fast CC/CV power supply take two

[iMo]

I wonder whether somebody tried a PSU with for example the TDA2020 - that is basically an opamp +/-22V, 3.5A output, 20W, with two pins for controlling the output transistors (two internal "power and current limiting circuits").

That could be a quick way to make a 4 quadrant PSU if CC mode could be added.

Like this PSU playing music

[xavier60]

I have combined my other two ideas into one idea for fast CC response limiting.
While in CV mode, Q5 keeps the non-inverting input of the CC opamp switched to ground, shunting away negative feedback from the compensation cap C4.
When the CC threshold is exceeded, the output of the CC opamp only sees the small capacitive loading of C4 so is able to slew down at close to it's full speed.
Q2 senses when the CC loop is in control and turns off Q5 via Q4, allowing C4 to provide feedback for normal CC compensation.
I took a photo of the CC opamp's output at the time the PSU's output was short circuited.

[Zoli]

I' playing around with simulations, with a radically different topology(sorry, no details yet). What can I recommend: use faster components; as example 2N5550, 2N5401 as control transistors, 2SCR586J as bypass transistor, OP37 as op-amp; all of them are part of LTspice standard distribution. Now about the pictures: first is is current limit set to 1.5A, the second current limit set to 3A; current sensing resistor 0.1Ω, reference 7V, output divider 360k/100k. Simulation command: .tran 0 4.5m 4.395m startup; load configuration: PULSE(1 2 4.4m 1u 1u 49u).

[xavier60]

Do you see an over-shoot across the CS resistor when the output is short circuited?

[Zoli]

Voltage over the CS resistor, current limit@1.5A; not nice, but does the job.
Edit bonus: output voltage with 330µF/50mΩ capacitor.

[xavier60]

This simpler version also has 3us of short circuit current overshoot.

[Zoli]

This simpler version also has 3us of short circuit current overshoot.

Overshoot where? On the CS resistor, or on the load? To understand the importance of the question, see attachments:
-Green: output load
-Blue: current sensing resistor
The first(Load&CS_W_RC(1+4.7).png) has an RC snubber over the output, 1µF+4.7Ω, exactly like in your drawing; current within 10% the target(1.5A) shortly after the 70µs mark.
The second(Load&CS_W_RC(082+22).png) RC snubber is 82nF+22Ω; current within 10% the target(1.5A) around the 15µs mark.
The third(Load&CS_WO_RC.png)has no RC snubber over the output; current within 10% the target(1.5A) around the 12µs mark, but with ringing extending up to the 40µs mark.
Note: the CS resistor is in series with the main supply, exactly like in your drawing; on my previous screenshots were in series with the load.
It takes around 5µs¹ for the CC circuit to take over the bypass transistor control, that's the spike around the 11µs mark.
Simulation command: .tran 0 4.5m 4.395m startup ; PULSE(1 2 4.4m 1u 1u 90u).
Schematics: I intend to release it soon, in public domain; since is niche of the niche, not worth trying to monetize IMNHO, even if it would be patentable.
¹5µs is the time required to discharge the compensation capacitors in the CV section.

[xavier60]

I always measure across the CS resistor. I have also included the Base of Q1.
I personally have no need for super fast CC limiting.
This is the design of the mainly used PSU on my bench, https://www.eevblog.com/forum/beginners/lm324-power-supply-with-variable-voltage-and-current/msg3582664/#msg3582664

[Zoli]

I've posted my design:
https://www.eevblog.com/forum/projects/voltage-regulator-in-current-feedback-topology/
Pay a visit for some reading and comment.
TIA,
Zoli

[luudee]

I've been following this thread with great interest.

Why not use an approach similar to high speed SERDES. They face the same
problem, trying to turn the output on or off really fast.

To avoid over and under shoots, the use a three stage driver:
1. Pre Driver
2. Main Driver
3. Post Driver

All three are very fast, the Pre and Post are weak drivers.

I wonder if you guys can adopt the same principle to this "fast PSU".

Good Luck,
rudi

[T3sl4co1l]

That's rather cryptic.  Could you provide a little more information or background on that?  Unqualified, "pre" "main" and "post" could be anything: any ratings, any connection.  It's also not clear if it would be a specifically digital solution, or at all amenable to analog function.  At the least, it doesn't sound like it needs to be reactive -- high speed digital signals transmit into controlled impedance and that's it, if the impedance is lumpy your signal is lumpy, end of story; whereas this circuit is intended to respond in a very specific way to different load conditions.

Tim

[luudee]

That's rather cryptic.  Could you provide a little more information or background on that?  Unqualified, "pre" "main" and "post" could be anything: any ratings, any connection.  It's also not clear if it would be a specifically digital solution, or at all amenable to analog function.  At the least, it doesn't sound like it needs to be reactive -- high speed digital signals transmit into controlled impedance and that's it, if the impedance is lumpy your signal is lumpy, end of story; whereas this circuit is intended to respond in a very specific way to different load conditions.

Tim

Hi Tim,

yeah sorry, I know it is a bit cryptic, lol.

Basically you have 3 drivers, first one and last one are "weak".

To turn on a signal (drive to logical '1'), you first engage the pre-driver, then the
main driver. The idea is that the pre-driver will charge up the line and prevent
overshoot.

To turn off the signal, you disable the main driver and enable the post-driver,
before disabling it as well. The idea here is to avoid undershoots.

It's a little bit of both digital and analog, but then at 56Ghz, nothing is digital nor
analog any longer, lol

Best Regards,
rudi