Raise switched mode PSU output voltage above nominal voltage - Page 1
Forum section: electronics / beginners

author: nour
author_rank: Frequent Contributor
timestamp: November 22, 2016, 07:17:50 pm
content: Hi,I have a switched mode PSU (an old laptop PSU) that have a nominal voltage around 20V @ 4.5A, of course this is a fixed outputMy question is can i modify it so I can raise the output voltage to 23V?I Know that if I can increase the switching frequency it would increase the output voltage(correct me if I am wrong), but sometime the switching component's frequency in the circuit is fixed internally!Is it doable !!The reason why I want to do it because I want to power up some LEDs in series but the total voltage for them is around 22V and I don't want to buy a new PSU just for this specially I want around 3.5 amps to have a enough power to run the LEDsI will try to reverse engineer the circuit of the PSU and see if I can find the switching component in it.

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: November 22, 2016, 07:45:09 pm
content: I am trying to do the same thing with an atx power supply. You tube videos say this can be done in a limited range. On the 12 volt rail you could change the pwm to make the output 14.4 volt to charge a car battery using a potentiometer. I guess lowering the voltage down woun't be much of a problem. To double the output voltage might exceed the rating on the output capacitors, and bang.

author: nour
author_rank: Frequent Contributor
timestamp: November 22, 2016, 07:54:47 pm
content: Quote from: joseph nicholas on November 22, 2016, 07:45:09 pmI don't want to double it I just want to increase it around 3 volts, also I have checked the output Caps and they be just fine, they are rated up to 25V!and also I have checked the switching IC and all I have found thisDAP6APNLNI have searched the above symbols and couldn't find Datasheet but found a lot of other Laptop PSU PCB images with the same IC, so presumably it is widely used in this application

author: Cliff Matthews
author_rank: Unknown
timestamp: November 22, 2016, 09:58:27 pm
content: Just throw in a 99-cent boost pcb and solder in a 10K panel pot. 5v in and ~6v to 37v out. It's adjustable and good for 10watts as outlined in Scullcom Hobby Electronics video #19**In your application, use a heatsink (he mentions it too) maybe you'll get out 23v at 1amp without making it work too hard.

author: nour
author_rank: Frequent Contributor
timestamp: November 22, 2016, 10:47:39 pm
content: Quote from: Cliff Matthews on November 22, 2016, 09:58:27 pmI don't have access for this right now and also that won't work for me at all, I need around 3.5A @22V that's obviously more than 10W

author: nour
author_rank: Frequent Contributor
timestamp: November 22, 2016, 11:00:11 pm
content: After doing some search I have found someone did the hard work for me, I have found the circuit diagram for the PSU, it seems like someone draw it by handNow I need to figure out which resistor should be changed to alter the feedback loop (assuming it has one).Can someone help over here!

author: nour
author_rank: Frequent Contributor
timestamp: November 22, 2016, 11:11:10 pm
content: According to this diagram the PAD6A is the NCP1200 with is PWM Current-ModeController.Searching for the datasheet was very easyhttp://www.onsemi.com/pub_link/Collateral/NCP1200-D.PDFNow after finding the datasheet I think it is much easier to change the values!Can someone help !

author: nour
author_rank: Frequent Contributor
timestamp: November 22, 2016, 11:20:03 pm
content: According to the datasheet the device has a fixed internal frequency, so is there still a chance to do it?I have no experience with switching mode power supplies so I think I am missing something

author: nour
author_rank: Frequent Contributor
timestamp: November 23, 2016, 02:26:07 am
content: Quote from: AcHmed99 on November 23, 2016, 01:01:40 amI have measured the voltage across R58 and it is 2.5 exactlyR58 is 3.83 indeed, I have removed it from the board and measured itNow I don't understand should I increase R59 value or decrease R58 value.I have tried to decrease R58 value to 3.6K and I have got 21.63Vbecause you have mentioned in the first paragraph increasing R59 but later in the second one you was talking about R58I am a bit confused.But really thanks a lot for your reply anyway you have put me on the right spot in the schematic after all.

author: nour
author_rank: Frequent Contributor
timestamp: November 23, 2016, 12:48:43 pm
content: I have decreased R58 down to 3.3K and now I have 23.3V which I consider very good. Also regarding to the caps, the space and the volume of the box wouldn't allow me to add higher rated caps, so I will just leave them like that for now and observe any unusual behavior until I can have another caps with higher ratings and the same volume.BTW would this affect the max. output current, this unit is capable of 4.5A @20V, and I will be using it around 3A @ 23V?

author: nour
author_rank: Frequent Contributor
timestamp: November 23, 2016, 03:04:11 pm
content: I will use this to power UV box that I have made and in my calculations I wouldn't run it for more than 2 minutes so It will be OK .also the caps are nichicon rated @ 105C so I think this is also going to be OK.BTW I only have 2 caps(25V@1000uF) not 3, I don't know why the circuit diagram has 3 instead of 2!!