| Electronics > Projects, Designs, and Technical Stuff |
| Is there any variable DC-DC converter suitable for pre-regulation? |
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| xavier60:
There could be complications. The Base of T1 needs to be pulled down to 1v. If you try the PNP follower idea, the op-amp might need a load resistor on the output pin to help get the voltage down. The transistor's B-E junction will also need to be protected from more than 6v reverse bias somehow. There is no negative control rail? |
| bloguetronica:
--- Quote from: bloguetronica on September 26, 2018, 12:26:29 am --- --- Quote from: T3sl4co1l on September 26, 2018, 12:07:06 am --- --- Quote from: xavier60 on September 26, 2018, 12:02:55 am ---Why not just have a diode between the CC op-amp and the Base of T1? Or a PNP Emitter follower. Then there will not be extra gain between the op-amp and the Darlington. --- End quote --- I guess that's not original enough. :-\ Tim --- End quote --- Never though of that. Kind regards, Samuel Lourenço --- End quote --- On second though, bad idea to use the diode. The op-amp will not not drive the pass transistor properly. Had to replace the op-amp with a 24V part, and still didn't work. Anyway, if HP, aka Agilent, aka Keysight (they are the same) uses this, it is probably patented. Kind regards, Samuel Lourenço |
| bloguetronica:
Hi, Did a new simulation using different values. Seems to be stable if I do a transient analysis for DC point operation. It shows oscillation if I do transient analysis based on the initial values or on zero values. At least, there is promise. Anyway, if I do a longer analysis, say, 100ms, I see that the voltage drifts upwards from the CC setpoint of 5V to the CV setpoint of 8V. Kind regards, Samuel Lourenço |
| xavier60:
I doubt that HP would mind someone using their idea for a one off project. It may not have originally been their idea anyway. I'm mainly curious as to how the simulator deals with it, the re-purposing of the Comp2 pin in that way. |
| bloguetronica:
Well, the simulator is not making any sense. There is no reason why the voltage should drift from 5V to 8V in the first place. I'm not blocking any DC with that added capacitor, except for the feedback loop. Thus, DC wise, the op-amp still works as a comparator of sorts. Thus, that behavior is impossible in real life. It seems I'll have to build this circuit myself and then tweak some component values. In the simulation I forgot to add a capacitor to the output, which is, in any case, essential. The power supply will oscillate without one. Attached is the circuit I'm going to test. The voltage generators will be replaced by DACs. There is no better simulator than real life. Kind regards, Samuel Lourenço |
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