SMPSU Modification  - Page 1
Forum section: electronics / repair

author: BullyBeef
author_rank: Newbie
timestamp: May 20, 2020, 09:35:52 pm
content: Hi all - my first post in this forum. I can't find anything like this on other posts. I'm no beginner but technology has changed and sometimes I find myself at a loss. I have a SL Power Electronics brick PSU that I want to repurpose as a laptop supply. The SMSPU is designed 12volts at up to 6.67amps and I tested it to 6.5 amps when I gave up as the load resistors were cooking. For the laptop, I need 19.5 volts at 4.5 amps - well within existing current capacity so all I need to do is up the voltage, right? . I couldnt find a circuit diagram anywhere, but the secondary controller is a TEA1761 so I got the application notes and identified the voltage control parts. Those SMDs are hard to solder for us old blokes. I have attached my scribbled circuit diagram and the one on the app note. This app note also gives component values to give my required output - very handy. I can now get 19.5 volts - easy enough - but - any load over 1 amp shuts it down. I know what you're thinking - but on my PSU, there is no load sense resistor and the Isense pin is connected to ground via a 220R resistor - I can't see how this is overload protection. The app note describes a procedure to set up the max current output by shorting Isense direct to ground, so I did this - I now got 1.5 amps before it shut down. I swapped out the 19.5V components for the original 12V components and it's giving 6.5 amps again. If there is no current feedback, why will it not give a lower current at a higher voltage? Hmm - What am I missing?

author: George Edmonds
author_rank: Frequent Contributor
timestamp: May 20, 2020, 11:13:40 pm
content: HiMost SMPSU's are designed for a specific output voltage, trying to increase it frequently makes them self destruct.George G6HIG

author: TheMG
author_rank: Frequent Contributor
timestamp: May 21, 2020, 01:29:05 am
content: It's probably shutting off because the duty cycle is becoming too great. In other words, as designed it is unable to provide the requested current and maintain voltage regulation while keeping the switching duty cycle within limits.The good thing is that the controller IC shuts down, instead of the switching transistors going pop which would be the likely outcome otherwise.You can adjust the output voltage of SMPS to some degree, but going from 12V to 19.5V is far too much of a change. That would require changing the number of turns in the transformer secondary for it to work correctly at that voltage.Also, that TEA1761 is a synchronous rectifier controller, not the switching controller for the SMPS (which would be on the primary side). This just means the SMPS uses transistors for secondary rectification, instead of diodes. This is more efficient when dealing with higher currents.In most modern small SMPS the current limiting is done on the primary side, as a function of the switching controller IC.

author: BullyBeef
author_rank: Newbie
timestamp: May 21, 2020, 07:26:56 am
content: Thanks for the replies. It gives the 19.5 volts with no problems, just not the voltage with a 2 amp current reduction. I don't know if it is the secondary controller shutting it down or the primary controller - a 1207A and I don't have the equipment to do a gradual load increase as I monitor the circuit. I'll decipher the primary side and see what I find there. Even if I can't get it to do what I want, it is teaching me about how these new fangled things work.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: May 21, 2020, 11:13:58 am
content: HiAs already pointed out its the turns ratio on the switching/isolating transformer that is causing your problemGeorge G6HIG

author: TheMG
author_rank: Frequent Contributor
timestamp: May 21, 2020, 10:56:28 pm
content: Quote from: BullyBeef on May 21, 2020, 07:26:56 amPower supply likely uses a flyback topology, using a single switching transistor. Each time the transistor switches on, current flows into the primary of the transformer, building up magnetic flux in the core. When the transistor switches off, the magnetic field collapses and energy is transferred into the secondary winding.Load regulation is achieved by controlling the pulse width (thus varying the amount of energy transferred with each pulse), or for very low loads, the controller will usually go into discontinuous mode, where it skips pulses.The transformer design is really the single most important aspect of the power supply. Everything from the turns ratio to winding inductance, parasitic capacitance, the ferrite core characteristics, etc.By trying to operate it at a much higher than designed voltage, you're forcing it to operate in an area where the required energy simply can not be properly transferred through the transformer, at duty cycles much higher than would normally be required for a given amount of load, thus it is no longer running optimally. There's a limit to how much flux can be built up with each pulse before the transformer core saturates, and that will effectively limit how high of a duty cycle the power supply can switch at.Every part of the design of an SMPS is a delicate balancing act, from switching frequency and duty cycle, to component choices, transformer design, etc. While there are some standardized transformers available for SMPS, most of the time they are designed and manufactured for a specific use.Even if you were to theoretically modify the transformer windings to suit the new voltage, you would only be able to get about 4.1A out of it at 19.5V, because the amount of power (watts) the transformer is capable of has not changed.Best way to see exactly what is going would be with an oscilloscope, but be careful, never hook up the scope to any part of the primary circuit unless you have either an isolated differential probe or the power supply connected to a suitable isolation transformer. Observing the switching waveform you would likely see that the duty cycle starts off rather high (as the power supply is attempting to maintain a higher output voltage), and as load is increased it would quickly reach a maximum and no longer increase, this is the point at which the output voltage starts to fall off as the load current is increased.I hope this explanation makes some sort of sense.

author: aqibi2000
author_rank: Regular Contributor
timestamp: May 22, 2020, 01:08:03 am
content: Or bolt on a dc dc boost converter from eBay

author: aqibi2000
author_rank: Regular Contributor
timestamp: May 22, 2020, 01:08:53 am
content: Although inputting 12V where the 3 cell battery would normally be will allow the laptop to boot and function of course lacking the battery

author: magic
author_rank: Super Contributor
timestamp: May 22, 2020, 08:27:01 am
content: There is another problem: if you increase the secondary voltage, the primary voltage which the switching transistor needs to withstand when off increases too and the magic smoke safety margin decreases.

author: shakalnokturn
author_rank: Super Contributor
timestamp: May 22, 2020, 11:52:31 pm
content: Quote from: magic on May 22, 2020, 08:27:01 amCould you explain how that happens please?

author: magic
author_rank: Super Contributor
timestamp: May 23, 2020, 07:27:07 am
content: The ratio of primary to secondary voltage equals the turns ratio of the windings. For n:m windings,1. when the transistor is on and X volts is applied to the primary, the secondary outputs x/n·m volts negative2. when the transistor is off and Y volts develops across the secondary, the primary outputs x/m·n volts in addition to the input voltageThat's how the flyback topology works, at least. Most low power SMPS bricks are flyback.

author: BullyBeef
author_rank: Newbie
timestamp: May 23, 2020, 03:28:45 pm
content: Update - I had thought about tying the 12volts to where the battery plugs in, however there are 8 - eight! connectors so that was far too many to choose from. The power supply is from a medical device so is well built and pretty beefy and is rated at 90 watts.I looked at the primary side as suggested - again using the typical circuit diagram from the app notes for the 1207A - attached. The current sense in the primary side is a 2R7 2 watt resistor and when the power supply secondary is set to 19.5volts , the increased volt drop across this resistor is setting off the early over current shut down of the 1207A - at just over 1 amp. I reduced this resistor and was rewarded with 2 amps before shutdown. Heading in the right direction. A bit more work and I got it to output enough to boot the laptop and Windows 7 started up. This laptop hasn't been switched on since 2015! The power supply seems happy, there are no signs (or smells) of overheating . The volt drop across the sensing resistor tells me there is about 130mA through the primary winding and the MOSFET. The PSU shuts down at just under 5 amps.Thanks for all the suggestions and information guys, - they made me go back and look again and I now know a lot more about these new fangled gadgets.