PSU – Experimental Engineering

Posted on May 4, 2017May 8, 2017 by The Engineer — Leave a comment

32A Bench PSU Build

Since I’ve discovered some nice high power PSUs in the form of Playstation 3 PSUs, it’s time to get a new Bench PSU Build underway!

I’ve gone for the APS-227 version as it’s got the 32A rail. This makes things slightly beefier overall, as the loading will never be anywhere close to 100% for long, more headroom on the specs is the result.

The case I’ve chosen for this is an ABS desktop instrument case from eBay, the TE554 200x175x70mm. The ABS is easy to cut the holes for all the through-panel gear, along with being sturdy enough. Aluminium front & back panels would be a nice addition for a better look.

The PSU board is removed from it’s factory casing & installed on the bottom shell half, unfortunately the moulded-in posts didn’t match the screw hole locations so I had to mount some brass standoffs separately. The AC input is also fitted here, I’ve used a common-mode filter to test things (this won’t be staying, as it fouls one of the case screw holes). The 40A rated DC output cable is soldered directly to the PCB traces, as there’s no room under the board to fit the factory DC power connector. (This is the biggest case I could find on eBay, and things are still a little tight). Some minor modifications were required to get the PCB to fit correctly.

I decided to add some limited voltage adjustment capability to the front panel, I had a 100Ω Vishay Spectrol Precision 10-turn potentiometer in my parts bin, from a project long since gone that just about fits between the panel & the output rectifier heatsink. The trimpot I added when I first posted about these PSUs is now used to set the upper voltage limit of 15 volts. (The output electrolytics are 16v rated, and are in an awkward place to get at to change for higher voltage parts). The binding posts are rated to 30A, and were also left over from a previous project.

This front panel potentiometer is electrically in series with the trimpot glued to the top of the auxiliary transformer, see above for a simple schematic of the added components. In this PSU, reducing the total resistance in the regulator circuit increases the voltage, so make sure the potentiometer is wired correctly for this!
After some experimentation, a 500Ω 10-turn potentiometer would be a better match, with a 750Ω resistor in parallel to give a total resistance range on the front panel pot of 300Ω. This will give a lower minimum voltage limit of about 12.00v to make lead-acid battery charging easier.
I’ve had to make a minor modification to the output rectifier heatsink to get this pot to fit in the available space, but nothing big enough to stop the heatsink working correctly.

Here I’ve got the binding posts mounted, however the studs are a little too long. Once the wiring is installed these will be trimmed back to clear both the case screw path & the heatsink. (The heatsink isn’t a part of the power path anyway, so it’s isolated).

To keep the output rectifier MOSFETs cool, there’s a fan mounted in the upper shell just above their location, this case has vents in the bottom already moulded in for the air to exit. The fan is operated with the DC output contactor, only running when the main DC is switched on. This keeps the noise to a minimum when the supply doesn’t require cooling. The panel meter control board is also mounted up here, in the only empty space available. The panel meter module itself is a VAC-1030A from MingHe.

The measurement shunt & main power contactor for the DC output is on another board, here mounted on the left side of the case. The measurement shunt is a low-cost one in this module, I doubt it’s made of the usual materials of Manganin or Constantan, this is confirmed by my meansurements as when the shunt heats up from high-power use, the readings drift by about 100mA. The original terminal blocks this module arrived with have been removed & the DC cables soldered directly to the PCB, to keep the number of high-current junctions to a minimum. This should ensure the lowest possible losses from resistive heating.

The panel meter module iself is powered from the 5v standby rail of the Sony PSU, instead of the 12v rail. This allows me to keep the meter on while the main 12v output is switched off.

here’s the supply with everything fitted to the lower shell – it’s a tight fit! A standard IEC connector has been fitted into the back panel for the mains input, giving much more clearance for the AC side of things.

With the top shell in place, a look through the panel cutout for the meter LCD shows the rather tight fit of all the meter components. There’s about 25mm of clearance above the top of the PSU board, giving plenty of room for the 40mm cooling fan to circulate air around.

Here’s the finished supply under a full load test – it’s charging a 200Ah deep cycle battery. The meter offers many protection modes, so I’ve set the current limit at 30A – preventing Sony’s built in over current protection on the PSU tripping with this function is a bonus, as the supply takes a good 90 seconds to recover afterwards. I’ll go into the many modes & features of this meter in another post.

