Electronics > Projects, Designs, and Technical Stuff
Just another DC Load
temperance:
If an LM358 fits your application depends. The offset current= op amp offset voltage / Rsense or 20 mA for 2 mV offset with a 0R1 sense resistor.
The model is fine. Unlike your MOSFET model, transconductance is non linear in a real MOSFET. But you've set Gm to be 30. Anything lower than 30 will only improve phase margin because the MOSFET voltage gain decreases. It is anyhow a trade off because the voltage gain depends on the wiring inductance and the snubber.
Crss dependen on Vds. You could try higher values for Crss as the value of 325 pF applies for Vds= 20 V. Crss is 700 pF with Vds is 5 V and 1 nF at 2.5 V Vds. (Crss is taken from Fig.11 from the data sheet) I think this is more realistic.
You've modeled two inductors in series with the supply under test. This is not required as the supply under test can be replaced by a short for AC signals (up to some frequency in real life).
Jay_Diddy_B:
Hi,
The LT1014 is really a trimmed version of an LM324.
From the ADI (Linear Technology) datasheet:
The LT®1014 is the first precision quad operational amplifier
which directly upgrades designs in the industry standard
14-pin DIP LM324/LM348/OP-11/4156 pin configuration.
It is no longer necessary to compromise specifications,
while saving board space and cost, as compared to single
operational amplifiers
The main feature is reduced input-offset voltage.
The other features that are common to between the LM324 and LT1014
The common mode input includes ground and the output can swing close to ground. By ground it means the negative supply voltage in single-supply applications.
The datasheet says:
Both the LT1013 and LT1014 can be operated off a single
5V power supply: input common mode range includes
ground; the output can also swing to within a few millivolts
of ground. Crossover distortion, so apparent on previous
single-supply designs, is eliminated. A full set of specifications
is provided with ±15V and single 5V supplies
The LT1013 is a better LM358
and the LT1014 is a better LM324
The LTspice library does not include the LM358 and LM324. The LT1013 is a suitable substitute.
In the Jay_Diddy_B original design I used dual power supplies, +/-9V. I did this to make summing of the static reference and the dynamic reference easier. The dual-supply allowed me to use a wide range of op-amps that don't need include the negative supply rail in their input common mode range.
Jay_Diddy_B
Jay_Diddy_B:
Hi group,
I am going to explore buffering the output of the op-amp using LTspice models. You can decide if the extra complexity is worth the results.
I replaced the transconductance model of the MOSFET with a real MOSFET model.
Unbuffered
Ideal Buffer
The voltage-controlled voltage source E1 has a gain of 1.
Practical Buffer - 4 BJTs - Diamond Buffer
This is based on the circuit of the National Semiconductor LH002
Regards,
Jay_Diddy_B
henmill:
--- Quote from: Kleinstein on January 30, 2025, 07:10:11 pm ---The DIP version of the LM324 would be no issue. For the MC34074 it looks like there was never a DIP version.
The main point is getting an OP-amp that is single supply and thus can work down to the negative supply.
Another option, a bit similar to the MC34074 is the TLC274.
I don't see a need for quad OP-amp - only 2 of the amplifiers are critical. As dual there is the LT1013 as a version with good accuracy, if this is wanted.
--- End quote ---
Thanks, I guess I was mistaken on the M340xx. Anyhow, if I feel the need to buy a new one I will likely start with the Digikey filter and work from there. Sometimes it's easier to start by filtering out what is available and in my price range, package, etc. TLC274 looks nice.
I mostly want a quad to keep my current board going, maybe make a new one but in the same style. I need one amp for buffered reference, one for the gate, and another for panel meter. The 4th I want available in case I implement some kind of overcurrent detection/prevention. Or I could have a second FET for more power. Etc.
--- Quote from: temperance on January 30, 2025, 07:45:01 pm ---If an LM358 fits your application depends. The offset current= op amp offset voltage / Rsense or 20 mA for 2 mV offset with a 0R1 sense resistor.
