Modify Voltage of SMPS Brick - Page 1
Forum section: electronics / repair

author: 0xPIT
author_rank: Regular Contributor
timestamp: August 09, 2017, 09:13:51 pm
content: Hello,I currently fail modding a LiteOn SMPS from 12V to 24V, though having modified countless other SMPSs successfully before.The relevant part of the circuit is attached to this post. (unpopulated parts removed)My approach was to modify the resistor divider formed by R62 + R64 (R65 = 0 Ohm).It's resulting voltage is compared with the internal 2.5V reference voltage.As mere calculation of the divider did not result in a working PSU, I've replaced R64 with a pot...Whenever the output voltage exceeds about 19V DC, the PSU shuts down – I fail to understand why.Can anyone give a hint?Note that I've also replaced R57 by double the value in order to not blow the LED in the OC, and C54 with a 25V type.Thanks!

author: glarsson
author_rank: Frequent Contributor
timestamp: August 09, 2017, 09:29:32 pm
content: A quick guess:Output on pin 1 is voltage feedback via the optocoupler.Output on pin 7 is over voltage protection via the optocoupler.D11 acts as an OR-gate.Resistors R68, R69 and R70 probably sets the over voltage point.When output reaches 19V output 7 triggers a shutdown.

author: Armadillo
author_rank: Super Contributor
timestamp: August 09, 2017, 09:35:13 pm
content: Countless ! LOLBy that token and your reasoning, making the divider variable, you can mod? into a benchtop power supply! ?Obviously one switching transformer cannot be taken to suit all voltages, it went into saturation and the inductance factor drops drastically, and the transistor went into pulling high amp and you are fortunate the management IC swung into protection mode quick enough otherwise it will bang & boom.

author: 0xPIT
author_rank: Regular Contributor
timestamp: August 09, 2017, 09:40:01 pm
content: Quote from: glarsson on August 09, 2017, 09:29:32 pmNo, this is over-current protection, as it's after R50 (which is 0.8R)Quote from: glarsson on August 09, 2017, 09:29:32 pmD11 is an AND gate.R68 is just Bias for the Reference, R69+70 are the voltage divider for the over-current comparator. (2.5Vref -> set to 0.33V)Also, I've lifted R60 to make sure over-current won't hit.

author: 0xPIT
author_rank: Regular Contributor
timestamp: August 09, 2017, 09:49:40 pm
content: Quote from: Armadillo on August 09, 2017, 09:35:13 pmYes, works with most fixed-voltage SMPSs, typically 3.3 to ~30V is not a problem at all. I've modifieddozensso far.I assume these devices are designed with that in mind, so they can cheaply produce 5/9/12/24V/etc. models by changing resistors only.Quote from: Armadillo on August 09, 2017, 09:35:13 pmQuite possible. Would be the first PSU in really many to do that, though.Evidence for your case is that LiteOn seems to offer this particular supply only in 12V configuration.Hm, I wonder how I can prove your hypothesis.

author: xavier60
author_rank: Super Contributor
timestamp: August 09, 2017, 09:53:54 pm
content: There is also likely to be a problem on the live side. Most control ICs monitor the voltage applied to the VCC pin for Over Voltage Protection. The higher output voltage will add to the PIV of the output diodes and to the peak voltage on the Drain of the likely present MOSFET connected to the primary winding. Saturation is not an issue.

author: Armadillo
author_rank: Super Contributor
timestamp: August 09, 2017, 10:07:02 pm
content: Quote from: 0xPIT on August 09, 2017, 09:49:40 pmWow! those are amazing!.

author: 0xPIT
author_rank: Regular Contributor
timestamp: August 09, 2017, 10:08:10 pm
content: That makes total sense, xavier60!I can't find a datasheet for the controller IC (LTA809FA). Do you think just doubling the OVP resistor can work?nvm, found it:http://www.datasheetcatalog.com/datasheets_pdf/S/G/6/7/SG6741.shtml

author: xavier60
author_rank: Super Contributor
timestamp: August 09, 2017, 10:51:04 pm
content: Z1 should be doubled. It looks like 20v, another 20v zenner could be added. allowing pin 7 VDD voltage to double is certain to cause problems. A 3 pin regulator IC could be add. Pick voltage close to the unmodified voltage on pin 7.The regulator is likely to interfere with startup. Normally C3 supplies enough energy to power the IC for 50ms or more after it is charged by current drawn by the HV pin.There are likely to be other surprise problems.

