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| Is 550uF too big for a power supply that has CC limit? |
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| bson:
--- Quote from: bloguetronica on January 29, 2019, 12:56:12 pm --- --- Quote from: bson on January 29, 2019, 05:43:47 am ---The IC4B based sense amplifier lacks external compensation. --- End quote --- The compensation is done externally, via shunts in J5. --- End quote --- I haven't had time to follow this thread, but an error amplifier can't have the same or greater bandwidth than the response of that which it controls, be it a voltage regulator or VCO. The way to control its bandwidth is through external compensation, and that needs to be local to the amplifier. It can't be shared. If it has excessive bandwidth it will swing wildly when the downstream stage can't respond. Edit: oh, and a shunt is a bad compensation mechanism; it works by presenting a load that makes the error amplifier operate current limited. Since it's shared by the input of the next stage, any change to the input impedance will affect the error amplifier's bandwidth! And then there's the potential for power dissipation in the error amplifier when it has to drive a large shunt cap. |
| Kleinstein:
--- Quote from: bson on February 04, 2019, 06:59:50 pm --- --- Quote from: bloguetronica on January 29, 2019, 12:56:12 pm --- --- Quote from: bson on January 29, 2019, 05:43:47 am ---The IC4B based sense amplifier lacks external compensation. --- End quote --- The compensation is done externally, via shunts in J5. --- End quote --- I haven't had time to follow this thread, but an error amplifier can't have the same or greater bandwidth than the response of that which it controls, be it a voltage regulator or VCO. The way to control its bandwidth is through external compensation, and that needs to be local to the amplifier. It can't be shared. If it has excessive bandwidth it will swing wildly when the downstream stage can't respond. --- End quote --- This limitation is not very strict: if the system to control is well known and well behaved it is allowed to have the controlled system to be slower than the regulator. However this is not the normal case. For a simple (dominant pole) compensation one needs one time constant to be much longer than the others - in theory this could be the system to control. Still it's likely no a good idea to have an OP without a local compensation - even if only to counteract varying speed of individual units. |
| bloguetronica:
--- Quote from: bson on February 04, 2019, 06:59:50 pm --- --- Quote from: bloguetronica on January 29, 2019, 12:56:12 pm --- --- Quote from: bson on January 29, 2019, 05:43:47 am ---The IC4B based sense amplifier lacks external compensation. --- End quote --- The compensation is done externally, via shunts in J5. --- End quote --- I haven't had time to follow this thread, but an error amplifier can't have the same or greater bandwidth than the response of that which it controls, be it a voltage regulator or VCO. The way to control its bandwidth is through external compensation, and that needs to be local to the amplifier. It can't be shared. If it has excessive bandwidth it will swing wildly when the downstream stage can't respond. --- End quote --- That makes sense. Kind regards, Samuel Lourenço |
| Marco:
A switching+linear power supply would have an advantage in getting fast CC with an emitter/source follower, it could use a smaller/faster output transistor. |
| bloguetronica:
I officially declare this project to be a failure. Assembled the board, and meanwhile the f*king regulator decided to deliver 24V to the whole 5V bus rail, so I had to replace many of the ICs. Second try, managed to compensate IC4 successfully using a 10nF capacitor in the feedback loop (a 3,3nF one would do, although marginally). However, when loaded, I was having interferences caused by the DC-DC pre-regulator, causing the output voltage to drift a few 10s of mV. Even soldering a 10uF capacitor in parallel with C16 and replacing R15 with a 22K resistor wouldn't solve the issue. Disconnecting the Vret line to the DC-DC made the issue go away. Meanwhile disconnected the 24V line feeding the regulator and pfff. Again! So, there are some frailties: - The DC-DC pre-regulator idea must go away; - The INA180A1 can surge the 5V line if the supply is disconnected, taking the regulator with it. Then, when the regulator is powered, it fries itself and everything on the 5V line; - The CC must go away, all of it, despite the fact that it works. A simple SC protection should work. The good news: - Apparently, a single 47uF capacitor at the output is enough to stabilize the supply, provided that IC4B is compensated using a 3,3nF to 10nF capacitor; - The temperature limiting works great. Learned the lesson. Project ditched. Thread closed! Kind regards, Samuel Lourenço |
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