The model is fine. Unlike your MOSFET model, transconductance is non linear in a real MOSFET. But you've set Gm to be 30. Anything lower than 30 will only improve phase margin because the MOSFET voltage gain decreases. It is anyhow a trade off because the voltage gain depends on the wiring inductance and the snubber.
Crss dependen on Vds. You could try higher values for Crss as the value of 325 pF applies for Vds= 20 V. Crss is 700 pF with Vds is 5 V and 1 nF at 2.5 V Vds. (Crss is taken from Fig.11 from the data sheet) I think this is more realistic.
You've modeled two inductors in series with the supply under test. This is not required as the supply under test can be replaced by a short for AC signals (up to some frequency in real life).
--- End quote ---
Thank you for pointing out the Crss behavior, I will definitely step through some values and see how performance changes.
About the inductor in series with the DUT return, I thought it just made sense if we were including it on the positive side. Often times a source will be hooked up with longish leads that aren't well coupled.
Or is it that there is just no interaction with the stray inductance beyond the control's reference? So it doesn't matter for the control stability? (thanks for helping my understanding in advance)
henmill:
Hi everyone,
Sorry for the extended lack of updates. Lost some steam via lack of free time, but I also detoured to rework the thermals and layout of the system to get it to it's temporary final state so I can get back to other work.
Recently I've spent an embarrassing amount of time dinking with the sims, but realized a mistake the other day that has unlocked my ability to move forward :D
Earlier in this thread I said one goal of the project was to use as much of what I have on hand and spend as little as possible. But I was lacking capacitor values between 200p and 10n, and also wanted some NTCs to incorporate, so I also shopped around and ordered some LM324 and TLV2374, just in case I have problems with the LMC6484. I still don't want to sink much $ into this project, but I will take any excuse to enhance my library of parts haha.
So, starting with the new thermal/layout concept:
I switched to a short, wide, finned heatsink, mostly because the other one I could not quite fit comfortably enough to close the lid. Not sure if this new sink is optimal but I would think I could dissipate 50W continuously. I need to do more research to back that claim up though, but will likely just test it empirically ;)
Now, onto the simulations. I've done some comparisons between the LMC6484 and TLV2374 with the simplified small signal model of my part IXTH80N075L2, as well as the model provided for a different IXYS part, used by a different user that Jay_Diddy_B also helped out. That part number is IXTN4650L, and you will notice it has even higher apparent capacitance Ciss compared to my part. So to me a reasonable hypothesis is that if my configuration is stable driving either part, there is a good chance it will be IRL.
In addition, I ran the simulation at Vin of 5V and 25V, and adjusted my model based on the datasheet values of Ciss, Crss vs. VDS:
To summarize, attached are sims showing phase margin for the following:
opamp | Mosfet | VDS
LMC6484 | IHXTH80N.. | 5V
LMC6484 | IHXTH80N.. | 25V
LMC6484 | IHXTN46N.. | 5V
LMC6484 | IHXTN46N.. | 25V
TLV2374 | IHXTH80N.. | 5V
TLV2374 | IHXTH80N.. | 25V
TLV2374 | IHXTN46N.. | 5V
TLV2374 | IHXTN46N.. | 25V
Long story short, I think I am safe to use the LMC6484, but might swap it out anyways to save it for something where its features can shine more. Please let me know any thoughts you have regarding my methodology and reasoning!
I will get around to running transient sims eventually...
Here is the schematic used for all these, in which I swapped the opamp and mosfet around to compare.
Another detail, I have decided to ditch the idea of using a big power resistor in series. With the parts I have, even at 1 ohm resistance I will be limiting my testable current to 5A at 5V (at best!) and I would like to have the ability to sink more at the low voltages, just in case! If anything, I might rig up a disconnect with overcurrent trip, or a plain old regular fuse.
There is probably more I could say, but I want to get this out here for comment.
Thanks as always for feedback!
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