PSU with discrete components. Any tips for CC control. - Page 1
Forum section: electronics / projects

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 06, 2022, 04:26:48 pm
content: HiI decided to challenge myself and build a PSU for my own use, using only discrete components.Vout = 0 ~10vImax = 2ACV and CC controlThe CV part is already well advanced, where I'm having good results in terms of stability and low noise. It remains to perform the transient test and thus be able to verify the phase margin.My other challenge is with the CC control, which I want to place on the high side. I see many projects using dedicated Opamps and CI. Already using only transistors, the information is kind of scarce. I received a tip that I could use a current mirror, which would then be unbalanced by the sensor resistance. Does anyone have a tip or example circuit, on how I could design this CC control, only using discrete components? Going from 50mA to 2A

author: magic
author_rank: Super Contributor
timestamp: October 06, 2022, 07:44:12 pm
content: This input stage has phase reversal. When Q3 base is at ground, the emitter is one diode above ground and the collector too. Q3 saturates and can't feed current into Q7 base, even more so if a couple mA coming from Q2 needs to pass through R13, so Q7 emitter is at +100mV or so. Then the output goes to the positive rail and there is smoke. This is exactly the same problem that TL072 is infamous for, also present in RC4558 and friends.LM358 solves it by adding PNP emitter followers in front of the input pair. Others solve it by replacing the NPN mirror with NPN folded cascodes and adding a PNP mirror above them. Or similar tricks.

author: magic
author_rank: Super Contributor
timestamp: October 06, 2022, 08:02:39 pm
content: As for current limiting, perhaps the most sane approach is to have a voltage reference and a second opamp which floats with the output voltage.The simplest way to get the above is using the base-emitter voltage of the output darlington to power the CC circuit, but then it takes some clever low voltage tricks to get it all done at 1.2V supply. In particular, when it comes to voltage references, you are limited to a crappy diode with its 1% per 3°C thermal drift. A bandgap reference could be attempted, but it's not clear if it would work much better.A different and maybe saner approach is to use a small transformer (or USB charger) to provide a floating supply. Then also the CV circuit can be powered from it, and they may even share a common and decent voltage reference, like a 5.1V zener.See here:https://www.eevblog.com/forum/beginners/shitty-chinky-psu-kit/

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 06, 2022, 09:20:31 pm
content: Quote from: magic on October 06, 2022, 08:02:39 pmMagic, my challenge will be to build the CC control on the high side and without appealing to opamps. I just found an article that mentions the tip I had received, which is to use a current mirror to read the sensor resistor. I'm going to replace the opamp in the example with a differential and see what happens.https://sound-au.com/articles/bench-supply.htm

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 06, 2022, 09:29:04 pm
content: Quote from: magic on October 06, 2022, 07:44:12 pmI have the circuit assembled here on the breadboard. Tomorrow I will do fault simulations, to try to better understand what you are reporting to me.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 06, 2022, 11:37:06 pm
content: HiI have finished devising a sketch of the CC control. I will first test with my electronic load.

author: magic
author_rank: Super Contributor
timestamp: October 07, 2022, 05:18:41 am
content: Quote from: Avelino Sampaio on October 06, 2022, 09:20:31 pmA discrete opamp is still an opampThe current mirror thing only transfers shunt voltage to ground, an opamp is still needed to compare it against the preset.Alternatively, the potentiometer could be connected to the positive rail and an opamp could compare it directly to the shunt.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 07, 2022, 03:33:26 pm
content: I was able to implement CC control with a good result.A quiescent current of 30mA flows through R1. With the adjustment of P2, this value was compensated in the current mirror.I managed to set the value of R27 so that P3 was in the range of 30mA to 2A.

