couldn't find a battery with a high voltage. Maybe a boost converter would be the best? I don't have experiance with designing SMPS so it would be nice to use something else if possible.
I attached a simulation from LTspice showing the overall design. The only change is going with 5v control logic, rather than 3.3v but that's not relevant here.
magic:
Rectified mains is 350V peak or more if you account for some mild overvoltage. 700V rail-to-rail for bipolar rectification :scared:
But it doesn't matter if you don't want mains anyway.
You could avoid bipolar supply by using H-bridge. Then you only need one HV rail.
Mind Safe Operating Area of the output transistors. Not every transistor can conduct 25mA while having a relatively high voltage on its collector/drain.
Do you really need serious precision? Perhaps a single resistor cascoded by a BJT/FET would suffice? AKA the "one transistor constant current source".
The opamp doesn't need to be HV if you take an LV opamp and add some circuitry to shift levels and drive HV transistors.
OM222O:
It's an actuator like a voice coil, so using an H bridge slows down the transition, I would like to use the bipolar approach as is. The precision is also important, I'll be using a 16 bit dac.
The last idea seems nice, but I'm not sure how to level shift with transistors :-// the buffer I used is the only one I have seen and it requires the op amp voltage to be the same as the driving transistors in order to have a fast response.
Edit: the transistors I plan on using are:
BUL216
MJE5852G
They have enough DC SOA and high enough CE and BC voltage tolerance
Zero999:
The class B output stage you have there, will have lots of crossover distortion, so no good for precision.
You're mistaken, an h-bridge will not slow down the transition. Why do think it will? A bridged configuration is used in audio amplifiers all the time and works very well. A bridge configuration should theoretically be twice as fast as single ended, because the voltage swing across the load is double the output of each amplifier, so the slew rate will also be doubled. For example, if each op-amp and output stage has a maximum slew rate of 1V/µs, the slew rate of the voltage across the load can reach double that at 2V/µs, because the positive side can be slewing at 1V/µs and the negative side at -1V/µs, giving 2V/µs across the load.
The supply voltage can be reduced and the power dissipated by the individual transistors can be reduced by using a bridged configuration. Here's an example.
https://www.eevblog.com/forum/projects/need-help-with-bi-directional-constant-current-source-(100ma)
OM222O:
I thought an H bridge similar to what is used to drive DC motors, where at 0mA, the opposing transistors turn on and connect the load in the opposite direction, that would slow things down when passing through 0 current.
I used R5 to take care of the crossover distortion by giving op amp direct control when Vbe is less than 0.7v
Sorry but I haven't designed audio amllifiers which seems to be what you are refering to.
I haven't seen that output model before. Can you please explain how it
works? For example what are R6 and R7 for? Where is the shunt resistor? Or maybe a link to a page where they detail this. Also does the op amp need to be high voltage with this config?
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