Psu Diy- Harrison topology - with problem in CV regulation, when load exceeds 2A - Page 1
Forum section: electronics / projects

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 09, 2023, 04:26:50 pm
content: I am designing this PSU for a maximum load of 3.5A, but a failure is occurring in the CV mesh when it exceeds the consumption of 2.25A. For example, when I set the current to 3A my output drops from 10v to 9.3v.- The LM334 (U12) has a current of 5.8mA configured, which is enough for the TIP142 to provide a maximum consumption of 3.5A.- The RC4558 (U7) is powered with +6v and -5V. Despite not being an RRo solution, I believe there is still a good margin for work.What I've discovered so far:- When short-circuiting the shunt resistors (R23 and R24) the regulation returns to normal, reaching 10v at 3.5A.- I then imagined that it could be the protection transistor BC337 (Q8) acting, but disabling it had no effect.- I removed the OP27 from the circuit and once again without any effect.What's in this CV mesh that I'm not seeing?

author: Warhawk
author_rank: Frequent Contributor
timestamp: October 09, 2023, 05:56:40 pm
content: What about Q8?

author: Kleinstein
author_rank: Super Contributor
timestamp: October 09, 2023, 06:13:40 pm
content: The resistor R26 at the fast current limit is somewhat odd. It don't see it having a correct effect as is increases the current limit with higher output voltage. For a SOA protection one would more have some current from the transistor drain side.2 A may be about the point where the fast current limit with Q8 engages and this can be a good thing.The TIP142 is a bit on the small side for more than 2 A anyway.The current meter part is at the wrong position - that part would likely not work as drawn and the drop in the meter internal shunt would not be compensated.

author: iMo
author_rank: Super Contributor
timestamp: October 09, 2023, 06:28:35 pm
content: His "Q6" is the Q8 (BC337).He removed the BC337 with no effect.The drop comes from the wrongly wired meter, it seems.The meter has to be floating, imho.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 09, 2023, 07:10:57 pm
content: Quote from: Kleinstein on October 09, 2023, 06:13:40 pmThis fast current limit is quite wrong. I'll fix it.Quote from: Kleinstein on October 09, 2023, 06:13:40 pmI removed the Q8 until I know what is going on with the CV control.Quote from: Kleinstein on October 09, 2023, 06:13:40 pmThe small deviation of this shunt does not justify the large compensation error after 2.25A. What is intriguing is that everything works very well up to 2.25A.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 09, 2023, 07:20:36 pm
content: Quote from: iMo on October 09, 2023, 06:28:35 pmCarrying out some measurements I realized that the RC4558 is not able to adequately compensate for the drop in the mesh (Shunt(R23 e R24) + Vbe + R29) greater than 0.7v.

author: iMo
author_rank: Super Contributor
timestamp: October 09, 2023, 07:26:45 pm
content: Try to remove the D5..D8 and D9 could limit the 4558 as well..

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 09, 2023, 08:06:46 pm
content: Quote from: iMo on October 09, 2023, 07:26:45 pmDidn't resolveAs I am carrying out the tests without the OP27, I decided to remove the D7 to test. To my surprise, the regulation started to have problems after 300mA. I imagined that the diodes, in front of the opamps, only had a switching function. I had to return with the D7.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 09, 2023, 08:19:12 pm
content: The voltage lost at D7 helps to shift the point where D14 engages and limits the output.D14 may start conducting if the output is driven hard. So one may consider having 2 diode in series for D14, maybe with a high resistor to GND from the center of them. As a side effect this can help with the precision.D14 limits the wind up of the regualtor, but it can also effect the precision as it starts to engage a bit soft and can have leakage.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 09, 2023, 09:35:37 pm
content: My joy is back! I added another diode in series with D7 and D13. I conclude that the compensation limit goes up to the diode voltage. My limit became 1.4v with the diodes in series, which is well above the maximum compensation (0.95v) necessary for my project.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 12, 2023, 07:36:35 am
content: I have another question with this project, now with CC control. I replaced the OP27 with the OP07, as I gained greater stability (drift) in controlling low currents. I believe that the DIP encapsulation of the OP07 contributed to less drift. The problem is that OP07 is much slower than OP27.How do you measure the CC control response? I did a test as described below but I think I'm wrong.Image 3- This is the circuit I used to try to discover this response time. I configured the CC to operate at 500mA.Image 1- When closing the switch over the 100R resistor, it took 1ms to discharge the capacitor over the 1R resistor.Image 2- I enlarged the top of the exponential to analyze and observed that there is a straight line of approximately 10µs. Would this be the CC control response time?

author: iMo
author_rank: Super Contributor
timestamp: October 12, 2023, 08:06:44 am
content: Years back an user (mike_mike) built a simple "psu shorter" with 555 + power-mosfet (there is the schematics as well) which does the same (ie. XX Hz switching between two currents, like X Amp and 100mA).Here it is:https://www.eevblog.com/forum/beginners/lm324-power-supply-with-variable-voltage-and-current/msg2281502/#msg2281502https://www.eevblog.com/forum/beginners/lm324-power-supply-with-variable-voltage-and-current/msg2283327/#msg2283327Also it is perhaps the longest CC/CV PSU thread with hundreds of measurements, imho

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 12, 2023, 10:01:02 am
content: Imo, thanks for the tip.I understood that I can then measure the CC response time on the shunt resistance (R23,R24). I took three measurements under different conditions. All tests with output voltage at 10v and CC control set to 500mA.image 1: I changed the resistance from 1R to 0R1 in my test circuit.Peak at 3.5A limited by Q8 (fast limiter).. CC control response in 20µsimage 2: I removed the test circuit and short-circuited it to the point of the 1m cable I was using.image 3: I used a very short 4mm cable.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 12, 2023, 02:37:55 pm
content: I conclude that the CC control with the OP07, combined with the Q8 quick limit, meets the project.Image 1:I carried out several transient tests and the project behaved very well.Thank you very much for the help I received!