author: magic
author_rank: Super Contributor
timestamp: October 07, 2022, 05:15:47 pm
content: I think the exact value of current limit changes with supply voltage.And if the supply is unregulated (trasformer + rectifier, for example), the output current in CC mode will ripple.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 07, 2022, 05:29:58 pm
content: Why use two shunt resistors (R1 and R15)I can understand wanting an extra gross current overload protection (Especially while experimenting), but I do not see a need to do this with two shunt resistors.If you want to keep the high side measurement, then increase R1 and remove R15. Move Q9 to R1 and add an extra transistor so you can shunt the base current of your power resistor to GND to squeeze it off on a gross overload.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 09, 2022, 08:59:14 am
content: Quote from: Doctorandus_P on October 07, 2022, 05:29:58 pmI've had previous experiences using MAX4080 and Opamp and they weren't fast enough. In my CC project I am using an integrator capacitor to decelerate and this can cause me the same problem. So I will keep R15.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 09, 2022, 09:28:08 am
content: Quote from: magic on October 07, 2022, 05:15:47 pmThe transformer I ordered has arrived (12 - 0 - 12 / 3A) and today I'm going to assemble the power supply to carry out more realistic tests. I intend to design the font with filtering at the minimum threshold. So I will have considerable ripple voltage at play when the current approaches 2A. I will return later today with the results, on how much this will affect CC.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 09, 2022, 05:27:53 pm
content: HiUsing my Electronic Load, I conclude that 3000uf will be enough to meet the project's demand.1 - Power supply2 - In the load of 2A, the ripple voltage in the limit to have a maximum regulated output of 10v.3 - Ripple value at 1A load.Now it's time to insert this source into the project and test the behavior of the CC module, with all this ripple voltage.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 10, 2022, 10:47:25 am
content: Quote from: Avelino Sampaio on October 09, 2022, 09:28:08 amMagic, I made two changes to the CC module. I added a current source, using the LM334 and moved the module actuation, which was under the VAS, to the reference voltage CV.To observe possible variations in CC mode, I monitored the voltage across resistor R15 (0.33R). And the results were satisfactory.1- Updated schema.2 - 1A current, without actuation of the CC Module. Oscilloscope reading on R15.3 - Actuation of the CC module. Oscilloscope reading on R15, without oscillations.4 - Minimum value I managed to regulate was 17.7mA

author: oz2cpu
author_rank: Frequent Contributor
timestamp: October 10, 2022, 04:44:41 pm
content: here is another all transistor PSUthis one also provide selectable foldback or CC modestart at 6:30https://youtu.be/Ut5VLmpI54oI am sorry i dont have the schematic, but it looks a lot more simple, compared to the schematics posted in this thread.by the way: tomorrow i release a new video about one more psu, made with the same transistor only design,i go thru its design a bit more deeply, so please come back :-)

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 14, 2022, 05:03:58 pm
content: HiContinuing, I performed the transient test.Output at 5v, maximum pulse of 1A with duration of 200Us1- Updated schema2- result with capacitor C3 = 1nf3- result with capacitor C3 = 2nf4- result with capacitor C3= 3nfAnother good behavior of this project..

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 15, 2022, 06:09:45 pm
content: Quote from: magic on October 06, 2022, 07:44:12 pmMagic, I forgot to mention that I performed tests and in fact the output goes to the positive rail. However, in my perception, this failure condition is of very low risk, given that c8 or q3 will have to fail for it to occur. Or is there some other situation that leads to the inversion, and I'm not noticing it?

author: magic
author_rank: Super Contributor
timestamp: October 15, 2022, 07:08:21 pm
content: What happens when you simply try to set output voltage to zero, by turning P1 all the way down?I'm referring to the latest (final) schematic.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 15, 2022, 08:56:29 pm
content: Quote from: magic on October 15, 2022, 07:08:21 pm2.8mv reading as you can see in the image. Same value if I hit the mute switch.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 16, 2022, 12:04:09 pm
content: Quote from: Avelino Sampaio on October 15, 2022, 08:56:29 pmMagic, this value of 2.8mv comes from the base of Q4, as transistor Q8(tip120) is off. This is a situation I don't want, so I added P4 to set the minimum output voltage value to 0.5v.I noticed that the CC control didn't work well at low voltage, so I returned it to the VAS. I also changed the position of the MUTE switch.