author: 0xPIT
author_rank: Regular Contributor
timestamp: August 09, 2017, 11:16:27 pm
content: Doubling is probably too much, as the IC accepts <= 30V, assuming both transformer-derived voltages are similar.It seems Z1 forms an additional over voltage protection by disabling the oscillator.Can't figure out what benefit this has as there's OVP built in the chip, also the reference design does not include that protection.

author: xavier60
author_rank: Super Contributor
timestamp: August 09, 2017, 11:22:02 pm
content: Because the IC has a wide operating voltage range, simply placing a zenner in series with D2 should do the trick.

author: xavier60
author_rank: Super Contributor
timestamp: August 10, 2017, 05:27:12 am
content: I'm expecting the MOSFET's peak drain voltage to rise by about 150v.

author: 0xPIT
author_rank: Regular Contributor
timestamp: August 10, 2017, 08:22:50 am
content: Quote from: blueskull on August 09, 2017, 11:39:00 pmWith my limited knowledge of power electronics, I don't understand how this is the case.Even if the Zener would be increased, the OVP of the IC would kick in and stop the PWM.Both OVP implementations (On-Chip & Zener+Transistor) are sourced from the same coil.I don't understand why they've implemented a 2nd protection.As your profile claims specialist knowledge, I'd be grateful for your wisdom.

author: capt bullshot
author_rank: Super Contributor
timestamp: August 10, 2017, 08:29:08 am
content: Quote from: 0xPIT on August 10, 2017, 08:22:50 amThat's for redundancy. If the (for example) optocoupler fails (this is a single point of failure), the controller goes out of regulation, output voltage rises. The zener / transistor OVP prevents further damage then.

author: Zucca
author_rank: Unknown
timestamp: August 10, 2017, 09:13:28 am
content: Quote from: blueskull on August 10, 2017, 08:47:51 amDo it and then increase the Vmain slowly with a variable trafo watching out if fireworks happen.Do you have one? If you are modding a lot of SMPS I suggest you to buy one.EDIT: also a bulb lamp in series between the SMPS and the mains can help.

author: capt bullshot
author_rank: Super Contributor
timestamp: August 10, 2017, 09:39:02 am
content: Quote from: 0xPIT on August 09, 2017, 09:49:40 pmI've walked a hundred times across the street without watching the traffic, so there's no problem at all doing so.Quote from: 0xPIT on August 09, 2017, 09:49:40 pmI believe the streets were designed with me walking carelessly across them in mind, so I'm safe.

author: 0xPIT
author_rank: Regular Contributor
timestamp: August 10, 2017, 09:58:54 am
content: Quote from: zucca on August 10, 2017, 09:13:28 amThanks, yes I have a variable isolation transformer.

author: Armadillo
author_rank: Super Contributor
timestamp: August 10, 2017, 10:01:47 am
content: Quote from: capt bullshot on August 10, 2017, 09:39:02 amWell said !

author: 0xPIT
author_rank: Regular Contributor
timestamp: August 10, 2017, 10:05:42 am
content: Ok, thanks a lot, all, I've learned stuff and also came to the conclusion that it's not worth modifying, as this model is not designed for voltages far from 12V.

author: 0xPIT
author_rank: Regular Contributor
timestamp: August 10, 2017, 10:07:33 am
content: Quote from: Armadillo on August 10, 2017, 10:01:47 amIndeed well said and true, but unfortunately in course of a forum discussion completely useless for anything but the poster's ego.

author: Armadillo
author_rank: Super Contributor
timestamp: August 10, 2017, 11:03:25 am
content: Quote from: 0xPIT on August 10, 2017, 10:07:33 amPride and the eagerness to show had inhibited the ability to "think" despite being directed and the lax of real intention to learn which defeated the purpose of asking.Because this is of safety concern, the adamant insistent of the mob may be hazardous to other young readers hence, the need to be poke and corrected to proper direction.The lesson is not to repeat such "stupidity". [and which you have indirectly and silently accepted in earlier posting].Cheers!.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: August 11, 2017, 06:11:04 am
content: You can modify the output of a flyback converter like this over quite a wide range so long as you don't go too far above its (12v) rating. Push-pull circuits have a more limited output range and are not generally suited to variable supplies.Main point is that raising the output also raises the reverse (flyback) voltage on the input winding. (Think, transformer effect) Eventually you will reach the point where the input switching transistor pops its clogs. So, 14v for charging lead acid (for example) would usually be OK, 24v is definitely not.