How to adjust SMPS output that has dual opamp and ref/volt chip? - Page 1
Forum section: electronics / projects

author: CHIPHEAD
author_rank: Newbie
timestamp: October 13, 2021, 12:40:17 pm
content: GOOD DAY...I'am still a noob when it comes to electronics,so if someone can show me what I'm doing wrong,or missing,I will appreciate it.Here goes:Culprit: Laptop charger 19.5v 65wI am trying to mod this smps so that the CC and CV is adjustable.I have modified other types of smps with no problems...but this one has a dual opamp with v/ref chip (data sheet in attachments) at the output to regulate CV and CC and I can't seem to crack it.(see attachments)I have made the CV adjustable,but the lower I make the output voltage,the less load I can pull from it before it cuts out.Things that I have done so far:1) Took images and revere engineered the circuit.(see attachment)2) Removed the 4.87k and 499k resistors and replaced them with a 5.6k resistor and 10k multi turn pot in series to make CVadjustable from 9.8v to 16.8v.3) Tried to make CC adjustable with multi turn pots at 4.64k resistor with no luck,and then at 115k resistor with no luck as well, thenresoldered resistors back as they were.As soon as I tried to lower the CC it just cuts out,and then saw it even cuts with a small loadat 10v.Basiclly the lower I make the voltage the less load before it cuts out.4) I then thought its got something to do with the supply voltage at pin 3 in the sens that there is a voltage drop,so changed the 10kresistor with a 6.8k,no luck and replaced the 10k.5) I then removed the 1k resistor going to pin 7 to disable the CC completely to rule out that the problem is not from that side and stillno luck,so it must be on the CV regulation side in my opinion.So I from what I can see is that I'am missing something on operation of opamp 1 with the ref/voltage.Thanks

author: Gyro
author_rank: Super Contributor
timestamp: October 13, 2021, 08:11:50 pm
content: Hi,A couple of thoughts...1. The opamp is (obviously) being powered by the secondary side. You may be running out of supply headroom.or2. You are regulating the output by adjusting the primary switching waveform, via the opto and switching controller. There will be a limit beyond which the duty cycle of the primary switcher cannot supply more current without causing the secondary voltage to rise at the same time, causing a conflict with the CV regulation.A laptop adapter is going to be designed to efficiently produce an output over a fairly narrow range. You're not going to get away with the sort of adjustability that can be achieved with specifically designed variable SMPSs which have auxiliary supply rails etc.

author: eblc1388
author_rank: Frequent Contributor
timestamp: October 14, 2021, 10:20:31 am
content: Quote from: CHIPHEAD on October 13, 2021, 12:40:17 pmPlease put a DVM between pin3 and pin4 of the Opamp while changing the power supply output voltage from 9.8V to 16.8V and record the results.The measured voltage should be around 2.5V steady, when the output voltage changes. If your measurement shows increasing pin voltage when output voltage goes up, then the Opamp is bad. It is likely to be a fake one. However, you can try replacing the 10K resistor(connects between pin3 and SMPS output+) to 2K2 or 3k3 to see you can get a steady 2.5V at pin3.

author: CHIPHEAD
author_rank: Newbie
timestamp: October 17, 2021, 11:15:42 am
content: Ok,the 1k resistor going to pin 7 is still removed,so CC is still disabled.Voltage between pins 3 and 4 is a perfect,stable 2.5v from 9.8v to 16.8v at out put without load.But when I conect a load (I used a 12v/21w globe) it starts cutting out at +/- 11v (see attached video)In the video I adjusted the voltage to 10.5v without a load and then connected the globe so that it starts cutting out.I then adjusted the pot slowly to increase the output voltage and as soon as the voltage reaches 11.11v the globe lights up and voltage is stable up to 16.8v.I then slowly decreased the voltage and the smps starts cutting out again at 10.9v.

author: eblc1388
author_rank: Frequent Contributor
timestamp: October 17, 2021, 01:34:12 pm
content: You have reduced the SMPS output voltage by nearly halve(19.5V to 10.5V) and it would mean a very low duty cycle switching pulse in the primary. To make matter worse, you have connect a lamp load which requires about 2A when the filament is HOT, meaning its internal resistance is much lower when cold. This large load on the SMPS output will quickly pull down its output voltage and affect the operation of the voltage feedback circuit. So the unstable output is pretty much expected.You can connect a large capacitor (2200uF or more) to the output and see if this improves the situation or not.

author: magic
author_rank: Super Contributor
timestamp: October 17, 2021, 02:14:48 pm
content: There is an error in the schematic. The feedback divider and optocoupler should be powered from POS rather than transfomer winding.As to the problem, I suspect that the primary side controller runs out of power. Primary circuits are typically powered by a third winding. By reducing secondary winding output, you also reduce the auxilliary winding output and the controller UVLO's.

author: CHIPHEAD
author_rank: Newbie
timestamp: October 17, 2021, 04:27:29 pm
content: You are correct Magic,I made a mistake there on the schematic but its fix now thanks.I'm still new to smps and still learning a lot,I have modified quite a lot of them so far with great success.From ATX power supplys to all kinds of charges.Whats getting me on this one is that it used opamps and not some type of zener regulation,and at 16v I can connect 2 of those globes with no problem,but the lower I make the voltage the less load I can conect.Thank you all for the help and feed back so far.I really appreciate it.

author: magic
author_rank: Super Contributor
timestamp: October 17, 2021, 04:49:29 pm
content: If the supply shuts down I am pretty sure it's the primary. In fact, I'm working right now on modifying a similar supply to provide 5V~15V range and I had to make mods on the primary to get that low.What ICs are on the primary? The datasheets will tell 90% of the story about what's the circuit there.

author: CHIPHEAD
author_rank: Newbie
timestamp: October 17, 2021, 05:50:48 pm
content: Aaaa I think that's where the problem lies,its using a SG6842 (see attachment)I just scanned thru the datasheet and saw some interesting stuff.I'm of to bed now I will study the data sheet tomorrow I see if I can find out what to do...Thanks

author: magic
author_rank: Super Contributor
timestamp: October 17, 2021, 06:05:37 pm
content: As expected. The PWM controller is expected to generate its own supply voltage using the third transformer winding. Reduce output voltage and this voltage also falls down, until the chip turns off at 10.5V.You could confirm that this is the problem by measuring the chip's VCC, but:- even if the voltage between "GND" and "VCC" is only 10~20V, both potentials are hundreds of volts RMS with respect to real ground - shock hazard- high power and voltages present, bad probe slip may blow up the PSU, accidentally using wrong range may blow up the DMMSuch chip needs some external power supply to work if output voltage is reduced. On ATX the chips typically run from the standby supplies so ATX are easy to mod. Wall warts and laptop bricks are not that easy.