author: Psi
author_rank: Super Contributor
timestamp: October 16, 2022, 12:41:41 pm
content: If a super simple CC with minimal components is desirable. You can use a P transistor with emitter and base across a high side current sense resistor and set the value of the resistor so the transistor starts to turn on when the current through the resistor is higher than your CC limit.Then you connect the collector through a resistor to your voltage feedback line. When the current is above a threshold it will start to reduce the output voltage to keep the current under that point.Being so crude it has some disadvantages.- It's not very accurate as you a relying on the transistors exact turn on voltage/current.- The resistor value needs to be quite large to get enough voltage drop, which means you have to burn off some power in the resistor.- It's only suited to a fixed current limit, rather than something adjustable

author: magic
author_rank: Super Contributor
timestamp: October 16, 2022, 01:08:40 pm
content: Quote from: Avelino Sampaio on October 15, 2022, 08:56:29 pmThank you for checking, I didn't expect this to work correctly.R9 must be helping by limiting how much current can be pulled out of the base of Q3 and preventing the base from really going all the way down to 0V. Perhaps a useful trick to remember...

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 16, 2022, 02:10:49 pm
content: Checking the behavior of the output voltage and the stabilization time, when energizing the PSU.1- Output voltage at 5v and no load - time of 57ms and no overshoot.2- Output voltage at 5v and with a load of 0.6A - time of 62ms and no overshoot.3 - Output voltage at 5v and no load - 1s time to discharge the 100uF capacitor.4 - Chinese power supply (30v/10A), with 5v output voltage and 0.6A load - my project is twice as fast as my switching power supply.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 16, 2022, 04:06:57 pm
content: Quote from: Psi on October 16, 2022, 12:41:41 pmThanks for suggesting this option to me, however the CC module presented is working very well!

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 17, 2022, 03:48:40 pm
content: New schematic update, adding the cooling system. I also added the Digital Meter.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 17, 2022, 08:57:47 pm
content: HiWith the PSU off, I connected a 6v source to its output. I expected a current of around 10mA but the value shown was 53mA. I have identified that most of the current is flowing through Q7 (43mA). I took some measurements and noticed that Q3 of the differential is saturated and Q4 in the cutoff. Any idea how I could stop Q7 from conducting or minimize its current flow?

author: magic
author_rank: Super Contributor
timestamp: October 17, 2022, 09:23:13 pm
content: Well, the reverse protection diode near TIP120 supplies power to the whole PSU from the output. The regulator works normally, and if it decides that the output is too high, it will try to pull it down.If you are concerned about Q7 overheating, you could limit its current. For example, adding a Schottky diode from Q3 collector to Q4 collector should limit R13 voltage to 300~400mV.editEven less than 300mV, because of R11 voltage drop. Not sure how much current is there under normal conditions? Maybe 1N4148 would be better fit. Or maybe R13 could be changed, it doesn't look like its value is very critical.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 17, 2022, 10:44:17 pm
content: Quote from: magic on October 17, 2022, 09:23:13 pmIt worked with the schottky diode. Did the MAGIC! I tested up to the intended 10v, with the maximum consumption of 43mA.Thank you very much!!

