SO can you give a modified scatch?The AD549 has an output that has short circuit current limit of 35 mA. So not best practice to
not use a base current limiting R, however circuit probably still work. But no R in this
case, transistor deep in saturation, aggravating the control loop delay problem.
The MOSFET can conduct considerable current at low Vgs
View attachment 176399
But the AD549 has crappy output swing at a Vs of 3.3V. Or raise its supply V. Would be better to use a rail-rail output OpAmp.
View attachment 176400
Regards, Dana.
Okah so for a while see the circuit like this. (attached).Hi,
The whole circuit makes no sense to me.
Please describe each part .. at least briefly.
I see many problems. like
* output of OPAMP short circuited to 3V3
* no power supply bypass capacitors
* no local feedback capacitors to ensure stability
* no base resistor
* no emittter resistor
* too high ohmic RG
* OPAMP supply voltage range
* ...
Klaus
SO can you give a modified scatch?
that schematic..Whats a scatch ?
Regards, Dana.
Cap will charge upto 42V.Whats a scatch ?
Youi could use the MOSFET as a swiutch, and in its source to cap create a
cons current source, say using a LM317.
View attachment 176408
Add this and add a diode to its output so that when MOSFET is off the LM317
does not discharge the Cap.
So you have MOSFET Source >> LM317 Current Reg >> Diode >> Cap >> ground
for the circuit. <MOSFET just acting as a switch.
How high a V does the Cap need to get charged to ?
Regards, Dana.
Cap will charge upto 42V.Whats a scatch ?
Youi could use the MOSFET as a swiutch, and in its source to cap create a
cons current source, say using a LM317.
View attachment 176408
Add this and add a diode to its output so that when MOSFET is off the LM317
does not discharge the Cap.
So you have MOSFET Source >> LM317 Current Reg >> Diode >> Cap >> ground
for the circuit. <MOSFET just acting as a switch.
How high a V does the Cap need to get charged to ?
Regards, Dana.
so first I don't have choice to use LM317 voltage regulator. And I am charging the cap to the supply voltage. while in your simulation cap will be charged upto 3V, while supply is 12v. That's not my intention.Whats a scatch ?
Youi could use the MOSFET as a switch, and in its source to cap create a
cons current source, say using a LM317.
View attachment 176408
Add this and add a diode to its output so that when MOSFET is off the LM317
does not discharge the Cap.
So you have MOSFET Source >> LM317 Current Reg >> Diode >> Cap >> ground
for the circuit. <MOSFET just acting as a switch.
How high a V does the Cap need to get charged to ?
Here is a simple sim. Not at 1A yiour 1`.5 uF cap charges quickly, is that what yiou want ?
Also not the LM317 takes a few mS to start up, but once it does its acting as a constant
current.
View attachment 176409
Here is same sim but with 1.5F cap, is this more what yiou are looking for ?
View attachment 176410
Regards, Dana.
Don't use any voltage regulator or LDO something.I do not like the sim results. The LM317 requires too much V across it to stay in
current regulation. I tried it with a LDO regulator, that worked better but also seemed
to be in oscillation.
When you pump up cap :
1. Whats max V you will pump it to ?
2. Are you pumping at a 1 Khz rate ?
3. Is cap just 1.5 uf ?
Regards, Dana.
@crutschowStill confused about what you think the circuit should be (seems rather convoluted) and what you actually want the circuit to do (not always the same).
Below is the LTspice simulation of a simple constant-current circuit that will charge the capacitor to the 42V supply voltage:
The charge current is approximately 0.7V / R1.
Transistor M2 turns the circuit on and off.
View attachment 176417
@FvMIn a short, the post #1 simulation circuit has several flaws that make it behave as it does. A few modifications make the simulation basically work, the control loop is however at the stability margin. At should be also noted that AD549 isn't specified to work with +/- 3.3V supply. You'd select a suitable OP instead.
If you want others to check and correct your LTspice simulation circuit, please post a .zip archive of the simulation files (.asc, .plt, nonstandard component libraries if any).
View attachment 176455
Since I still don't understand (and you have not clearly stated) the exact purpose of your funky circuit (why charging in steps?), and why you insist on using it (other than it's the one you designed), I'll leave dealing with that to others.So any suggestion?
To keep within the SOA region of NMOS. As per the SOA, at high voltage we need less current so what i am looking is continuous current steps.Since I still don't understand (and you have not clearly stated) the exact purpose of your funky circuit (why charging in steps?), and why you insist on using it (other than it's the one you designed), I'll leave dealing with that to others.
AFAIK N-channel MOSFETs don't have any advantage over P-channel devices regarding SOA limits, so why do you want to use N-channel?@crutschow
Thanks for this but same phenomena I would like to achieve with NMOS(M1). There you can use an opamp for gate control and npn for fast discharge.
So any suggestion?
An NMOS will not charge the cap to the full 42V unless you have a higher bias voltage for the MOSFET, so a P-MOSFET would be better.To keep within the SOA region of NMOS
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