Posted on April 11, 2017May 1, 2017 by The Engineer — 9 Comments

Sony PS3 APS-231 Power Supply Voltage Mod

I was recently given a Sony PS3 with a dead disc drive, and since I’m not a console gamer I figured I’d see if there were any handy parts inside. Turns out these units contain a rather nice SMPS, the Sony APS-231 with a high power 12v rail, rated at 23.5A. A bit of searching around discovered a thread on the BadCaps Forums about voltage modding these supplies for a 13.8v output, suitable for my Ham radio gear.
These supplies are controlled by a Sony CXA8038A, for which there is very little information. Active PFC is included, along with synchronous rectification which increases the efficiency of the supply, and in turn, reduces the waste heat output from the rectifiers.

Like many of the SMPS units I’ve seen, the output voltage is controlled by referencing it to an adjustable shunt reference, and adjusting the set point of this reference will in turn adjust the output voltage of the supply, this is done in circuit by a single resistor.

Here’s the regulator section of the PSU, with the resistors labelled. The one we’re after changing is the 800Ω one between pins 2 & 3 of the TS2431 shunt reference. It’s a very small 0402 size resistor, located right next to the filter electrolytic for the 5v standby supply circuit. A fine tip on the soldering iron is required to get this resistor removed.

Once this resistor is removed from the circuit, a 1KΩ 18-turn potentiometer is fitted in it’s place, from the Anode (Pin 3) to the Ref. (Pin 2) pins of the TS2431 shunt reference. I initally set the potentiometer to be the same 800Ω as the factory set resistor, to make sure the supply would start up at a sensible voltage before I did the adjustment.

The pot is secured to the top of the standby supply transformer with a drop of CA glue to stop everything moving around. The supply can now be adjusted to a higher setpoint voltage – 13.8v is about the maxumum, as the OVP cuts the supply out at between 13.9v-14v.

After doing some testing at roughly 50% of the supply’s rated load, everything seems to be stable, and nothing is heating up more than I’d expect.

Posted on October 27, 2016 by The Engineer — Leave a comment

LG Flatron 22EA53VQ-P Power Issue

I was recently given a pretty nice LED backlit 1080p LG monitor, with the instruction that it wouldn’t power on correctly. The monitor would power on as far as the standby light, but when fully powered on, would flash the backlight momentarily then shut down. A power supply issue was immediately suspected.

I popped the covers off the monitor itself first, thinking that it was an electrolytic gone bad in the backlight DC-DC converter. Not to mention the fact that cracking into a wall-wart type of PSU is only occasionally possible without the use of anger & large hammers. (Cracking the glue with the handle of a screwdriver doesn’t work so well when the factory went a bit nuts with the glue/ultrasonic welder). As can be seen in the photo, there’s not much inside these monitors, the logic is a single-chip solution, the rest of the PCB is dedicated to supplying the power rails for the various circuits. On the left is the power input & the DC-DC converter for the backlight, along with the DC-DC converter supplying the logic circuits. None of the capacitors here are damaged, everything looks good.
I then measured the output of the PSU, which under no load was the correct 19v DC. However applying any load caused the output voltage to drop like a proverbial brick. Applying a full load of 1.3A saw the output voltage drop so severely that the PSU tripped on it’s UVLO.

At 200mA of load the factory PSU is already dropping to 18v, with a 5.3kHz switching frequency appearing.

At higher load the frequency increases to 11.5kHz & the output voltage has dropped to 11.86v!

750mA was as high as I could make the supply go without it tripping itself out – the UVLO circuit trips at 9v. 12.6kHz is now riding on the severely low DC at this point.

The power supply is supposed to be rated at 1.3A at 19v, however with this fault it’s getting nowhere near that. The LG brand is on this PSU but it’s contracted out to Shenzen Honor Electric Co. Ltd.

Here’s the problem with this PSU. The output electrolytic has ballooned. I don’t have an ESR tester, but this cap has gone way past it’s sell-by date. It’s position right next to the heatsink with the output rectifier diodes has probably cooked it. The PSU isn’t that badly built for a Chinese one – there’s plenty of creepage distance on the PCB & even a couple of isolation slots.