author: magic
author_rank: Super Contributor
timestamp: October 18, 2022, 05:34:19 am
content: 43mA?That's exactly what you reported before, so where is the improvement?I expected less...editBTW, what I wrote yesterday is nonsense.It appears that differential stage current is 4mA. When all of it flows out of Q3, the new diode turns on the current mirror and Q5,Q6 both sink half of 4mA or 2mA each. So there is 200mV at R12, 900mV at Q6 base, 1200mV at Q7 base, 500mV at Q7 emitter and ~23mA Q7 current assuming 22Ω value of R13.That's what I would expect.It's also possible to connect the schottky elsewhere. For example, schotky+1N4148 from Q7 base directly to ground. Then Q7 base voltage is guaranteed not to exceed ~1V. Arguably a more straightforward solution, but more components.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 18, 2022, 11:10:32 am
content: Quote from: magic on October 18, 2022, 05:34:19 amSorry, I was too vague.See the values on R13, after inserting the schottky diode between Q3 and Q4. Note the red part, the values without the diode.Source >> Vr13 >> IQ74.00v >> 0.18v >> 8.2mA6.00v >> 0.26v >> 12mA (0.96v >> 43mA)8.00v >> 0.34v >> 15mA10.0v >> 0.40v >> 18 mASee the values with the second option (schottky + 1n4148).Source >> Vr13 >> IQ74.00v >> 0.14v >> 6.4mA6.00v >> 0.18v >> 8.2 mA8.00v >> 0.22v >> 10mA10.0v >> 0.24v >> 11 mAI will opt for the two diodes in the base of Q7. Maximum current was 35mA at 10.0v.Update: the(schottky + 1n4148)option in the base of Q7 had to be discarded, as it presented current consumption, unbalancing the differential. I returned to the schottky diode option between Q3 and Q4 collectors.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 18, 2022, 02:16:19 pm
content: In my last test I decided to check how fast the Dropout system is. I don't know if the way I applied the tests is valid.Test 1:Output voltage at 5v with load of 0.6A. I applied a short between collector and emitter of Q8 (tip120) and the result is in the image below. I have no idea if 0.5ms of this overshoot would be fatal for some components (opamp, PIC, ...)Test 2:Turning on the power supply with transistor Q8 shorted. I couldn't get a reading with the oscilloscope, about the value on the output. I put an LED on the output and repeated the test. The LED took no damage.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 23, 2022, 10:41:04 am
content: HiI made two more changes to the project (green lines) and had the pcb manufactured. I will return to present the end of the project.Thank you very much for the help of the collaborators.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 30, 2022, 04:44:32 pm
content: HiWhile I wait for the PCB from China, I decided to build a preamp, with a gain of 100 (40db), in order to analyze the noise level of my new Linear PSU project.1 - Scheme. Configured with 100khz width.2 - Input at 0R, to test the noise of the preamp itself. Reading around 100µV. Amount that I should deduct from the next tests.3- Battery without charge, with noise reading around 76µv4- Battery with charge of 80mA. Reading around 108µvI will perform tests on two sources and bring the result.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 31, 2022, 01:37:28 pm
content: More testing with the preamp.1- psu (LM723) no load: 9.8mVpp noise2- psu (LM723) with 0.6A load: 12mVpp noise2- switched psu (lw-k3010D) no load: 7.2mVpp noise4- switched psu (lw-k3010D) with 0.6A load: 13.8mVpp noiseDo the values shown appear to be within a tolerable range for a general purpose PSU?Can anyone with experience tell me if the measurement method I'm applying is appropriate, taking into account cabling, preamp noise floor + DSO noise floor?

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: October 31, 2022, 03:55:55 pm
content: In the book "The Art of Electronics", in the noise chapter, the author states that the RMS value is between 1/6 and 1/8 of Vpp, read on the oscilloscope. I decided to testify with my Brymen and in fact the value is consistent.1 - Psu (LM723) with a load of 0.6A. Value 12.8mVpp on the oscilloscope and 2.08mV RMS on the Brymen.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: November 19, 2022, 03:55:24 pm
content: Hi!I'm back with the final assembly of the project. I will perform one more round of tests (transients, noise, cc, ..) and come back with the results to finish the topic.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: November 19, 2022, 06:08:34 pm
content: I went to measure the noise and got an unexpected surprise. A ripple of 120hz with a voltage of 6mVpp. This ripple increases with increasing load. On my protobord this didn't happen so I conclude that I may have a grounding problem. Please, could you offer me a tip regarding the trace (white) GND of my pcb? Note that the top line is where most of the load current flows. In the lower row flow the lower currents. The part on the left is the fan current path towards the transformer. I can't figure out where I went so wrong.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: November 20, 2022, 11:30:27 am
content: I discovered my pcb error by reading an article about shielding and grounding. It caught my attention when the author comments on ground loops (figure 10). I cut the pcb at the point marked with the green X and my joy returned. It follows the noise measurements, with very good results.https://www.smar.com/en/technical-article/tips-on-shielding-and-grounding-in-industrial-automation1- cut point on pcb;2- background noise, with PSU off:400µV(measurement over the output capacitor);3- noise, with output at 8v/1A:80µV(measurement over the output capacitor);4 - background noise, with PSU off:1.12mV(measured at the output terminal, which includes the meter and wiring);5- noise, with 8v/1A output:720µV(measured at the output terminal, which includes the meter and wiring).

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: November 21, 2022, 10:29:58 am
content: I performed the transient test. I had to provisionally mount an electronic load, based on a 9v battery. My other electronic load is very noisy.Output at 8v, maximum pulse of 1A with duration of 200Us. Note in the images, the differences between measuring directly on the output capacitor, measuring at the output terminal (shunt + wiring) and measuring at the electronic load (shunt + wiring + wiring).1- Measurement on the output capacitor. Ri=10mΩ2- Measurement over the output terminals.3- Measurement of the electronic chargeAnother good behavior from this project..

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: November 21, 2022, 12:33:21 pm
content: Verification of the behavior of the output voltage and the stabilization time, when energizing the power supply.1- Output voltage at 6v and with a load of 0.7A - start with a time of 50ms and without overshoot.2- Output voltage at 6v and no load - 0.5s time to discharge the 100uF capacitor.3- Updated schematic.Below is the EasyEDA file.Project completed. My thanks to the participants!

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: November 23, 2022, 02:10:32 pm
content: HiI forgot to test the behavior of the CC module, when the output goes to a short circuit condition. Current monitored over resistor R15(0R33).1- Output voltage at 10v and no load. CC module configured for 1A. Shorting the output terminals with a screwdriver. Very good reaction from the dc module, no overshoot.2- Output voltage at 5v with a load of 0.6A. CC module configured for 1A. Shorting the output terminals with a screwdriver. No surprises.3- Checking CV behavior when leaving the short circuit condition. Output voltage set to 5v. No overshoot.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: November 24, 2022, 06:49:43 am
content: One more adjustment. I had to change the position of C11 to before switch S1, because the switch was suffering damage (smoking) when energizing the transformer.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: November 24, 2022, 09:02:48 pm
content: Quote from: oz2cpu on October 10, 2022, 04:44:41 pmI'm almost sure that it is THIS circuit. (The version published at the same time in FUNKSCHAU magazine had 3 pass transistors and was nominally 30V/3A but otherwise identical). Although it does not do foldback, but cutoff to the minimum current. One can improve it by using real double transistors and adding a discrete negative regulator for the auxiliary supply.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: November 26, 2022, 11:14:14 am
content: Hi* I added to the project, the monitoring of the heatsink temperature.* The fan starts at 49ºc.* After several tests, within the limits of the project, I observe that the maximum temperature of the heatsink can reach 62ºc.* By adjusting P5, I determine the maximum temperature allowed on the heatsink. I set it to 65ºc as the maximum limit.* When exceeding the temperature of 65ºC, TR2 is activated, taking the output to MUTE mode.* Follow the updated EasyEDa project.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: December 10, 2022, 01:54:42 pm
content: HiI'm back to report how pleased I am with the CC module of this project. With a careful adjustment of P2 I can reach 1mA of control. I used an LED to check if the CC module was being effective.Note:The images are too dark to see the LED under test.4- I set the voltage to 8v and the CC to 10mA. The intention was to test whether the LED would survive the accumulated charge of the 100uf output capacitor.1- LED OK at 10mA. I believe that the LED has survived in all tests, due to the constant current source (Q10) of 23mA.2- CC module configured for 5mA. LED OK.3- CC module configured for 1mA. LED OK.